Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, northwestern Argentine Andes

Journal of $outhAmerican Earth Sciences, Vol. 7. No. 2, pp. 179-207, 1994 Copyright O 1994 Elsevier Science Ltd& Earth Sciences & Resources Institute Printed in Great Britain. All rights resctwed 0895-9811/94 $7.00 + 0.00

Pergamon 0895-9811(94)E0016-T

Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, northwestern Argentine Andes R.A. MARRET1"l*. R.W. ALLMENDINGER1. R.N. ALONSO2 and R.E. DRAKE3 ]Dept. of GeologicalSciences,CornellUniversity,Ithaca,New York 14853-1504 USA; 2UniversidadNacional de Salta, (4400) Salta, Argentina;31389 GrizzlyPeak Blvd., Berkeley,California94708 (Received September, 1993; Revision Accepted December 1993) Abstract--Kinematic analysis of ~ 1500 fault-slip measurements from the Puna plateau and adjacent foreland of northwestern Argentina suggests that two regional kinematic regimes characterize late Cenozoic deformation: a thrust phase with ~WNW-ESE shortening and subvertical extension followed by a strike-slip phase with ~ENE-WSW shortening and ~NNW-SSE extension. Radiometric dating combined with field relationships demonstrate that thrust faulting started by 13 Ma and lasted, at least locally, until 1 Ma, and that strike-slip faulting started by 2 Ma and is still active. The shortening direction of the thrust phase, which accounts for most of the Andean shortening, differs from the coeval plate tectonic convergence direction and probably cannot be explained by later oroclinal bending, Paleostructural control of deformation kinematics and/or strike-slip faulting along the thermally weakened volcanic arc might explain the Mio-Pliocene shortening direction. The subhorizontal extension direction of the strike-slip phase is evident at all elevations studied, suggesting that local body forces do not drive it. A decrease in South America-Nazea plate convergence rate and/or complex three-dimensional effet:ts, possibly including kinematic variation with depth in the crust, might provide a satisfactory explanation.

Resumen--An~lisis de ~1500 indicadores cinemtiticos en el altiphmo de Puna y areas adjacentes de antepais en el noroeste Argentino sugieren que dos regfmenes cinemtiticos caraetefizan la deformaci6n de el Cenozoico tardio: una fase de cabalgamiento con aeortamiento ~WNW-ESE y extensi6n ~ENE-WSW. Datacioncs radiom6tricas y relaciones de eampo demuestran que el eabalgamiento empezo hate 17 M a y termino al menos localmente hate 1 M a y el fallamiento de rumbo comenzo hace 2 M a y permanece activo. La direcci6n de acortamiento difiere de la direcci6n de convergencia tect6nica contemporania y probablemente no puede set explicada por el tiltimo corvamiento oroclinal. Control paleoestructural y/o fallamiento rumbo-deslizante a lo largo d e z o n a s t e r m a l m e n t e d 6 b i l e s d e l a r c o v o l c ~ n i c o p o d r i a n e x p l i c a r la d e i r e c e i 6 n d e a c o r t a m i e n t o d u r a n t e el M i o - P l i o c e n e . L a

direcci6n de extensi6n subhorizontal de la fase rumbo-deslizante es evidente en todas las elevaciones estudiadas sugiriendo que no existe control local. Un eomplejo efecto tri-dimencional posiblemente con variaciones Cinemttticas con profundidad en la corteza podria proveer una satisfactoria explicaci6n.

INTRODUCTION

1986; Marrett and Allmendinger, 1987; S6brier et al., 1988b; Allmendinger et al., 1989) suggested that gravitational extension might be active in both mountain belts.

IDENTIFYING THE MECHANICS of continental plateau evolution is essential to understanding the processes of mountain building. We investigate two aspects of platean evolution in the Puna plateau of northwestern Argentina (Fig. i): the active mechanical state, and the tectonic development during the mid-Miocene to Recent. New fanlt-slip data, compilation of new and existing chronologic data, new analyses of existing paleomagnetic data, and synthesis of existing seismic data suggest that previously proposed models do not accurately describe the tectonic evolution of the Puna. Several models proposed in recent years predict that locally thickened continental crust, such as that in high mountain belts, can reach a gravitationally unstable mechanical state which produces horizontal extension (e.g. Dalmayrac and Motnar, 1981; Froidevaux and Isacks, 1984; Burchfiel and Royden, 1985; Molnar and LyonCaen, 1988). Previous interpretations of limited fault data from the Himalaya (Burchfiel and Royden, 1985; Herren, 1987) and the Andes (S~brier et al., 1985; Allmendinger,

Fault-slip data from the Puna plateau and adjacent foreland suggest that the gravitational effect of thickened crust in the region is more complicated than that predicted by simple models. The data do not show a significant correlation of young extensional surface faulting with high elevations, but hint that there may be important kinematic variation with depth. Complex three-dimensional effects related to the southward narrowing of the thick Puna crust might explain the areal distribution of young extensional faulting. The construction of a regional kinematic chronology for the Puna and adjacent foreland based on kinematic and chronologic data clarifies the tectonic development of the region. Mio-Pliocene thrust faults probably accommodated the majority of upper crustal shortening. Their shortening directions, however, are statistically distinct from the late Cenozoic South America-Nazca plate convergence direction and also from the shortening directious

A d d r e s s all c o r r e s p o n d e n c e a n d reprint r e q u e s t s to R i c h a r d W. AUmendinger, Cornell University, Ithaca, N Y 14853-1504, U S A P r e s e n t a d d r e s s o f R a n d a l l A. Marrett: A m o c o Production R e s e a r c h , P.O. B o x 3385, Tulsa, O K 74102, U S A S/~£S7:2-F

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R.A. MARRETI",R.W. ALLMENDINGER,R.N. ALONSO and R.E. DRAKE

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occurs over an approximately 30° east-dipping segment of the subducted Nazca slab (Barazangi and Isacks, 1976; Bevis and Isacks, 1984). At both ends of the plateau, Benioff zone contours indicate that the subducted slab shallows to subhorizontal dips (Barazangi and Isacks, 1976; Bevis and Isacks, 1984). The Nazca and South American plates have converged about 10 cm/yr in an ENE-WSW direction during the late Cenozoic (Pilger, 1984; Pardo-Casas and Molnar, 1987). Crustal seismicity in and around the Puna plateau attest to continuing deformation (Chinn and Isacks, 1983; Cahill et al., 1992). Compilation of fault-plane solutions from intracrustal earthquakes in the region indicates that thrusting predominates, although a few events have important components of oblique movement (Chinn and Isacks, 1983; Dziewonski et al., 1983a, b, 1985, 1987a, b, c, d, 1988).

16 c

20 c

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28 °

32 °

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74 °

70 °

66 °

62 °

Fig. 1. Map of the Central Andes showingthe region above3 km average elevation (the Altiplano-Punaplateau) in gray and the Sierras Pampeanas basement uplifts in horizontal ruled pattern. The contours show the depth to the Benioff Zone in kilometers (fromIsacks, 1988). Large arrow shows averagedirectionof late Cenozoic plate convergence. of Plio-Quatemary faults and modem earthquakes. Isacks (1988) applied Carey's (1958) concept of oroclinal bending to the central Andes, concluding that clockwise vertical axis rotation has occurred during the Cenozoic at the latitude range of the Puna. The rotation might be accommodated by in situ block rotations (Beck, 1988) and/or variation in the amount of crustal shortening along the Andean strike (Isacks, 1988). Analysis of limited paleomagnetic data suggests that vertical axis rotation cannot account for the anomalous shortening directions of Mio-Pliocene faults. Thus, late Cenozoic kinematics in the Puna and adjacent foreland differed from the plate kinematics. Paleostrucmral control of deformation kinematics in the Puna and adjacent foreland and/or strike-slip faulting along the thermally weakened volcanic arc might explain the differences between late Cenozoic plate motions and the kinematics in the Puna and adjacent foreland.

REGIONAL TECTONIC SETTING The Altiplano-Puna plateau (Fig. 1) is about 300 km wide, about 2000 km long, and is characterized by an average elevation of nearly 4 km (Isacks, 1988). The plateau

Previous syntheses of structural and chronologic data from the Puna and adjacent foreland (Fig. 2) agree that most Andean deformation took place during the Neogene (e.g. Schwab and Lippolt, 1976; Mingramm et al., 1979; Turner and M6ndez, 1979; Turner and Mon, 1979; Coira et al., 1982; Allmendinger et al., 1983; Salfity et al., 1984b; Allmendinger, 1986; Jordan and Alonso, 1987; Jordan and Gardeweg, 1989). In the northern Puna and in the Cordillera Oriental, the late Miocene is considered the period of most intense deformation, although prior and subsequent late Cenozoic deformation has been recognized at least locally. In the Subandean belt, the Santa Barbara System, the Sierras Pampeanas, and the southern Puna, Pliocene deformation is regarded as dominant. The structural geometries of the foreland south of 24 ° S (Fig. 3) are strongly influenced by pre-existing Cretaceous rift basins and the lack of an important wedge of undeformed Paleozoic strata (Allmendinger, 1983; Grief et al., 1991). Virtually all Andean volcanism in the Puna occurred during the Neogene (e.g. Coira et al., 1982, Pilger, 1984; Salfity etal., 1984b; Jordan and Gardeweg, 1989; Coira et al., 1993). During the latest Pliocene and Quaternary, small but widespread basaltic-andesite cinder cones and lava flows erupted mostly on the southern Puna (H6rmann et al., 1973; Coira and Pezzutti, 1976; Schwab and Lippolt, 1976; Coira and Paris, 1981; Deruelle, 1982; Gonz~ilez, 1983; Alonso et al., 1984a, b; Thorpe et al., 1984; Viramonte et al., 1984a; Knox et al., 1989; Schreiber and Schwab, 1987; Fielding, 1989; Kay et al., in press). The basaltic-andesites are commonly associated spatially and kinematically with faulting (Schwab and Lippelt, 1976; V'tramonte et al., 1984a; Marrett and Allmendinger, 1987; Mort et al., 1988; Allmendinger et al., 1989; Marrett and Emerman, 1992). In general, the basalticandesites are increasingly shoshonitic with distance above the subducted slab at depth (Knox et al., 1989). S. M. Kay has interpreted these mafic rocks and their association with the extensional structures described here as evidence of delamination of the mantle lid beneath the southern Puna (Kay and Kay, 1993; Kay et al., in press; Whitman et al., 1993).

Late Cenozoic tectonic evolution of the Ptma Plateau and adjacent foreland, NW Argentine Andes

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STUDY AREAS Field study in the Puna plateau and adjacent foreland (Fig. 2) consisted of measurement of fault-slip data, measurement of sextimentary bedding orientations, observation of stratigraphic constraints on the ages of faulting and folding, and (in some areas) lithologic mapping. The fault-slip data were analyzed using the graphical kinematic techniques described in Marrett and Allmendinger (1990). In this method, "P" and 'q'" axes - - in reality infinitesimal shoteaxing and extension axes, respectively, located at 450 to the striae and the pole to the fault plane - - are calculated and plotted on lower hemisphere stereographic projections. Faults of all different scales, from centimeters to kilometers of slip, were measured. We have shown elsewhere (Marrett and Allmendinger, 1990) that the fractal nature of fault kinematics allows the integration of data due to faults of all sizes. The original fault data may be obtained in Marrett (1990) or by writing to the senior author. Bedding orientations were used to infer fold kinematics by construction of r~ diagrams. Assuming that layer-parallel slip is the dominant deformation mechanism, folds will have parallel cylindrical geometry. For parallel cylindrical folds with upright axial planes, shallowly plunging axes, and open interlimb angles, the strike of the re-plane describing the poles to bedding will approximate the trend of the shortening direction. We determined the r~-planes using Bingham statistics which is a least squares algorithm. Fig. 2. Generalized geologic map of northwest Argentina and surrounding areas, showing the locations of the study areas described in text, Grayed box locations along Argentine-Bolivian border are described in Cladouhos et al. (this volume). Thick, white dashed line shows 3 km average elevation contour which generally delimits the Puna plateau from the foreland provinces. Boxed locations in the southern part of the map which do not appear in subsequent figures are described in detail in AUmendinger(1986), Allmendinger et al. (1989), and Grier et al. (1992).

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ician flysch deposits of low metamorphic grade (Ramos, 1972; Coira. 1973) are in fault contact with the Upper Cretaceons to lower Tertiary deposits of the Salts Group (Turner, 1958; Reyes and Salfity, 1973; Alonso, 1986). Both of these units are locally unconformably overlain by a small basin of clastic, volcaniclastic, and pyroclastic

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Fig. 3. Interpretive structure section of the northwest Argentine Andes at 25030' S. Only the major stratigraphic units and structures are shown. Faults in the eastern half of the section are interpreted as reactivated Cretaceous extensional structures. Bold faults in the Puna are thought to be Andean; faults drawn with lighter lines, pre-Andean. The existence and depth of the decollement beneath the Eastern Cordillera are entirely hypothetical. Modified from Grier (1990).

