Lake patterns as related to neotectonics in subsiding basins: The example of the Ucamara Depression, Peru

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Tecto~~phys~s, 222 (1993) 69-78 Elsevier Science Publishers B.V., Amsterdam

Lake patterns as related to neotectonics in subsiding basins: the example of the Ucamara Depression, Peru J.F. Dumont

*

ORATOR, CP 9214 La Paz, Boliuia

(Received April 21992; revised version accepted November 6.1992)

ABSTRACT Dumont, J.F., 1992. Lake patterns as related to neotectonics

in subsiding basins: the example of the Ucamara De 3~ sion,

Peru. Tecto~ophysics, 222: 69-78. TWO Northeast trending elongated lakes from the Andean foredeep (Ucamara Depression, south Maraiion Basin) are analyzed in relation to basement structures, neotectonic and seismotectonic data from the surrounding areas. Lake Puinahua (25 x 60 km) lies over pre-Cretaceous basement faults and Lake Punga (9 x 35 km), which subsided in a historical times, lies over a Paleozoic basement uplift. P axes from earthquake focal mechanisms located east and west of the Ucamara Depression are parallel to the lakes. The trend of lineaments, together will neotectonic data from the craton border, suggest that lakes develop along extensional faults. The western limit of the lakes are lie over folds from the front of the Sub-andean Zone. ~mpa~son of eIongated lakes from the Ucamara Depression with other lakes from the Andean foredeep suggests close relationships between lake pattern and geodynamics. Ria lakes are located along the front of the upper foothills and are possibly related to subsidence due to tectonic overloading of the sub-Andean border. Elongated lakes from the Ucamara Depression are situated in front of the wide, low foothills of the Sub-andean Thrust and Fault Belt. Subsidence is related to block faulting, and tectonic overloading is weak. These results are compared with previous studies of rectangular lakes from the Beni Basin, which appear to be related to the bending of the craton border. Analysis of lake pattern is suggested as a compiementary approach for the neotectonic study of active subsiding areas.

Introduction Neotectonic studies of subsiding areas are scarce because the standard methods of morphology and fault analysis have not yielded results. Generally, subsidence phenomena are only substantiated by the long-term accumulation of sediments. In the case of the Andean foredeep, active tectonics in the adjacent mountain have been well documented (Sebrier and Soler, 19911, but active subsidence in the foredeep has not been re-

* Present address: Laboratoire de GCologie Dynamique Inteme, Universite de Paris-Sud, Bat. 509,91405 Orsay Cedex, France.

ported. Sternberg (1950) first suggested a relationship between lake patterns and neotectonics in the Amazonian lowlands. Plafker (1964) suggested that the two major trends of elongated lakes in the Beni Basin of Bolivia are probably controlled by a regionally oriented system of fractures in the crystalline basement. Allenby (1988) suggested that these lakes trend parallel to the direction of principal stress in the basement. Such hypothesies are speculative because no related basement structures have been reported and because neotectonic and seismotectonic studies of the area are lacking. In contrast, the neotectonics (SCbrier and Soler, 19911, seismotectonics (Suarez et al., 1983) and main structures of Central and Northern Peru (Laurent and Pardo, 1975; Pardo, 1982;

0@45-1951/93/$06.00 0 1993 - Eisevier Science Publishers B.V. Ah rights reserved

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MCgard, 1984, Laurent, 1985) are relatively well known. These provide a background for comparing lake patterns and neotectonics in the Maranon foredeep basin. Surface landforms in the basin have been studied using radar and MSS LANDSAT images, while MSS LANDSAT images alone were used for the upland, due to the poor quality of radar images. The basin surface exposes two types of extended lakes: (1) two large, elongated lakes located in the southeastern part of the Marafion Basin (Ucamara Depression) which cover a total area of 1,624 km2 (Fig. 1, numbers 5 and 6); and (2) ria lakes, located along the front of the upper foothills (Fig. 1, no. 4), in the Pastaza Depression and in the Ucayali Basin (Rhtinen et al., 1987). The case of ria lakes will be considered in the discussion. Lakes originating from abandoned fluvial morphology, such as oxbow lakes, will not be considered here.

Geological background The structural framework of the Sub-andean Zone of Peru comprises (Fig. 1) the