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R.A. MARRETT, R.W. ALLMENDINGER, R.N. ALONSO and R.E. DRAKE

rocks containing an interbedded airfaU tuff radiometrically dated at 6.99 + 0.18 Ma (Alonso, 1986). Deformation of uncertain kinematics must have occurred after the deposition of the Upper Cretaceous to lower Tertiary Salta Group and before the deposition of the upper Miocene strata toexplain their angularly unconformable relation. The upper Miocene strata are unconformably covered by laterally extensive welded tuff (Alonso, 1986). Analyses of minor fault kinematics and fold geometries indicate that at least two deformations of distinct kinematics have occurred at Loma Blanca since the late Miocene. First, poles to bedding in the upper Miocene strata and the Salta Group constrain subhorizontal NNE-SSW trending fold axes (Fig. 4a), suggesting post-6.99 + 0.18 Ma deformation with subhorizontal NNW-SSE shortening and subvertical extension. One of the minor faults studied has kinematics similar to those inferred for the late Tertiary folding. The second deformation produced dominantly strike-slip faults with subhorizontal ENE-WSW to subvertical shortening and subvertical to subhorizoutal NNW-SSE extension (Fig. 4b). A thrust fault with the latter style of kinematics cuts modem travertine deposits at the Loma Blanca mine (Figs. 4 and 4c), so this fault is inferred to have been active during the Quaternary. The shortening direction observed for the strike-slip faults (Fig. 4b) coincides with that of the thrust fault (Fig. 4c), suggesting that both sets of faults might represent the same phase of deformation. Because the thrust fault is inferred to have a Quaternary age, the strike-slip faulting may be younger than the folding as well.

Chorrillos. The Puncoviscana Formation of upper Protero-

zoic or Lower Cambrian age is a low-grade metamorphic flysch (Turner, 1960; Aceflolaza, 1973; Omarini, 1983) which comprises the oldest unit recognized in the area around San Antonio de los Cobres and Olacapato (Fig. 5). Ordovician catazonal granitoids (M~ndez et al.o 1972; Omarini et al., 1984) and Ordovician low-grade meta_rnorphic flysch and volcanic deposits (Rolled and Mingramm, 1968; Koukharsky and Mirre, 1974) are in fault contact with each other and with the Puncoviscana Formation (Turner, 1960; Vilela, 1969; Amengual et al., 1979). The Upper Cretaceous to lower Tertiary deposits of the Salta Group unconformably overlie older rocks (Turner, 1958; Vilela, 1969; Reyes and Salfity, 1973). Tertiary continental clastic strata southwest of Olacapato unconformably overlie Ordovician flysch (Vilela, 1969) and are correlative with the Geste Formation to the south (see Salar de Pastos Grandes, below) in which Eocene mammal fossils have been found (Alonso et al., 1982; Pascual, 1983). Eruption of dacitic and andesitic tufts and lavas in the area began by 17.2 + 0.5 Ma (S.F. Olson, personal communication, 1987) and lasted into the Quaternary (Schwab and Lippolt, 1974; Coira and Paris, 1981). A welded tuff radiometrically dated at 10.1 + 0.5 Ma (S. F. Olson, personal communication, 1987) unconformably covers the continental clastic strata southwest of Olacapato. Continental clastic strata east of San Antonio de los Cobres at Corte Blanco lie unconformably on the Salta Group (Turner, 1958; Reyes and Salfity, 1973; Viramonte et al., 1984b) and contain an airfall tuff radiometrically dated at 9.6 + 0.35 Ma (J. G. Viramonte, personal

Fig. 4. Geologic map of the Loma Blanca area (modifiedfrom Alonso, 1986). Lithologic contacts indicated with fine lines, fold axes with medium lines, faults with bold lines (dotted where covered, dashed where approximately located). Towns indicated with solid squares. Length of entire scale bar shown. 3(a) Poles to bedding in the upper Tertiary strata near Loma Blanca (N = 11). Great circle represents replane (296° , 82°) and grey square represents fold axis. (b) Kinematic axes of faults in upper Tertiary strata near Loma Blanca (N = 20). Contour interval = 2.0 sigma. Solid dots represent shortening axes, small open squares represent extension axes, and large squares represent Bingham distribution axes: shortening = 246°, 9% extension = 155°, 10°. (c) Kinematic axes of fault in travertine near Loma Blanca (N = 2); shortening (closed) = 58 °, 17°; extension (open) = 224°, 73 °.

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Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, NW Argentine Andes

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Fig. 5. Geologic map of the ChorriUos-CorteBlanco area (modifiedfrom Amengual et al.. 1979). See Fig. 4 for explanation of symbols. communication, 1988). Late Pleistocene eruptions of two basaltic-andesite cinder cones and lava flows west of San Antonio de los Cobres were radiometrically dated at 0.78 + 0.1 Ma and 0.2 + 0.08 Ma (HOrmann et al., 1973; Schwab and Lippolt, 1976; Coira and Paris, 1981; Deruelle, 1982; Schreiber and Schwab, 1987; Knox et al., 1989; Coira et al., 1993). The lower Tertiary strata southwest of Olacapato are folded about subhorizontal NNE-SSW trending axes (Figs. 5, 6a), suggesting subhorizontal WNW-ESE shortening and subvertical extension. The folding must have begun before the deposition of the 10.1 _+0.5 Ma welded tuff, which is angularly unconformable over folds, but may have been ongoing when the tuff was emplaced because fiamme in the tuff locally dip as steeply as 64*. The lower Tertiary strata of the Salta Group at Corte Blanco are folded about subhorizontal NNE-SSW trending axes (Fig. 6b), suggesting subhorizontal WNW-ESE shortening and subvertical extension. The folding began before the development of the angular unconformity beneath the 9.6 :t: 0.35 Ma continental clastic strata, but may have continued during subsequent deposition because the upper Miocene elastic strata are folded with similar kinematics (Fig. 6c). Minor faults in the 9.6 + 0.35 Ma continental elastic strata have subhorizontal WNW-ESE shortening and mostly subhorizontal NNE-SSW extension (Fig. 6d). The coincidence of shortening directions of faulting and folding suggest that both processes may represent the same phase of deformation. The Chorrillos fault is marked by numerous scarps and sag ponds, indicating Pleistocene activity (Strecker et al., 1985). One scarp is developed in the 0.78 + 0.1 Ma old basaltic-andesite flow and deflects flows from the 0.2 +

0.08 Ma old basaltic-andesite flow, so some fault movement must have occurred between these dates. The Chorrillos fault defines one segment of the so-called "El ToroOlacapato-Calama lineament" (e.g. Mon, 1979; Allmendinger et al., 1983; Salfity, 1985) which extends from the foreland in the southeast (see Quebrada del Toro below) to the Chilean forearc in the northwest. However, the Chorrillos fault loses geomorphic expression in the map area (Fig. 5) aleng strike both to the northwest and to the southeast, so the El Toro-Olacapato-Calama lineament is not a single, through-going Quaternary fault. The Chorrillos fault separates structurally bounded outcrops of Salta Group strata by 10-20 km in a left-lateral sense (Allmendinger et al., 1983), however some of this displacement could be Tertiary. To the northwest, the Chorrillos fault terminates near a set of ESE-dippiag thrust faults affecting Tertiary strata (Schwab, 1980). If the ChorriUos fault was active at the same time as the thrust faults, it might have served as a lateral ramp during the Tertiary. Quaternary movement on the Chorrillos fault is oblique normal-left lateral, with moderately ENE-plunging shortening and subhorizontal NNW-SSE extension (Fig. 6e). A minor thrust fault between the two basaltic-andesite flows cuts the Chorrillos fault and has similar subhorizontal ENEWSW shortening but subvertical extension (Fig. 6t). The average shortening direction of the Chorrillos fault forms an angle of about 38 ° with the average shortening direction of the faults studied at Corte Blanco (Figs. 6d and e). This angle is nearly double the sum of the 95% confidence cone half apical angles for the two distributions, so the distributions are statistically distinct, Although it is possible that the faults in both areas represent the same phase of deformation, we prefer to interpret

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R.A. MARRETI', R.W. ALLMENDINGER, R.N. ALONSO and R.E. DRAKE

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Fig. 6. Chorrillos-Corte Blanco structure data. (a) Poles to bedding in the lower Tertiary strata near 01acapato (N = 18). Great circle represents n-plane (287°, 83°) and grey square represents fold axis. (b) Poles to bedding in the lower Tertiary strata near Corte Blanco (after unfolding upper Tertiary strata; N = 6). Great circle represents n-plane (125°, 81°) and grey square represents fold axis. (c) Poles to bedding in the upper Tertiary strata near Corte Blanco (N = 9). Great circle represents n-plane (285°, 88°) and grey square represents fold axis. (d) Kinematic axes of faults in upper Tertiary strata near Corte Blanco (N = 17). Bingham distribution (solid squares): shortening = 103°, 8°; extension = 12°, 6 °. (e) Kinematic axes of Chorrillos fault (N = 27). Bingham distribution (solid squares): shortening = 79 °, 42°; extension = 345°, 4 °. (f) Kinematic axes of fault cutting the Chorrillos fault (N = 4). Bingham distribution: shortening (closed) = 253°, 4°; extension (open) = 359°, 77°. the faults at Corte Blanco as representing Tertiary deformation distinct from Quaternary deformation along the Chorrillos fault. Such an interpretation would be consistent with better constrained temporal changes in fault kinematics observed in adjacent areas (e.g. Negra Muerta, Pastos Grandes, Quebrada del T~o). Also, this interpretation would be consistent with the hypothesis that the late Miocene strata at Corte Blanco and the welded tuff southwest of Olacapato were deposited during a single phase of folding. The Quaternary basaltic-andesites in the hanging wall of the Chorrillos fault are closely related in time to extensional movement along the fault, as demonstrated by their

relationships with scarps (Fig. 5). Similar relationships between Quaternary basahic-andesites and fault kinematics have been found in other areas (see Bequeville, Salar de Hombre Muerto, Salar de Antofalla, La Poma, and Pasto Ventura-E1 Pefl6n below). Marrett and Emerman (1992) present a mechanical model to explain the surface eruption of basaltic-andesites along faults during the Quaternary.

Negra Muerta. At Negra Muerta (Fig. 7) the Puncoviscana Formation, composed of upper Proterozoic or Lower Cambrian low-grade metamorphic flysch (Turner, 1960; Ace~olaza, 1973; Omarini, 1983), is thrust over strata of the Upper Cretaceous to lower Tertiary Salta Group and

Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, NW Argentine Andes

185

Fig. 7. Geologic map of the Negra Muerta area 6(a) Poles to bedding in the Tertiarystrata near Negra Muerta(1'4 = 9). Great circle represents n-plane (107° , 85°) and grey square represents fold axis. (b) Kinematic axes of fault cutting dike near Negra Muerta (N = I). Shortening axis = 13°, 57°; extension axis = 145°, 23°. (c) Kinematicaxes of faults near Negra Muerta (N = 7). Bingham distribution (solid squares): shortening = 2510,480; extension = 346° , 4° .

Tertiary continental clastic strata of uncertain age (Turner, 1958; Reyes and Salfity, 1973; Mdndez, 1975; Llambfas et al., 1986). Neogene volcanic and subvolcanic rocks unconformably overlie or intrude older rocks (Llambias et al., 1986). A dike associated with these volcanic rocks intrudes the Toro Muerto fault. The Toro Muerto and the Calchaquf faults have thrust movement with subhodzontal WNW-ESE shortening and subvertical extension (see La Poma below). The Calchaqui fault affects the Acay monzonite stock (Llambias et al., 1986) which was radiometrically dated at 26 + 1 Ma (Linares, 1979), so thrusting postdates the Oligocene. Folds developed in the Tertiary continental clastic strata at Negra Muerta have subhorizontal NNE-SSW trending axes (Fig. 7a), consistent with the kinematics of the Toro Muerto and Calchaqui faults, and are unconformably covered by volcanic rocks (Fig. 7). Furthermore, the dike intruding the Toro Muerto fault has not been deformed by subsequent movement along that fault, indicating that thrusting had ended before emplacement of the dike. The dike stitching the Toro Muerto fault is cut by a minor normal fault with moderately N-plunging shortening and subhorizontal NNW-SSE extension (Fig. 7b). Minor faults with mostly normal slip in the Tertiary continental clastic strata have similar extension directions (Fig. 7c) and may represent the same phase of deformation. Salar de Pastos Grandes. Santa Rosa de Pastos Grandes and Sijes (Fig. 8) lie in a NNE-SSW trending depression flanked on the east by ranges composed of Ordovician catazonal granitoids (Mdndez et al., 1972; Omariul et al., 1984) and on the west by ranges composed of Ordovician low-grade metamorphic flysch (Turner, 1964; Rolled and Mingramm, 1968). Continental clastic strata of the Geste Formation sit unconformably on the Ordovician flysch

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Fig. 8. Geologicmap of the Salar de Pastos Grandes area, modified from Alonso, 1986.

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R.A. MARRETr, R.W. ALLMENDINGER,R.N. ALONSO and R.E. DRAKE

(Turner, 1964; Alonso, 1986) and contain Eocene mammal fossils (Alonso et al., 1982; Pascual, 1983). Eruption of dacitic and andesitic tufts and lavas in the northern part of the area began by the end of the early Miocene and ended in the late Miocene (S. F. Olson, personal communication, 1987). The Singiiel Formation is dominated by conglomerates (Alonso, 1986; Alonso and Guti6rrez, 1986) and contains a welded tuff that may be a lateral equivalent to an intracaldera tuff just north of the map area (based on spectral interpretation of satellite imagery), which was radiometrically dated at 17.2 + 0.5 Ma (S. E Olson, personal communication, 1987). West of Sijes, a conglomerate unit that may be a lateral equivalent of the Singiiel Formation sits unconformably on folded strata of the Geste Formation. Also resting unconformably on the Geste Formation southwest and east of Santa Rosa de Pastos Grandes is a welded tuff radiometrically dated at 10.6 + 0.4 Ma (S. E Olson, personal communication, 1987). The same tuff is unconformable on the Singtlel Formation to the northeast of Sijes (Alonso and Guti6rrez, 1986). The Pozuelos Formation consists of massive halite with intercalations of fine-grained clastic strata (Turner, 1964; redefined by R. N. Alonso and T. E. Jordan, pers. comm., 1989) and contains a 7.6 + 1.1 Ma old airfall tuff (Alonso et al., 1991); however, the relationship between the Pozuelos and older strata is uncertain (Alonso, 1986). The Sijes Formation, consisting of finegrained clastic strata intercalated with borate bearing evaporites in the lower part of the section and in the upper part of the section (as considered here) consisting of conglomerates, conformably overlies the Pozuelos Formation (Turner, 1964; Alouso, 1986). An airfall tuff near the change fr~a fine- to coarse-grained sediments in the Sijes Formation was dated at 4.0 + 1.2 Ma (Alonso et al., 1991). Lake sediments of the Blanca Lila Formation lie unconformably on the Sijes Formation and have been radiometricaUy dated at 1.57 + 0.74 Ma (Strecker, 1987). Two faults and associated folds in the Eocene Geste Formation east of Santa Rosa de Pastos Grandes must have developed by the late Miocene because they are covered by the 10.6 + 0.4 Ma old welded tuff. The faults, which have displacements of a few 100 m in the Geste Formation but do not similarly affect the welded tuff, have subhorizontal WNW-ESE shortening and subvertical extension (Fig. 9a). The associated folds have subhorizontal NNE-SSW trending axes (Fig. 9b), suggesting similar kinematics to those of the faults. Folds with similarly trending axes affect the Geste Formation west of Sijes and are unconformably covered by the Singtiel Formation (Turner, 1964), so deformation may have begun before 17.2 + 0.5. Other NNE-SSW trending folds deform the upper Miocene to Pliocene Sijes Formation south of Sijes (Turner, 1964; Alonso, 1986), so the pre-late Miocene phase of deformation either lasted into the Pliocene or a second phase of deformation with similar kinematics occurred after 4.0 + 1.2 Ma. Similar kinematics also characterize a fault east of Santa Rosa de Pastos Grandes with displacement of about one kilometer, as well as minor faults in Cenozoic strata of uncertain stratigraphic position near Ochaqui (Fig. 9c).

Minor faults in the 1.57 + 0.74 Ma old Blanca Lila Formation at Sijes have kinematics distinct from those of the Mio-Pliocene deformation(s) described above, with steeply ENE-plunging shortening and subhorizontal NNW-SSE extension (Fig. 9d). A fault cutting poorly consolidated colluvium southwest of Cerro Aznfre (Fig. 8), with ~15 m displacement inferred, has moderately NEplunging shortening and subhorizontal NW-SE extension (Fig. 9e). Numerous minor faults in the Sijes Formation south of Sijes and a fault southwest of Santa Rosa de Pastos Grandes displacing the 10.6 + 0.4 Ma old tuff ~100 m have kinematics similar to the above Quaternary faults (Fig. 9f); these normal and strike-slip faults are too small to show on the geologic map (Fig. 8). Cerro Pircas. Cerro Pircas (Fig. 10) is composed of the Eocene Geste Formation (Turner, 1964; Alonso et al.,

1982; Pascual, 1983) and is flanked on the northeast and southwest by ridges composed of Ordovician low-grade metamorphic flysch (Turner, 1964; Aceflolaza and ToseUi, 1971). The Ordovician strata are deformed in regionally consistent folds with subhorizontal N-S to NNE-SSW trending fold axes (Mon and Hongn, 1988). The folds are believed to have formed during the Ordovician because Silurian strata locally rest unconformably on the Ordovician strata (Mon and Hongn, 1988). Folds in the Geste Formation near Cerro Pircas have subhorizontal NNE-SSW trending axes (Fig. 10a), suggesting subhorizontal WNW-ESE shortening and subvertical extension. Because the basal unconformity of the Geste Formation is folded (Fig. 10), folding during the Tertiary must have affected the Ordovician strata. The Tertiary folding of the Ordovician strata may have been accommodated by amplification of older folds because the Tertiary folds are similar in orientation to pre-Silurian folds in the Ordovician strata. Two large faults truncate the folds and, thus, are younger (Fig. 10). Both of these faults have subhorizontal NE-SW shortening (Fig. 10b), but the fault northeast of Cerro Pircas has mostly subvertical extension (indicating thrust movement) whereas the fault southwest of Cerro Pircas has mostly moderately SE-plunging to subhorizontal NW-SE extension (indicating oblique thrust-right lateral movement). The kinematic and sequential similarities between the deformation at Cerro Pircas and deformation in adjacent areas (e.g. Salar de Pastos Grandes, Sierra de Bequeville) suggest that at Cerro Pircas the folding represents Mio-Pliocene deformation and the faulting represents Plio-Quaternary deformation. Sierra de Bequeville. The Sierra de Bequeville (Fig. 11)

has two parallel ridges, one on the west composed of or underlain by Ordovician low-grade metamorphic flysch and another on the east composed of, or underlain by. multiply deformed gneiss, schist, and pegmatite of uncertain age (Turner, 1964; Aceflolaza et al., 1975). Continental clastic strata of the Eocene Geste Formation unconformably overlie the Ordovician flysch at Esquina Colorada (Turner, 1964). The lower Tertiary strata are in turn unconformably overlain in Quebrada Tar6n by a unit of massive welded tuff (Bequeville Formation of Turner, 1964) with moder-

Late Cenozoic teet_nnic evolution of the Puna Plateau and adjacent foreland, NW Argentine Andes

$11ettm~ am*

187

ftmMen ~*l*

(d)

( ~eltw~

Axm

~

Fig. 9. Pastes Grandes structure data. (a) Kinematic axes of faults covered by welded tuff east of Santa Rosa de Pastos Grandes (N = 5). Bingham distribution: shortening (closed) = 127°, 22°; extension (open) = 325°, 67°. (b) Poles to bedding in the lower Tertiary strata covered by welded tuff east of Santa Rosa de Pastos Grandes (N = 10). Great circle represents r~-plane (289°, 82°) and grey square represents fold axis. (e) Kinematic axes of fault east of Santa Rosa de Pastos Grandes (N = 19). Bingham distribution (solid squares): shortening = 115", 00; extension = 208", 87°. (d) Kinematic axes of faults near Sijes (N = 6). Bingham distribution: shorterring (dosed) = 62°, 64°; extension (open) = 167°, 7 °. (e) Kinematic axes of fault near Cerro Azufre (N = 16). Bingham distribution (solid squares): shortening = 39°, 43°; extension = 138°, 9 °. (f) Kinematic axes of faults near of Sijes and southwest of Santa Rosa de Pastos Grandes (N = 40). Bingham distribution (solid squares): shortening = 15°, 84°; extension = 162°, 5°. ately to steeply dipping fiAmma and a minimum thickness of -500 m. Associated with the tuff are subvolcanic intrusions near the Bequevine fault at the upper reaches of Quebrada Carro Grande. The thickness of the tuff, the orientation of fmmme, And the association with intrusions suggest that the massive tuff represents proximal or intracaldera deposits. Also unconformable on the lower Tertiary clastic strata are volcaniclastic strata and interbeAded welded tufts in Quel~ada Fabian, which are interpreted to be correlative with the massive tuff to the north and have been dated at 16.7 + 0.1 Ma (VAndarvoort and Zeitler, pers. comm., 1990). Unconformable on the volcaniclastic strata in the southern part of the Sierra de Bequeville and on the

high-grade metamorphic rocks in the northern part are continental clastic strata from which we obtained an age of 13.21 + 0.26 Ma (Table 1). Basaltic-andesite cinder cones and lava flows along the BequeviUe fault are geochemically akin to the widespread basaltic-andesites of the Ptma (B. Coira and S. M. Kay, personal communication, 1989)', which have yielded Plio-Quaternary radionaetric ages (Schwab and Lippolt, 1974; Coira and Pezzutti, 1976; Coira and Paris, 1981; Gonz~llez, 1983; Alonso et al., 1984a). The volcaniclastic unit lies unconformably on the lower Tertiary Geste Formation in Quebrada Fabian (Fig. 11), however insufficient data were collected to confidently

188

R.A. MARRETr, R.W. ALLMENDINGER, R.N. ALONSO and R.E. DRAKE

Shortening Axes

Extension Axes

Fig. I0. Geologic map of the Cerro Pircas area. (a) Poles to bedding in the lower Tertiary strata near Cerro Pircas (N = 17). Great circle represents n-plane (283° , 76°) and grey square represents fold axis. (b) Kinematic axes of faults near Cerro Pircas (N = 26). Bingham distribution (solid squares): shortening = 232°, 18°; extension = 126°, 40 ° describe the kinematics of the pre-16.7 + 0.1 Ma deformation of the Geste Formation. The upper Tertiary clastic strata unconformably cover folds in the volcaniclastic unit (Fig. 11), which have subhorizontal NNE-SSW trending axes (Fig. 12a) suggesting deformation with subhorizontal WNW-ESE shortening and subvertical extension between 16.7 _+0.1 and 13.21 + 0.09 Ma. The upper Tertiary clastic strata are gently folded and steeply tilted with a geometry consistent with folds in the volcaniclastic strata (Fig. 12b), suggesting that subhorizontal WNW-ESE shortening and subvertical extension was also active after deposition of the upper Tertiary clastic strata at 13.21 + 0.09 Ma. These relations indicate that either the volcaniclastic unit and/or the upper Tertiary clastic strata were deposited syntectonically or three/two phases of deformation with similar kinematics bracketed their deposition, all during the midMiocene. Minor faults in the lower Tertiary clastic strata, the volcaniclastic strata, and the upper Tertiary clastic strata have similar kinematics (Fig. 12c), and may be products of the same phase(s) of deformation. The WNW-dipping Bequeville fault may have formed originally as a thrust fault during the phase(s) of deformation that produced the folding and faulting described above, because presently in Quebrada Fabian and near Vega Chuculaqui the Bequeville fault juxtaposes Ordovi-

Fig. 11. Geologic map of the Sierra de BequeviUe area. cian flysch in the hanging wall with Cenozoic rocks in the footwall. However, a scarp indicating Quaternary oblique normal-right lateral movement marks its trace almost continuously for over 30 km through the axis of the Sierra de BequeviUe (Fig. 11). Analysis of fault-slip data from the fault indicate moderately ENE-plunging shortening and subhorizontal NNW-SSE extension (Fig. 12d). Minor faults in the upper Tertiary clastic and volcaniclastic strata with similar kinematics (Fig. 12e) may correspond to the same Quaternary phase of deformation. The basaltic-andesites near the generally NNE-SSW striking Bequeville fault are associated with more nearly NE-SW striking segments in the fault (Fig. 11). Right-lateral movement along the Bequeville fault should produce extensional pull-apart deformation at the NE-SW striking segments, which may explain the localization of eruptive vents in these areas and is consistent with the interpretation of a Plio-Quaternary age for the basaltic-andesites. South of the map area the Bequeville fault may be related to the Acazoque fault (see Salar de Antofalla below) which strikes NE-SW and is associated with extensive basaltic-andesite volcanism, including that in the Salar de Hombre Muerto area which is discussed in the following section. Salar de Hombre Muerto. Although the ages of the various

lithologic units in the Salar de Hombre Muerto area (Fig. 13) are reasonably well constrained, their structural relationships are still poorly understood (Alonso, 1986). Ordovician low-grade meta-flysch deposits (Acefiolaza et al.,

Late Cenozoic tectonic evolution of tb," Puna Plateau and adjacent foreland, NW Argentine Andes

Sllo~enulg Axes

Exter~on Axes

Shoflermg ~a~s

~ i ~

Axes

Sho~ening Axes

Extension Axes

Fig. 12. (a) Poles to bedding (not unfolded) in the volcaniclastic unit in the Sierra de BequeviUe (N = 13). Great circle represents r~-plane (287 °, 83 °) and grey square represents fold axis. (b) Poles to bedding in the upper Tertiary clastic strata in the Sierra de Bequeville (N = 7). Great circle represents n-plane (275°, 72°) and grey square represents fold axis. (c) Kinematic axes of faults in the Sierra de Bequeville (N = 23). Bingharn distribution (solid squares): shortening = 118°, 7°; extension = 256°, 81 °. (d) Kinematic axes of the Bequeville fault (N = 14). Bingham distribution (solid squares): shortening = 64°, 27°; extension = 334°, 0°. (e) Kinematic axes of faults in the Sierra de BequeviUe (N = 7). Bingham distribution (solid squares): shortening = 65°, 80; extension = 331°, 28°.

189

1975) are unconformably overlain by the continental clastic strata of the Eocene Geste Formation (Alonso, 1986; Alonso and Fielding, 1986). The base of the clastic, volcaniclastic, and pyroclastic Catal Formation is unknown (Alonso, 1986; Alonso and Guti6rrez, 1986), however interbedded airfall tufts near the bottom and top of the section exposed on Farall6n Catal yielded ages of 15.0 + 2.4 Ma and 7.2 + 1.4 Ma, respectively (Alonso et al., 1991). The base of the clastic, evaporitic, and pyroclastic Sijes Formation is also unknown (Alonso, 1986), however interbedded alrfaU tuff was dated at 5.86 + 0.14 Ma (Alonso et al., 1984a). The ages of composite volcanoes bordering the northeastern and southern edges of the map area are uncertain; because of their similarities with other dated volcanoes in the region they are inferred to have late Tertiary ages (Alonso et al., 1984b; Sparks et al., 1985). A thick welded tuff in the southern part of the map area corresponds to the Galan ~nimbrite sheet dated at 2.03 + 0.07 Ma (Gonzalez, 1984; Sparks et al., 1985). Basaltic-andesite cinder cones and lava flows (HOrmann et al., 1973) unconformably overlie the Catal and Sijes Formations and have been radiometrically dated at 0.754 + 0.028 Ma (Alonso et al., 1984a). The strata of the Sijes Formation are folded about dominantly NNE-SSW trending axes (Alonso, 1986), suggesting deformation after 5.86 + 0.14 Ma with subhorizontal WNW-ESE shortening and subvertical extension. This deformation must have taken place before 0.754 + 0.028 Ma when basaltic-andesites were unconformably deposited on the folded strata. Strata of the Catal Formation dip homoclinally to the ESE and are unconformably covered with basaltic-andesites (Alonso, 1986), consistent with deformation of similar kinematics and timing as the folding of the Sijes Formation. The 2.03 + 0.07 Ma old Cerro G a l ~ ignimbrite is cut by a fault with a conspicuous scarp (Alonso et al., 1984b; Gonz~Uez, 1984) in the southeastern part of the map area (Fig. 13). Limited measurements of minor faults in the ignimbrite near the scarp indicate subhorizontal ENEWSW shortening and subvertical extension (Fig. 13). A shallowly SSW-dipping fault with a thrust component of movement cuts an upper Pleistocene basaltic-andesite cinder cone west of the Tincalayu borate mine and, thus, postdates folding of the Sijes Formation. This fault probably has kinematics similar to those of the fault cutting the Cerro Gal~in ignimbrite, suggesting that the latter fault may also post-date folding of the Sijes Formation. Minor faults in the Sijes Formation at the Tincalayu mine have steeply S-plunging shortening and subhorizontal WNWESE extension (Fig. 13). Although the kinematic axes are distinct from the above mentioned thrust faults, the same stress field could have produced both, given appropriately oriented, pre-existing heterogeneities; thus, these faults may represent the same phase of deformation as the thrust faults. The Quaternary basaltic-andesites in the Salar de Hombre Muerto area (Fig. 13) may be related to Quaternary extensional pull-apart deformation associated with the Bequeville fault discussed in the previous section and the Acazoque fault discussed in the following section.

190

R.A. MARRETr, R.W. ALLMENDINGER, R.N. ALONSO and R.E. DRAKE

Fig. 13. Geologic map of the Salar de Hombre Muerto area, modified after Alonso et al. (1984b). Lower right: Kinematic axes of fault cutting the Gal~in ignimbrite (N = 3). Bingham distribution: shortening (closed) = 51 °, 8°; extension (open) = 202 ° , 81 ° . Upper right: Kinematic axes of faults near Tincalayu (N = 9). Bingham distribution : shortening (solid) = 176°, 69°; extension (open) = 297° , 11° . Gal&n Ignimbnte Sties Formation CataJFormation [[~

GesteFormation

B

~"=

Salar de Antofalla. East of the Salar de Antofalla (Fig. 14), Ordovician low-grade meta-flysch deposits (Aceflolaza et al., 1975) are unconformably overlain by continental clas-

tic strata, possibly correlative with the Eocene Geste Formarion (Turner, 1964; Alonso and Fielding, 1986). Similar relationships are observed west of the area shown in Fig. 14 (Coira and Pezzutri, 1976). Middle Miocene to Pliocene volcanic complexes border the salar on the west (Coira and Pezzutti, 1976; Gonz~ilez, 1983) and a volcanic complex east of the salar is probably of a similar age (Sparks et al., 1985). West of the map area, welded tufts dated at 3.0 + 2 Ma are angularly unconformable on the Tertiary clastic strata (Coira and Pezzutti, 1976). The Tertiary clastic strata and volcanic rocks are unconformably overlain by widespread basaltic-andesite lava flows and cinder cones 03. Coira and S. M. Kay, personal communication, 1989) which may be correlative with basaltic-andesite west of the map area dated at 2.0 + 1.0 Ma (Coira and Pezzutti, 1976).

Shortening Axes

Extension Axes

Insufficient data were collected to confidently characterize deformation of the Tertiary clastic strata prior to deposition of the basaltic-andesites. However, west of the map area NNE-SSW striking thrust faults and folds with NNE-SSW trending axes developed in the Tertiary clastic strata are unconformably covered by 3.0 + 2 Ma old tufts, suggesting prior deformation with approximate kinematics of subhorizontal WNW-ESE shortening and subvertical extension. The Acazoque fault (Segerstrom and Turner, 1972; Acefiolaza et al., 1976) dips to the SE and is marked over much of its length by a prominent scarp developed in the basaltic-andesites, so it probably has been active during

Fig. 14. Geologic map of the Salar de Antofalla area, modified after Alonso et al. (1984b). Kinematic axes of Acazoque fault (N = 16). Bingham distribution (solid squares): shortening = 52 °, 50°; extension = 142°, 0°.

Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, NW Argentine Andes

191

the Quaternary. Minor faults in the Tertiary elastic strata near the scarp have moderately N-E-plunging shortening and subhorizontal N-W-SE extension (Fig. 14). The Acazoque fault has been interpreted previously as a right step in a system of regional faults generally striking N-S to NNE-SSW (Segerstrom and Turner, 1972; Alonso et al., 1984b), one of these being the Bequeville fault shown above to have dominantly right-lateral movement during the Quaternary. The concentration of basalticandesites near the Acazoque fault (Alonso et al., 1984b) can be explained by the Quaternary fault kinematics. Right-lateral movement along the regional fault system should produce extensional pull-apart deformation at right steps, consistent with observations of Quaternary movement along the Acazoque fault, and may explain the localization of eruptive vents in these areas. Foreland Study Areas Quebrada de Humahuaca. Flanking the Quebrada de Humahuaca (Fig. 15), upper Proterozoic or Lower Cambrian low-grade metamorphic flysch of the Puncoviscana Formation (Turner, 1960; Aceflolaza, 1973; Omafini, 1983) are unconformably overlain by marine clastic strata of the Cambrian and Ordovician Mes6n and Santa Victoria Groups and bounded by west-dipping thrusts (Turner, 1960; Ramos et al., 1967; Amengual and Zanettini, 1974). Upper Cretaceous to lower Tertiary deposits of the Salta Group overlie the lower Paleozoic and Precambrian rocks (Turner, 1958; Ramos etal., 1967; Reyes and Salfity, 1973; Amengual and Zanettini, 1974). At the top of the section, upper Tertiary to lower Pleistocene continental elastic strata and pyroclastic rocks include the Mairnar~i Formation (Salfity et al., 1984a). which contains late Miocene to early Pliocene mammal fossils (Berman, 1987), and the Uqu+a Formation, the base of which has been dated at 2.78 + 0.09 Ma and the top of which is ~1.5 Ma based on magnetic polarity stratigraphy (Marshall et al., 1982). NNE-SSW trending folds in the Salta Group south of Uquia are unconformably overlain by the Uquia Formation (Fig. 15a), suggesting subhorizontal WNW-ESE shortening and subvertical extension prior to 2.78 :!: 0.09 Ma. In the same area, the strata of the Uquia Formation are cut by an oblique reverse-right lateral fault with hundreds of meters inferred displacement. This fault must be younger than 1.5 Ma and has shallowly ENE-plunging shortening and shallowly SSE-plunging extension (Fig. 15b). Near Tilcara, a fault juxtaposes the Maimar~i Formation with poorly consolidated alluvium inferred to be of Quaternary age (Salfity et al., 1984a). Fault striae developed in the Maimar~i Formation generally indicate thrust movement with shallowly WNW-plunging shortening and steeply ESE-plunging extension (Fig. 15c). However, striae developed in the alluvium and at the contact between the alluvium and the Maimar~i Formation indicate right-lateral movement with subhorizontal NE-SW shortening and subhorizontal NW-SE extension (Fig, 15d). The dip-slip faulting that affects the Mairnar~i Formation indicates the thrust kinematics must postdate the late Miocene

Fig. 15. Geologic map of the Quebrada Humahuaca area (modified after Amengual et al., 1979). (a) Poles to bedding in the Salta Group coveredby the Uqufa Formation near Uqufa (N = 7). Great circle represents n-plane (104°, 80°) and grey square represents fold axis. (b) Kinematic axes of fault cutting Uqufa Formation near Uquia (N = 11). Bingham distribution: shortening (solid) = 60°, 18°; extension (open) = 161", 30°. (c) Kinematic axes of fault cutting Maimar~iFormation near Tilcara (N = 10). Bingham distribution: shortening (solid) = 279°, 25°; extension (open) = 101°, 65°. (d) Kinematic axes of fault cutting conglomerate near Tilcara (N = 7). Bingham distribution: shortening (solid) = 40°, 4°; extension (open) = 131°, 9°. (e) Kinematic axes of fault cutting Maimar~ Formation near Maimarfi (hi = 8). Bingham distribution: shortening (solid) = 228°, 62°; extension (open) = 330", 6°. to early Pliocene. Because only strike-slip faulting affects the alluvium, we infer that thrust kinematics pre-date the alluvium and that later fault reactivation produced the right-lateral kinematics. Thus, the thrust kinematics are probably of Pliocene age and the right-lateral kinematics are probably of Quaternary age. Minor faults in the Maimar~i Formation near Maimar~ have moderately SWplunging shortening and subhorizontal NW-SE extension (Fig. 15e), which are consistent with Quaternary deformation observed at Tilcara and Uqufa.

192

R.A. MARRETI', R.W. ALLMENDINGER,R.N. ALONSO and R.E. DRAKE

Quebrada del Toro. Mountain ranges surrounding the Quebrada del Toro (Fig. 163.46) are composed of the upper Proterozoic or Lower Cambrian low-grade metamorphic flysch deposits of the Puncoviscana Formation (Turner, 1960; Ace/lolaza, 1973; Ramos, 1973; Omarini, 1983). North of Affarcito, the Tastil Granite of late Proterozoic or early Cambrian age (Kilmurray and Igarz,'ibal, 1972; Ramos 1973; Omarini et al., 1985) intrudes the Puncoviscana Formation. Marine clastic strata of the Cambrian and Ordovician Mes6n and Santa Victoria Groups unconformably overlie the Puncoviscana Formation northeast of G61gota (Vilela, 1956; Turner, 1960; Schwab and Schttfer, 1976). The Upper Cretaceous to lower Tertiary deposits of the Salta Group unconformably overlie the Cambro-Ordovician strata northeast of G61gota and the Puncoviscana Formation southwest of G61gota (Vilela, 1956; Turner, 1958; Reyes and Salfity, 1973; Schwab and Schger, 1976). Younger continental clastic strata (inferred to be upper Cenozoic because of interbedded pyroclastic and volcaniclastic strata) are approximately concordant with the underlying Salta Group (Vilela, 1956; Schwab and Schiffer, 1976). The upper Tertiary strata are mapped as two units (at the upper limit of a prominent interbedded welded tuff) to illustrate late Cenozoic structural geometries and strati~aphic relations. The lack of the lower unit northwest of the San Bernardo fault and the presence of unconformities within the upper Tertiary (Schwab and Sch~er, 1976) suggest that important relief may have developed before and/ or during deposition of the upper Tertiary. We dated a tuff interbedded with a golden-colored conglomerate near the top of the upper Cenozoic section at 0.98 + 0.02 Ma (Table 1). The San Bernardo, G61gota, Carachi, and associated minor faults have subhorizontal NW-SE shortening and subvertical extension (Fig. 17a). Folds related to this faulting affect the upper Tertiary strata, including the 0.98 + 0.02 Ma tuff, so this deformation was active after deposition of the upper Tertiary section. However, northwest of G61gota strata downsection from the 0.98 + 0.02 Ma conglomerate unconformably cover N-S trending folds in upper Tertiary rocks. These folds are associated with the G61gota fault, so deformation with NW-SE shortening both pre- and post-dates 0.98 + 0.02 Ma. The Solti fault and associated minor faults have subhorizontal WSW-ENE shortening and subvertical extension (Fig. 17b), and several observations suggest that they are younger than the faults with NW-SE shortening. The Agujas fault (Fig. 17c) cuts otherwise undeformed alluvial rocks that appear to be correlative with a partly dissected alluvial fan to the southwest, suggesting that it is very young. The Agujas fault also truncates the Caraclai fault, which has NW-SE shortening (Fig. 16). Southwest of Estaci6n SoIL the Sol~ fault cuts poorly consolidated alluvium that unconformably overlies folded upper Tertiary strata. Additionally, near G61gota where the Sol~i fault curves to a N-S strike and decreases in displacement (Fig. 16), the G61gota fault appears to have been folded. Adjacent to the G61gota fault and unconformably overlying folded upper Tertiary strata, alluvium tilts 20 ° to the northeast, suggesting large-scale drag folding that is consistent

Fig. 16. Geologic map of the Quebrada del Toro area.

with the kinematics of the Solfi fault but not with the kinematics of the G61gota fault. The Solfi fault marks the El Toro-Olacapato-Calama Lineament in this area (e.g. Mort, 1979; Allmendinger et al., 1983; Salfity, 1985). Southwest of Estaci6n SoIL the Sol~i fault has at least 3 km of Cenozoic dip-slip movement. Some of this movement is probably Quaternary, however much of it could be older. The Quaternary thrust kinematics of the Sol~ifault differ markedly from the Quaternary oblique normal-left lateral kinematics of the Chorrillos fault previously discussed. As with the Chorrillos fault, field relations suggest that along strike in both directions the Soht fault decreases displacement and terminates not far from the map area, so the El Toro-OlacapatoCalama lineament is not a single through-going Quaternary fault. This interpretation can be tested kinematically by assuming that the Solti and Chorrillos faults are in fact the same fault, but that a third structure accounts for their distinct kinematics. The most likely candidates for a third structure are the N-striking faults in the La Poma valley, which are discussed in the following section. A third structure south of the E1 Toro-Olacapato-Calama lineament would have to accommodate left lateral movement along a N-S striking plane to reconcile simultaneous Qua-

Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, NW Argentine Andes

Shortening Axes

~ens#on Axes

~lortenmg Axes

~en,~on Axes

Shortoning Axes

Exlensmn Axes

Fig. 17. Structure data from the Quebrada del Toro. (a) Kinemarie axes of San Bernardo, Gtlgota, Carachi. and associated minor faults in the Quebrada del Toro (N = 128). Bingham distribution (solid squares): shortening = 311°, 11°; extension = 91°, 75°. (b) Kinematic axes of Sol~ifault and associated minor faults in the Quebrada del Toro (N = 80). Bingham distribution (solid squares): shortening= 61°, 3°; extension = 307°, 82°. (c) Kinematic axes of Agujas fault in the Quebrada del Toro (N = 14). Bingham distribution (solid squares): shortening = 237°, 0°; extension = 329°, 87°. ternary movement on the Sol~i and Chorrillos faults. As shown below, the N-S striking faults in the La Poma valley have accommodated right-lateral movement during the Quaternary, confirming the interpretation that at least Quaternary activity on the Sol~t and Chorrillos faults is distinct. La Poma. The valley of La Poma (Fig. 18) is flanked on

both sides by fault bounded ranges composed in the map area of upper Proterozoic or Lower Cambrian low-grade metamorphic flysch deposits of the Puncoviscana Formation (Keidel, 1934; Vilela, 1956; Turner, 1960; Acefiolaza, 1973; MOadez, 1975; Omarini, 1983). Within the valley, strata of the Upper Cretaceous to lower Tertiary Salta

193

Group unconformably overlie the Puncoviscana Formation (Keidel, 1934; Vilela, 1956; Turner, t958; Reyes and Salfity, 1973; Mtndez, 1975). Continental clastic strata devoid of volcanic material, and thus possibly correlative with the Eocene Geste Formation in Salar de Pastos Grandes to the west, overlie the Salta Group in approximate concordance (Keidel, 1934; Vilela, 1956; Mtndez, 1975; Marrett and AllmeDdinger, 1987). The Tertiary strata grade upwardly into a conglomerate composed entirely of clasts derived from the Puncoviscana Formation. Two cinder cones and lava flows of basaltic-andesite (I-IOrmann et al., 1973; Knox et al.. 1989) overlie other lithologies including alluvium (Keidel, 1934; Vilela, 1956; Mtndez, 1975; Marrett and Allmendinger, 1987) and, based on their morphology, are probably of Quaternary age. The dominant kinematics of the Calchaqui, Toro Muerto, and associated minor faults (Marrett and Allmendinger, 1987) are subhorizontal WNW-ESE shortening and subvertical extension (Fig. 18a). North of the map area, the Calchaqui fault affects the Acay monzonite (Llambfas et al., 1986) which was radiometrically dated at 26 + 1 Ma (Linares, 1979), so some, if not all, of the activity on the Calchaqui fault is post-Oligocene. By inference, the same can be said of the other faults with kinematics similar to those of the Calchaquf fault. The Trigal fault (Fig. 18) has developed a scarp in a partly dissected fan of poorly consolidated alluvium that unconformably overlies folds in the Tertiary strata. Displacement on the Trigal fault is about 10 m in both the alluvium and the Tertiary strata. Thus, the Trigal fault must be younger than the thrust faulting described above and is probably of Quaternary age (Fig. 18). The Trigal fault is characterized by moderately SSW-plunging shortening and shallowly NW-plunging extension (Fig. 18b), kinematically incompatible with the thrust faulting described above. Both the Toro Muerto and Calchaquf faults locally have NW-SE striking thrust segments or are cut by NW-SE striking thrust faults (Marrett and Allmendinger, 1987), one of which is marked by a scarp developed in poorly consolidated alluvium southeast of Palermo Oeste (Fig. 18). Also, both have populations of shallowly-plunging striae indicating fight-lateral movement on their N-S striking segments. These data, combined with data from associated minor faults, indicate subhorizontal ENE-WSW shortening and a subvertical maximum of extension axes in a diffuse girdle (Fig. 18c) compatible with kinematics of the Trigal fault. The compatibility with the kinematics of the Trigal fault and the association with a scarp suggest that faults with WSW-ENE shortening and/or NNW-SSE extension were produced during the same phase of deformatien, which has been active in the Quaternary and postdates the fault movements with WNW-ESE shortening and vertical extension (Marrett and Allmendinger, 1987). Both of the Quaternary basaltic-andesites in the map area are associated with right steps along major N-S striking faults (Marrett and Allmendinger, 1987) (Fig. 18). The inferred Quaternary fault kinematics should produce extensional pull-apart deformation at the right steps, which may explain the localization of eruptive vents in

194

R.A. MARRETr, R.W. ALLMENDINGER,R.N. ALONSO and R.E. DRAKE

these areas and supports the interpretation of Quaternary right-lateral movement along the N-S striking fault segments. PucardL Near Pucar,'i (Fig. 19) upper Proterozoic or Lower Cambrian low-grade metamorphic flysch deposits of the Puncoviscana Formation (Turner, 1960; Aceflolaza, 1973; Omarini, 1983) and Paleozoic granitic intrusions CRapela, 1976; Villanueva Garcfa, 1988) are unconformably overlain by the Upper Cretaceous to lower Tertiary Salta Group (Turner, 1958; Reyes and Salfity, 1973; Villanueva Garcia, 1988). Continental clastic strata representing the lower part of the Payogastilla Group (Vilela and Garcfa, 1978; Diaz and Malizzia, 1983; Villanueva Garcia, 1988; Grier, 1990), which was radiometricaUy dated at 13.4 :!: 0.4 Ma (Grier and Dallmeyer, 1990), overlie the Salta Group strata in slight angular unconformity. Angularly unconformable on older rocks, including the upper Tertiary clastic strata, are a welded tuff and an overlying lzh_ardeposit which we dated at 12.11 +0.11 Ma (Table 1).

Folds affecting the upper Tertiary elastic strata have subhorizontal NNE-SSW trending axes (Fig. 19a), suggesting subhorizontal WNW-ESE shortening and subvertical extension. The folds are truncated by the Jasiman~i fault and are unconformably overlain by the welded tuff and lahar, constraining the age of folding between 13.4 + 0.4 Ma and 12.11 + 0.11 Ma ago. Alternatively, the angular unconformity between folded Tertiary and the volcanic unit could die out eastward into a paraconformity or the Tertiary sediments could be growth strata, suggesting that deformation is older than or synchronous with the 13.4 Ma strata. In any case this deformation, though similar in age to some of the deformation in the l~ma, is the oldest of our foreland sites. Kinematics similar to those of the folds characterize minor faults in the Salta Group along the syncline south of Pucmi (Fig. 19b), so these faults may cormspend to the same phase of deformation as the folding. The Jasimana fault, which cuts the 12.11 :t: 0.11 Ma old welded tuff and lahar, appears to be en echelon with the

(a)

Q~

Alluvium, riverterraces, lakesediments

]

Maficlava Conglomerate

ShorteningAxes

Extension

Continental elasticstrata SaltaGroup ]

Puncoviscana Fon~ation

5 km

(c)

Palewao O~le

ShorteningAxes

ExtensionAxes

Fig. 18. Geologic map of the upper Valle Calchaqu+ near La Poma. (a) Kinematic axes of Calchaqui, Tore Muerto, and associatedfaultsin the La Poma valley (N = 58). Bingham distribution(solidsquares): shortening = 299 °, 80; extension = 67 °, 77 °. (b) Kinematic axes of Trigal faultin the La Poma valley (N = 3). Bingham distribution: shortening (closed) = 203 °, 460; extension (open) = 320 °. 24 °. (c) Kinematic axes of N W - S E strikingthrustsand associatedfaultsin the La Poma valley (hl = 55). Bingham distribution (solid squares): shorte~g = 60°, 60; extension = 302 o, 78 °.

Late Cenozoic tectonic evolution of the lhma Plateau and adj~r.,entforeland. NW Argentine Andes Table 1. Ar40/Ar39 Single Crystal Dates

Sample

Min.

19-9-89/3 Pssto Venture

Bk)t Blot Biot Biot Plag Plag Rag Rag Hbe HIDe Hbe

4 0 ( v ) 40/39

0.36 2.74 3.85 2.04 1.28 0.37 0A1 0.68 0.77 0.68 0.76

2.61 2.77 2.80 2.81 15.85 2.72 3.03 2.76 2.76 2.77 2.85

37/39

0.02 0.02 0.03 0.02 3.98 2.37 2.48 2.55 3.83 3.74 4.84

36/39

.0070 .0048 .0012 ,0009 .0115 .0016 .0012 .0(~1 .0128 .0059 .0081

K/Ca

1.51 1.51 1.18 1.39 -0.91 -0.69 -0.70 .0.72 .0.89 -0.88 -1.00

Z 39At

0.021 0,203 0.544 0.729 0.782 0.816 0.852 0.912 0.763 0.957 1.000

% 4 0 A t " 40A~

55.8 66.3 89.1 91.5 83.7 90.4 94.7 86.7 44.2 65.5 58.5

Age

0.56 0.66 0.89 0.91 0.82 0.84 0.89 0.80 0.39 0.58 0.50

hmchron Age= I 19,,9-89/7 PsstoVentuzl

Plag Plag Plag WR WR

0.06 0.21 0.84 0.66 0.25

-2.03 -0.43 9.52 0.56 0.11

8.10 5.73 6.41 2.13 2.41

.0518 .0420 .0318 .0035 .0038

-1.22 -1.07 -1.12 -0.64 -0.69

0.007 0.030 0.089 0.690 1.000

-16.0 -3.7 51.9 39.4 10.8

.0.21 -0.08 0.49 0.27 -0.08

Isochron Age= I SAF-13 Junca Grande Plag Plag WR WR

J

SAF'14

0.21 .001992 0.95 0.95 0.03 0.32 contain. 0.12 0.12 0.95 0.18 0.15 0.18

10.04:1:.05

-7.13 -1.52 33.48 1.95 0.39

]

n=9

2.97 .001925 1,26 0.51 contain.

0.05 0.95

1.3 + .6

I

r1=4

1.46 0.84 3.10 0.98

1.62 1.38 1.51 1.48

4.15 3.51 2.04 2.17

.0158 .0183 .0058 .0043

-0.93 -0.86 -0.62 -0.65

0.244 0.308 0.810 1.000

27.2 20.9 49.1 57.4

0.22 0.17 0,44 0.51

5.37 4.49 4.92 4.113

0.13 .001814 0,20 0.03 0.95

0.74 0.52 2.16 0.81

1,29 2,21 1.49 1.47

16.13 22.46 1.59 1.82

.07:._'7 .1569 .00~3 .0(~7

-1.52 -1.67 -0.51 -0.57

0.030 0.040 0.762 1.000

5.9 4.6 56.6 48.9

0.00 0.01 0,52 0.44

4.42 7.57 4.99 4.91

0.85 .001858 2.62 0.05 0.95

Isochm Age=

Juncal Grande] ~ WR

10.08 2.97

1.21 1.20

0.26 0,26

.0403 0 ` 2 7 .0329 0 . 2 7

0.457 0.606

9.3 11.0

I

0.09 0.11

3.82 3.76

I

2.38

113

0.25

.0~

0~9

0708

86

0.09

3~

0`20

5.63

I,25

0.26

.0311

0,28

1.000

12.0

0.12

3.94

0.09

Plag WR WR

2.40 1.34 4.53 3.19

1.79 1.62 1.99 2.02

2.63 2.78 2.00 2.11

.0341 .0467 .017,2 .0073

-0.73 -0.75 -0.61 -0.63

0.107 0.153 0.585 1.000

15.3 10.6 36.5 50.5

0.14 0.09 0.34 0.46

Ilmchron Age= [

4.3,

11=4

0.95 .001747 0.13

WR

h,och,-o,, As,,= [

AM,42

4.97 :l: .07

WR

I Juncal.,=.7 GrandeJ] Plag

.5

]

8.77 5.21 6.33 6.43

,~,

0.21 .001769 0.37 0.95 0.05

6.64 + .07

]

n=4

]

Pucani

Blot Biot Biot Pla9 Anorth Plag

10.83 11.08 11.53 8.97 8.51 8.69

0.07 0.05 0.04 0,91 0.01 0.48

.0082 .00~5 .00:)8 .0OZ2 .0007 ,0020

77.7 77.3 74.9 93.7 97.5 93.6

12.12 12.33 12.44 12.11 11.95 11.72 Age=

AM-B09 Bequeville

I Biot Bk)t Rag

Rag

12.44 11.15 9.40 20.69

0.06 0.05 0.63 4.95

.0104 .0071 .0024 .0392

75.3 81.1 93.1 46.0

AM-291 Qda del Tom I Blot Biot

0.88 1.22

0.40 0.01

.0010 .0016

70.3 60.4

K 4Q/39 = 0.026

I

I

0.07 .000801 0.10 0.95 0.07 0.95

0.07

12.11 t-0.11

ISEM(n=6)

0.95 .000801 0.95 0.95 0.16

13,21 ~.2S 0.00 1,06

Age= £'0.001

[

13.49 13.01 12.8 13.57 Age=

DIsc Cow. = 1.0075 Cs 39/37 : 0.0007 Ca 36/37 = 0.0003 K 38/39 ,= 0.0108

SAE$ 7:2-6

9.47 10.04 10.14 10.17 55.45 9.63 10.72 9.78 8.88 9.91 10`21

I

Juncal GrandeJ Plag Rag WR WR

I

+ Ma

ISEM(n=4)

0.02 .000801 0.01

0.95 ~.12

ISEM(n:4)

195

196

R.A. MARRETr, R.W. ALLMENDINGER, R.N. ALONSO and R.E. DRAKE

Fig. 19. Geologic map of the Pucar~iarea. (a) Poles to bedding in the Payogastilla Group near Pucarti (N = 17). Great circle represents n-plane (120 °, 79 °) and grey square represents fold axis. (b) Kinematic axes of minor faults in Salta Group near Pucarti (N = 12). Bingham distribution (solid squares): shortening = 308°, 1°; extension = 207°, 86°. (c) Kinematic axes of Jasiman~i and Vallecito faults near Pucar~ (N = 27). Bingham distribution (solid squares): shortening = 62°. 9°; extension = 181°, 72°.

Shortening Axes

Vallecito fault. They have subhorizontal ENE-WSW shortening and subvertical extension (Fig. 19c). Choromoro. West of the Choromoro valley (Fig. 20), mountain ranges composed of upper Proterozoic low-grade metamorphic flysch deposits of the Medina Formation (Keidel, 1947; Bossi, 1%9; GalvAn, 1981), which is thought to be correlative with the Ptmcoviscana Formation further north (Mon and Hongn, pers. comm., 1991). Lateral equivalents of the Upper Cretaceous to lower Tertiary Salta Group unconformably overlie the Medina Formation (Turner, 1958; Bossi, 1969; Reyes and Salfity, 1973; Galv~n, 1981). Conformable on the Salta Group are continental clastic strata thought to be upper Tertiary because of intercalated airfall tufts (Bossi, 1%9). The India Muerta Formation, the youngest of this sequence, contains fossils (Bossi, 1969) now considered to be representative of the late Miocene (Marshall et al., 1982). A fault with approximately 100 m inferred displacement in the India Muerta Formation northeast of La Higuera and minor faults in the Salta Group south of Gonzalo have similar kinematics with subhorizontal WNW-ESE shortening and subvertical extension (Fig. 20a). The Salta Group east of Gonzalo (Galv(m, 1981) and the India Muerta Formation east of the map area (Bossi, 1969) are affected by NNE-SSW striking thrust faults and folds with NNE-SSW trending axes, suggesting kinematics similar to those determined for faults affecting the same strata. Folds in the India Muerta Formation east of

Extension Axes

the map area are unconformably overlain by alluvial deposits containing Pleistocene fossils (Bossi, 1969), suggesting that the deformation represented by the folds and faults described above occurred during the Pliocene. The thrust fault immediately south of Gonzalo has been reactivated with largely strike-slip movement that affects poorly consolidated alluvium, probably of Quaternary age. The right-lateral movement has shallowly WSW-plunging shortening and shallowly SSE-plunging extension (Fig. 20b).

Previously Described Areas Northern Puna. West of Salar de Olaroz and the Salar de Cauchari (Fig. 2), N-S striking reverse faults are covered by continental clastic and pyroclastic strata radiometrically dated at 9.5 + 0.3 Ma, but the faults cut older upper Tertiary strata (Schwab and Lippolt, 1976; Schwab, 1980). In the same area, the 9.5 + 0.3 Ma old continental strata are folded with approximately N-S trending axes and covered unconformably by welded tuff radiometrically dated at 4.8 + 0.3 Ma (Schwab and Lippolt, 1976; Schwab, 1980). Thus, the approximate kinematics of late Tertiary deformation in this area are subhorizontal E-W shortening and subvertical extension. This deformation occurred before 9.5 + 0.3 Ma and also between 9.5 + 0.3 Ma and 4.8 + 0.3 Ma, although a single phase of deformation spa_oning 9.5 + 0.3 Ma could have produced both sets of structures.

Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, NW Argentine Andes

Fig. 20. Geologicmap of the Quebrada Rearte area, west of the Choromoro Valley. (a) Kinematic axes of faults near La Higuera and Gonzalo (N = 9). Bingham distribution: shortening (solid) = 287°, 2°; extension (open) = 172 °, 85 °. (b) Kinematic axes of fault cutting alluvium south of Gonzalo (N = 1). Solid dots represent shortening axes = 241 ° , 17°; open squares represent extension axes = 141°, 31°.

197

26 ° 25'S

East of the Salar de Olaroz and the Salar de Cauchari (Fig. 2) late Tertiary clastic and pyroclastic strata radiometrically dated at 8.9 + 0.7 Ma are deformed by NNESSW striking reverse faults and folds with NNE-SSW trending axes (Schwab and Lippolt, 1976; Schwab, 1980), suggesting approximate kinematics with subhorizontal WNW-ESE shortening and subvertical extension. Pasto Ventura-El Pefi6n. In the southern Puna, structures at Pasto Venmra and E1 Pefl6n (Fig. 2) have been described previously (Allmendinger et al., 1989), but here we report new geochrouologic results from this previously undated area. Continental clastic strata containing an ash fall tuff dated at 10.04 + 0.05 Ma (Table 1) were affected by WNW to NW-dipping thrust faults. This structural geometry suggests approximately subhorizontal WNW-ESE to NW-SE shottenin~ and subvertical extension during the late Miocene and/or Pliocene. These faults were reactivated and other apparently new faults formed, both types affecting litde eroded basaltic-andesite cinder cones and lava flows, one dated at 1.3 + 0.6 Ma (Table 1), as well as alluvial surfaces inferred to be of Quaternary age. The reactivated thrust faults show evidence for oblique normal-right lateral movement during the Quaternary (Allmendinger et al., 1989), suggesting approximate kinematics with moderately NE to ENE-plunging shortening and subhorizontal NNWSSE to N-S extension. Where studied, the newly formed Quaternary faults near E1 Pefi6n and Pasto Ventura show geomorphic evidence only for normal movement (Allmeadinger et al., 1989). Minor faults associated with the E1Pett6n fault indicate subverdcal sh(xtening and shallowly N-plunging extension, however minor faults associated with the Pasto Ventura fault indicate shallowly W-plunging shortanlnE and shallowly N-plunging extension (Allmendinger et al., 1989). Thus, thrust faulting at Pasto Ventura occurred sometime after 10.04 + 0.05 Ma and before 1.3 + 0.6 Ma; normal and strike-slip faulting is younger than 1.3 + 0.6 Ma.

Amblayo-VaUe CalchaquL In the Amblayo fold and thrust belt and in the Valle Calchaquf of the southern Puna foreland (Fig. 2), continental clastic strata that range in age from at least 13.4 + 0.4 Ma to approximately 3 Ma (based on correlation with the Santa Maria Group; see Santa Maria below) lie unconformably on older strata (Grier, 1990; Grier and Dallmeyer, 1990). The upper Tertiary strata are deformed by folds with dominantly N-S trending axes and dominantly N-S striking thrust faults, some of which have kilometers of displacement (Grier et al., 1991). Many of these structures are covered unconformably by alluvium. The fault kinematics of the N-S striking structures are consistent throughout the area, with subhorizontal E-W shorteating and subvertical extension (Grier, 1990). The N-S striking structures are systematically cut by younger structures, some of which affect poorly consolidated alluvium considered to be Quaternary in age. None of the younger structures have displacements demonstrated to exceed 100 m. Most of the younger faults have strike-slip movement with subhorizontal NE-SW shortening and subhorizontal NW-SE extension or thrust movement with subhorizontal N-S shortening and subvertical extension. Santa Marfa. In the southern Puna foreland near Santa Maria valley (Fig. 2), continental clastic strata of the Santa Maria Group span at least 10.7 + 1.7 Ma to 2.97 + 0.6 Ma in age and unconformably overlie older rocks (Strecker et al., 1989). The upper Tertiary strata are deformed by folds with dominantly NNE-SSW trending axes and dominantly NNE-SSW striking thrust faults, some of which have kilometers of displacement (Strecker et al., 1989). Many of these structures are covered unconformably by several pediment deposits ranging in age from at least 2.51 + 0.6 Ma to 0.28 + 0.1 Ma (Strecker et al., 1989). The geometries of the NNE-SSW striking structures suggest that during the time period of most intense deformation in the area, namely between 2.97 +_.0.6 Ma and 2.51 + 0.6 Ma, approximate kine-

198

R.A. MARRETr, R.W. ALLMENDINGER,R.N. ALONSO and R.E. DRAKE

matics with subhorizontal WNW-ESE shortening and subvertical extension were active. The pediments and younger alluvium have been affected by subsequent folding and faulting, including a fold with an interlimb angle of 46 ° developed in a 0.6-1.2 Ma old pediment and a scarp developed in a Holocene floodplain deposit (Strecker et al.. 1989). A thrust fault affecting poorly consolidated alluvium indicates subhorizontal WNW-ESE shortening and subvertical extension (Allmendinger et al.. 1989; Strecker et al., 1989). Rio Bolstn-La H oyada. In the southern Puna foreland near

Rio Bolstn and La Hoyada (Fig. 2), continental clastic strata correlative with the Santa Maria Group lie unconformably on basement rocks (Navarro Garcia, 1984; Allmendinger, 1986; Allmendinger et al., 1989). Radiometric dating indicates that deposition occurred from at least 6.70 + 0.05 Ma to 3.53 + 0.04 Ma (Butler et al., 1984), and, assuming a constant sedimentatiou rate, the top of the exposed upper Tertiary section in the area would be approximately 2.35 Ma old (Allmendinger, 1986; Ailmendinger et al., 1989). A coarse conglomerate unit unconformably overlies the upper Tertiary clastic strata (Penck, 1920; Allmendinger, 1986; Allmendinger et al.. 19~9) and may be correlative with the 2.51 :!: 0.6 Ma old pediment described in the Santa Maria area (Strecker et al.. 1989). The upper Tertiary clastic strata are folded about NE-SW trending axes and cut by NE-SW striking thrust faults, both of which are unconformably covered by the coarse conglomerate, indicating subhorizontal NW-SE shortening and subvertical extension (Allmendinger, 1986; Allmendinger e t al. . 1989). The coarse conglomerate is in turn cut by faults with a variety of orientations, some of which are associated with scarps (Allmendinger, 1986; Allmendinger et al., 1989). Some are thrust faults indicating subhorizontal WNWESE shortening and subvertical extension and others are strike-slip faults indicating subhorizontal WNW-ESE shortening and subhorizontal NNE-SSW extension (Allmendinger, 1986; Allmendinger et al., 1989). REGIONAL KINEMATICS AND DEFORMATION CHRONOLOGY Puna Plateau

In all of the Puna study areas, two or more local events of late Cenozoic deformation are recognized. In all cases, the older, Mio-Pliocene events have regionally homogeneous kinematics defined by subhorizontal NW-SE to E-W shortening and mostly subvertical extension (Fig. 21). Similarly, the Plio-Quaternary event has regionally homogeneous subhorizontal E-W to NE-SW or subvertical shortening and subvertical or subhorizontal N-S to NW-SE extension (Fig. 22). Deformation with Mio-Pliocene kinematics produced significant shortening in each of the study areas. In contrast, faults with Plio-Quaternary kinematics nowhere have demonstrated displacements in excess of 500m.

Timing constraints on the Mio-Pliocene and Plio-Quaternary deformational events in the Puna are regionally consistent (Table 2). In several areas (Abra Pampa. Loma Blanca, west of Salar de Olaroz and Salar de Cauchari, Olacapato, Corte Blanco, Salar de Pastos Grandes, Sierra de Bequeville), there is evidence for deformation with the Mio-Pliocene kinematics prior to 7-13 Ma and possibly before 17 Ma, locally. However, deformation with the Mio-Pliocene kinematics was active after 7-13 Ma in several areas as well (Loma Blanca, west of Salar de Olaroz and Salar de Cauchari. east of Salar de Olaroz and Salar de Cauchari. Corte Blanco, Salar de Pastos Grandes, Sierra de Bequeville, Tmcalayu, Farall6n Catal) and locally continued until at least 4 Ma. Faults with the Plio-Quaternary kinematics are observed in most of the areas where fault-slip data were collected and appear to have moved after 4 Ma. These faults affect rocks radiometrically dated as young as 0.2 Ma and commonly are associated with scarps. The one earthquake focal mechanism in the southern Ptma is consistent with this younger phase suggesting continued activity to the Present. Deformation with horizontal extension. characteristic of the Plio-Quateraary kinematics, might be necessary for ascending basaltic-andesite magma (Chorrillos. Sierra de Bequeville, T'mcalayu, Farali6n Catal, Salar de Antofalla. El Petl6n-Pasto Ventura) to reach the surface (Marrett and Emerman, 1992). Dated basaltic-andesites near the Salar de Antofalla suggest that. at least locally, the Plio-Quatemary kinematics were active by 2 Ma. Foreland Study Areas

In all of the foreland study areas, two or more local events of late Cenozoic deformation are recognized. The events have kinematics similar to those in the Puna. The Mio-Pliocene event has regionally homogeneous kinematics defined by subhorizontal NW-SE to E-W shortening and mostly subvertical extension (Fig. 21). Similarly, the Plio-Quatemary event has regionally homogeneous subhorizontal E-W to NE-SW or subvertical shortening and subvertical or subhorizontal N-S to NW-SE extension (Fig. 22). As in the plateau, the younger deformation in the foreland is generally minor in comparison to the older thrust deformation (with the notable exception of the Sol~t fault in the Quebrada del Toro). Timing constraints on the events with different kinematics in the foreland are regionally consistent (Table 3), however they differ in some ways from those for the Puna. In most areas, significant shortening with the MioPliocene kinematics must be post-late Miocene although at Pucar~i it occurred before 12 Ma. In several areas (Quebrada del Toro, Pucar,'i. Amblayo-Valle Calchaqu+. Choromoro). Neogene strata are concordant or nearly so with Paleogene strata, suggesting that only minor deformation occurred in the foreland before about 10(5.9) Ma. Deformation with older kinematics (i.e.. Mio-Pliocene elsewhere) in the foreland is as young as 1 Ma in the Quebrada del Toro and is younger than 2-3 Ma farther south at Santa Maria and Bolstn-Hoyada.

24.5

24.3

24.4

24.7

24.6

24.6

25.1

25.1

25.3

25.3

25.2

25.2

25.6

25.8

25.8

25.8

25.8

25.8

Quevar

Quevar

Acay

Sijes

Sijea

Sijes

Bequeville

Bequeville

Faralldn Catal

Farall6n Catal

"llncalayu

"rincalayu

Antofalla

Antofalla

Juncal Grande

Juncal Grande

Juncal Grande

Juncal Grande !

67.3

67.3

66.7

67.7

67. 7

67.7

67. 7

67.1

67.1

67.0

67.0

66.9

66.9

66.7

66.6

66.7

66.2

66.4

66.7

66.7

66.5

66.4

66.2

66.4

66.7

*see Table 3 for explanation of abbreviations

~,~ o

24.2

Quevar

~--:__ ,,._.ura I

24.2

San Geronimo

26.8

24.3

Chorilloe

Peeto Venture

24.2

26.0

68.3

23.1

Loma Blanca

Corte Blanco

Galen

68.4

23.8

Olaroz/Cauchari

67.2

23.9

Olaroz/Cauchari

67.0

66.2

Longitude ow

23.6

22.8

Latitude os

Olaroz/Ca".ch~,ri I

Abra Pampa

Area

Ar(sc)

Rb/Sr

Ar(.~)

Ar(sc)

Ar(sc)

Ar(sc)

WAr

WAr

K/Ar

WAr

FT

FT

Ar(sc)

Ar/Ar

FT

FT

FT

WAr

WAr

WAr

WAr

WAr

WAr

K/Ar

WAr

WAr

K/Ar

K/A,

WAr

Method*

this paper

10.04 :~ 0.05

Puna Summary:

this paper

Sparks et aL, 1985

this paper

this paper

this paper

this paper

Coira & Pezzutti, 1976

Coira & Pezzutti, 1976

Alonso et al,, 1984a

Alonso et aL, 1984a

Alonso et al., 1991

Alonso et al., 1991

this paper

Vandervoort, p. c. 1990

Strecker, 1987

Alonso et al., 1991

Alonso et al., 1991

Unares, 1979

Olson, p. c., 1987

Olson, p. c., 1987

Olson, p. c., 1987

Co±re & Paris, 1981

Coira & Paris, 1981

Viramonte, p. c., 1988

Alonso, 1986

Schwab & Lippolt, 1989

Schwab & Lippolt, 1989

Schwab & Lippolt, 1989

Coira, 1978

Reference

1.3 ± 0.6

2.03 + 0.07

4.3 ± 0.5

4.97± 0.07

5.03 ± 0.08

6.64 ± 0.07

3.0+2.0

2.0 ± 1.0

0.754 + 0.028

5.86 ± 0.14

7.2 ± 1.4

15.0 + 2.4

13.21 ± 0.09

16.7 ± 0.1

1.57 ± 0.74

4.0 ± 1.2

7.6 ± 1.1

26 ± 1

17.2 ± 0.5

10.6 ± 0.4

10.1 ± 0.5

0.78 ¢ 0.1

0.2 ¢ 0.08

9.6 ± 0.35

6.99 ± 0.18

8.9 ± 0.7

4.8 ± 0.3

o 5 i 0.3

10 + 2

Age (Me)

J

younger than

4.0

13.2 (17.2?)

)

•

•

•

•

•

?

•

•

•

•

older than

•

•

•

•

•

•

•

•

?

?

•

•

•

?

•

•

Older Kinematics Predates unit P.ost-detes unit •

Table 2. Summary of Puna Geochronology

0.2 (2.0?)

older then

0.8

younger than

•

•

?

• ?

•

?

?

Younger Kinematics Predates unit Post-dates unit

[,4

o..

R.A. M_~ETL R.W. ALLMF_,NDINGER,R.N. ALONSO and R.E. DRAKE

200

In all of the study areas, deformation with the Plio-Quaternary kinematics is observed. Relative timing relations demonstrate that structures with Plio-Quatemary kinematics everywhere post-date those with Mio-Pliocene kinematics. As the latter affect Plio-Quaternary rocks in several areas, we conclude that the youngest deformation probably began during the Quaternary. Faults with the Plio-Quaternary kinematics affect probable Quaternary deposits in most areas and some faults are associated with scarps. The sparsity of scarps in the foreland compared with the Puna may reflect greater precipitation and erosion rather than less tectonic activity. In fact, the foreland east of the areas studied is much more seismically active than the Puna (Chinn and Isacks, 1983; Cahill et al., 1992). Whether late Cenozoic deformation occurred in shortlived pulses or in long-lived phases, the kinematic and chronologic data from the Puna and the adjacent foreland are consistent. The Mio-Pliocene and Plio-Quaternary kinematics observed in the foreland are statistically indistinguishable from those in the Puna. Available chronologic constraints suggest that the timing of the change from Mio-Pliocene to Plio-Quaternary kinematics in the foreland may be slightly younger than in the Puna. Substantial shortening of the Puna had occurred by the middle Miocene, occurred during the middle and late Miocene,

and occurred as late as the Pliocene. Subsequent deformation with distinct kinematics probably produced only minor strain. In the foreland, substantial shortening probably began in the late Miocene and ended in the late Pliocene in most areas studied, although further east it probably is ongoing. Thus, deformation generally appears to have begun later in the foreland than in the Puna and probably is continuing at a faster rate. This is consistent with generally eastward propagation of the locus of most active deformation.

Comparison With Other Regions The same style and relative timing of deformation kinematics is also observed in the northernmost Puna but with important differences in absolute timing. West of Abra Pampa (Fig. 2), upper Tertiary volcanic rocks and older strata are affected by NNE-SSW striking reverse faults and folds with NNE-trending axes (Coira, 1978, 1979), suggesting kinematics with approximately subhorizontal WNW-ESE shortening and subvertical extension. These structures are unconformably covered by a laterally extensive welded tuff radiometrically dated at 10 + 2 Ma (Coira, 1978, 1979). NW-SE shortening predominantly along thrust faults occurred prior to deposition of deposits on a

~entina

prnary ~lcs 68 °

67 °

660

65 °

64 °

63 °

Fig. 21. Summaryof Mio-Pliocenefault kinematics, Resultsof linked Binghamanalysesrepresented as pseudo-fault-planesolutions. Black dihedra are extension, white are shortening. All except for one (Corte Blanco) are thrust mechanismswith NWSE shorteningand vertical extension. Shadedarea is above 3 km average elevation.

68 °

67 °

66"

65 °

64 °

63*

Fig. 22. Summaryof plio-Quaternaryfault kinematics. Results of linked Bingham analyses represented as pseudo-fault-plane solutions. Black dihedra are extension, white are shortening. Note widevariation in faultingstyle. Shadedarea is above 3 km average elevation.

Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, NW Argentine Andes

201

Table 3. Summary of Foreland Geochronology Area

Latitude

Longitude

o s

Method

Age (Ma)

Reference Marshall et al., 1982

Uquia

23.3

65.4 - -

WAr

2,78±0.09

Oda. del Toro

24.5

65.8

Ar (sc)

0,98±0.~

25.7

~6.1--

Ar/Ar

13.4±0.4

Grier & Dallmeyer, 1991

25.9

" ~ 3

Ar (SC)

12.11±0.07

this paper

Angastaco Puce~ I

this paper

Santa Mafia

26.0

65.8

FT

2.51~0.6

Strecker et al., 1989

Santa Mar(a

L765

65.8

FT

0.28±0.1

Strecker et al., 1989

66.2

FT

10.7~1.7

Strecker et al., 1989

66 1

FT

2.97~0.6

Strecker et al., 1989

66.9

K/Ar

6.70±0.05

Butler et al., 1984

66.9

WAr

3.53±0.04

Butler et al.. 1984

NW Quilmes

t_262_.6

Santa Marfa

27.1

Bola6n-Hoyada

I 27.~

Bolsdn-Hoyada I

Method Key Ar/Ar - Ar (so) - FT WAr -Rb/Sr - -

!

27.1

,

Older Kinematics Predates unit Post-dates unit •

•

?

•

Foreland Summary: older than 12.1 03.4?) 40Ar/39Ar stepwise heating, plateau age single crystal laser fusion age (see Table 1 for analytical data and number of crystals dated) fission track age conventional potassium/argon age Rubidium/Strontium age

regionally extensive erosion surface which has been dated at 8-9 Ma (Gubbels et al., 1993; Cladouhos et al., this volume). The youngest deposits comprising the surface are locally younger than 3.5 Ma. The surface itself is deformed by scarps related to faults which exhibit the younger kinematics described here: WSW-ENE shortening and subhorizontal extension. Thus, it appears that the older deformation ceased at a much earlier time in the northern Puna (pre-8-9 Ma) than in the southern Puna (post-4 Ma). The reasons for this difference may be found in lateral variations in lithospheric structure and thickness (Allmendinger et al., 1993; Whitman et al., 1993). In southern and central Bolivia, Andean deformation began in the late Oligocene to mid-Miocene (Sempere et al., 1990; Gubbels et al., 1993). However, in northern Bolivia and Chile and southern Perd, important Cenozoic deformation first occurred in the middle to late Eocene (Mtgard, 1984; Stbrier et al., 1988a). As many as five subsequent pulses of deformation have been reported (Mtgard, 1984; Stbrier et al., 1988a; Ellison et al., 1989). An Oligocene pulse affected the forearc, Cordillera Occidental, and western Altiplano, although it was of different ages in different areas (S~brier et al., 1988a; EUison et al., 1989). At least three pulses of Miocene deformation affected all geologic provinces in the region, although again they were of different ages in different areas (Mtgard, 1984; Stbrier et al., 1988a; Ellison et al., 1989). A late Pliocene to early Quaternary pulse of deformation also affected all geologic provinces in the region at various times (Stbrier et al., 1988a). Seismicity and fault scarps indicate that deformation is presently active (Chinn and Isacks, 1983; Su~irez et al., 1983; Grange et al.. 1984; Stbrier et aI., 1985). Whereas late Pliocene to early Quaternary, and probably most previous deformation, was dominated by thrust kinematics, more recent fault kinematics at high elevations and along the Pacific coast are characterized by N-S trending subhorizontal extension (Stbrier et al., 1985). In the flat-slab region south of the Puna, late Cenozoic deformation occurred during the middle Miocene in the

younger than 1.0

Younger Kinematics Pre-datas unit Post-datas unit

older than --

younger than 1.0

areas of highest present topography (Jordan and Gardeweg, 1989). Upper crustal shortening shifted to the east in the late Miocene when movement began on the most important thrust and reverse faults (Jordan and Gardeweg, 1989). Seismicity indicates that deformation is ongoing in this region (Chinn and Isacks, 1983). However, some faults associated with scarps indicate that subhorizontal extension has occurred since the Pliocene at least locally (Whitney and Bastias, 1984; Bastias and Bastias, 1987; Allmendinger et al., 1988). VERTICAL AXIS ROTATION(?) AND HORIZONTAL EXTENSION Comparison of Mio-Pliocene and Plio-Quaternary fault kinematics in the Puna plateau and the adjacent foreland, modern earthquake kinematics in the central Andes, and late Miocene to present South America-Nazca plate kinematics reveals several interesting characteristics (Fig. 23) that disagree with models proposed for Andean dynamics (e.g. Assump~ao, 1992; Dewey and Lamb, 1992; Assump~ao and Araujo, 1993): 1) The cones of 95% confidence for Plio-Quaternary fault shortening, modern earthquake shortening, and late Miocene to present South America-Nazca convergence direction overlap with each other, but not with the confidence cone for Mio-Pliocene fault shortening. 2) The cones of 95% confidence for Plio-Quatemary and Mio-Pliocene fault extension do not overlap. 3) The cones of 95% confidence for Plio-Quateruary fault extension and modern earthquake extension do not overlap. The differences between the Mio-Pliocene fault shortening and the other shortening/convergence directions indicate two possibilities: either the stress field responsible for the fault movements has changed its orientation or the region coatainin~ the measured faults has rotated about a vertical axis. The simplest solution, a shift in direction of convergence between Nazca and South America, can be most quickly ruled out. The known plate convergence

202

R.A. MARRETI', R.W. ALLMENDINGER,R.N. ALONSO and R.E. DRAKE

direction, 77 + 12° (Pardo Casas and Molnar, 1987), is averaged over the last 10 Ma so that, even though itparallels the youngest shortening direction, it was active during both older and youngest kinematics. Several recently proposed models for central Andean orogeny, oroclinal bending (e.g. Carey, 1958; Isacks, 1988) and in situ block rotations (Beck, 1988), require rotations about vertical axes. Existing paleomagnetic evidence at latitude range of the Puna are sparse but uniformly indicate clockwise rotations of 10 to 300 (Butler et al., 1984; MacFadden, 1992; reviews in Isacks, 1988 and Beck, 1988), even though data from the foreland farther south do not (see review in Marrett, 1990). However, analyses of the paleomagnetic data suggest that vertical axis rotation alone cannot account for the change in shortening direction observed in the Ptma and the adjacent foreland. None of the paleomagnetic data sets indicate rotations sufficiently large (a minimum of -40 ° is necessary) to rotate the uncertainty cone of the Mio-Pliocene shortening direction into intersection with that of the PlioQuaternary shortening direction. Furthermore, if vertical axis rotation was accompanied by faulting, then a gradation between Mio-Pliocene and Plio-Quaternary shortening directions should be observed. The Mio-Pliocene and Plio-Quaternary shortening directions determined for most fault-slip study areas are statistically distinct. Thus, the kinematics of Mio-Pliocene deformation in the Puna and adjacent foreland were distinct from the South America-Nazca plate kinematics. The MioPliocene fault shortening direction is roughly perpendicular to the widespread Ordovician folds in the Puna (Mon and Hongn, 1988), suggesting that paleostructural features might have controlled deformation kinematics. Reactivation of pre-existing structures (such as demonstrated at Cerro Pircas) may have been feasible only with kinematics similar to those of the pre-existing structures. Alternatively, the crust below the late Cenozoic volcanic arc, which is ro, ghly perpendicular to the Mio-Pliocene shortening direction, might have been too thermally weakened to support shear stress of large magnitudes. If only compressional normal stress was transmitted through the volcanic arc, then the principal compression direction would be subparallel with the Mio-Pliocene shortening direction. This may have required right-lateral movement along the volcanic arc or further outboard in the forearc region. It is tempting to interpret the fault-slip kinematic change from Mio-Pliocene subvertical extension to PlioQuaternary subhorizontal extension in the Puna (Fig. 23) as a result of gravitationally driven extension due to thickened crust, such as previously proposed for the AltiplanoPuna and the Andes of central Perti (Dalmayrac and Molnar, 1981; Froidevaux and Isacks, 1984; S6brier et al., 1985; Mohiar and Lyon-Caen, 1988; S6brier et al., 1988b). However, several lines of evidence suggest that, at least in the Puna segment of the central Andes, gravitational extension alone fails to account for the change to subhorizontal extension (Allmendinger et al., 1989). The predominantly NNW-SSE trending extension directions for Plio-Quaternary faults in the Puna and adjacent foreland are not perpendicular to the regional topographic contours

•"••"•

plate convergence tion

Fig. 23. Comparisonof kinematic directions and their 95% confidence cones on lower hemisphere equal angle net. Open dots represent plate convergence direction (from Pardo-Casas and Molnar, 1987), solid dots represent shortening axes. and squares represent extension directions. Earthquake kinematics determined using linked Bingham analyses of fault-plane solutionsfor shallow intraplate events between 16 and 36° S (Chinn and Isacks, 1983; Dziewonski et al., 1983a, b, 1985. 1987a, b, c, d, 1988).

surrounding the Puna (Allmendinger et al.. 1989), in contrast to extensional structures observed in the Himalaya (Burctdiel and Royden. 1985; Herren. 1987). The extension directions are, however, sub-perpendicular to the South America-Nazca plate convergence direction (Allmendinger et al., 1989), suggesting a possible mechanical relationship. The change from subvertical to subhorizontal extension might reflect a possible decrease in the South Amedca-Nazca plate convergence rate during the Pliocene or Quaternary. Although poorly constrained, plate reconstructions suggest that the average normal component of convergence rate during the Pliocene and Quaternary at the latitude range of the Puna was only one half to two thirds of the average rate during the early and middle Miocene (Pilger, 1984), It is possible that the relatively low Plio-Quaternary average normal convergence rate reflects a larger drop during a portion of the Plio-Quaternary. This hypothesis might also explain why the shortenhag rate determined from moment tensor summation of earthquakes is more than an order of magnitude smaller than the average late Cenozoic rate one can infer from geologic data (Marrett. 1990). A major change in South America-Nazca kinematics during the Plio-Quatemary may be related to the inferred reorganization of Pacific plate movement during the Pliocene (Cox and Engebretson, 1985; Harbert and Cox. 1989). Although systematic field investigations have not been conducted in the central part of the Altiplano-Puna plateau, sparse seismicity (Chinn and Isacks, 1983) and the results of a reconnaissance survey of satellite imagery

Late Cenozoic tectonic evolution of the PumaPlateau and adjacent foreland, NW Argentine Andes (Fielding, 1989) suggest that little tectonic activity presently occurs in this region. Extensional faulting appears to be restricted to the northern and southeru ends of the plateau (S~brier et al., 1985; Isacks, 1988; Allmendinger et al., 1989). In contrast to the positive correlation between extensional faults and high elevation observed in the northern Altiplano and adjacent foreland (S~brier et al., 1985). the kinematics of Plio-Quaternary fault populations in the Puna are similar to those at lower elevations in the foreland (Fig. 24). Plio-Quaternary faults with subvertical extension occur above 4 km elevation in the Puua (Loma Blanca, Chorrillos, Salar de Hombre Muerto) and PlioQuaternary faults with subhorizontal extension occur below 1 km elevation in the foreland (Choromoro). Nevertheless, it is possible that a systematic change in fault kinematics occm's at even lower elevations. Thus, at least in the vicinity of the most important relief surrounding the southern and eastern margins of the southern Puma, the distribution of extensional faults fails to support gravitational extension driven in a simple way by the thickened crust of the Puna plateau. Complicated three-dimensiooal effects related to the southward narrowing of the Puna high relief, and presumably of the thickened crustal root as well, might better account for the distribution of extensional faults (Isacks, 1988; Allmendinger et al., 1989). Evidence for the complex spatial nature of kinematics is provided by comparison of the Plio-Quaternary fault kinematics with the kinematics of modern earthquakes (Fig. 233.67). Although it is possible that the Plio-Quaternary fault kinematics do not reflect the active mechanical state of the Puna region, the common association of scarps with the Plio-Quaternary faults suggests otherwise. The dominantly subvertical extension directions characteristic of the earthquake kinematics are distinct from the dominantly subhcfizontal extension directions of the Plio-Quaternary faults and might reflect their typically more eastern locations. However, the extension direction of the one fault-plane solution available for the Puna (Fig. 14) plunges moderately and is generally steeper than the extension directions of exposed faults near the epicenter (see Salar de Antofalla and Pasto Ventura-El Pefl6n). This suggests that kinematics may depend on crustal depth (Allmendinger et al., 1989) in a manner analogous to the dependence on elevation of exposed faults in the northern Altiplano (S~brier et al., 1985). If some active upper

203

crustal shortening in the foreland ramps down to the lower crust below the Puna, then the depth dependence of kinematics is consistent with the hypothesis that the crust of the Puna has reached a steady-state thickness. In order to balance horizontal shortening and consequent thickening in the lower crust of the Puna, horizontal extension and consequent thinning or lateral extrusion in the upper crust is necessary. CONCLUSIONS Kinematic and chronologic data from the Puaa plateau and adjacent foreland are internally consistent and indicate that late Cenozoic deformation in the region is broadly correlative with deformation reported from the flat-slab region to the south and from the northern Altiplano region to the north. In the Puna, WNW-ESE directed thrusting began by 13 (177) Ma and was active continuously or episodically until after 4 Ma. NE-SW shortening due to strike-slip faulting of minor magnitude began in the Puna by 0.2 (2?) Ma and is still active. Kinematics in the foreland are indistinguishable from kinematics in the Puna. Significant thrust deformation began after 10 (5?) Ma at most localities in the foreland, later than in the Puna. Thrusting until after 2-3 Ma in some areas, but at least locally it lasted until 1 Ma, and east of the areas studied it is probably ongoing. Thus, the locus of most intense deformation appears to have generally migrated eastward during the late Cenozoic. The change from thrusting to strike-slip faulting in the foreland probably occurred in the Quaternary, and strike-slip faulting remains active. Paleomagnetic constraints on vertical axis rotation suggest that some oroclinal bending may have affected the Puna and adjacent foreland, but not regions further south. The amount of vertical axis rotation probably is insufficient to account for the Plio-Quatemary change in shortening direction. The difference between Mio-Pliocene fault kinematics and South America-Nazca plate kinematics may reflect paleostructural control of deformation kinematics in the Puna and adjacent foreland and/or strike-slip faulting along the thermally weakened volcanic arc or forearc. The Plio-Quatemary change from subvertical to subhorizontal extension might be linked to a decrease in normal

Fig. 24. Normalizedhistogramsof averagekinematic axis plunges for Plio-Quatemaryfaults observed at differentelevations. t 0 o- 0 o >4kin 3-4 300-0

< 3 km

6 0 ° . 310 9 0 °. 61 °

°

204

R.A. MARRETr, R.W. ALLMENDINGER,R.N. ALONSO and R.E. DRAKE

South America-Nazca plate convergence that in turn is related to a reorganization of Pacific plate movement, but existing plate reconstructions are too imprecise to be conelusive. Plio-Quaternary extensional faulting is not restricted to regions of high elevation on or near the Puna, so simple models of gravitationally driven extension due to thickened crust fail to explain the distribution of extensional faults. Rather, complex three-dimensional effects related to the southward narrowing of the thick Puna crust might better account for the distribution. Possible kinematic variation with depth is poorly constrained, but limited data are consistent with coeval thickening of the lower crust of the Puna and thinning of the upper crust. Acknowledgments--We are indebted to many colleagues in South and North America for sharing with us their ideas and unpublished data, and for pointing us to key localities for study. We would particularly like to recognize the contributions of B. Coira, E. Fielding, M. Grier, E Hungn, B. Isacks, T. Jordan, S. Kay, R. Mon, S. Olsen, R. Omarini, J. A. Saltity, M. Strecker, D. Vandervoort, and J. Virarnonte. Logistical support for the field work was provided by BHP-Utah International, Boroqufmica, and the Martin Flores family of Salta. We thank James Helwig for his review of the manuscript. This research was funded by National Science Foundation grants EAR-8519037 and EAR-8816287 to R. W. Allmendinger, a Fulbright Fellowship to R. Marrett, grants from the Geological Society of America, American Association of Petroleum Geologists, and Sigma Xi to R. Marrett, and CONICET funds to R. Alunso.

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Late Cenozoic tec~tie evolution of the Pana Plateau and adjacent foreland, NW Argentine Andes CahiU, T., Isacks, B.L., Whitman, D., Chatelain, J.-L., Perez, A. and Chiu, J.-M., 1992. Seisznioity and tectonics in Jujuy Province, northwestern Argentina. Tectonics 11, 944-959.

205

for 515 earthquakes. Physics of the Earth and Planetary Interiors 50, 127-154.

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Dziewonski, A. M., Franzen, J. E., and Woodhonse, J. H., 1983b, Centroid-moment tensor solutions for April-June, 1983. Physics of the Earth and Planetary Interiors 33, 243-249.

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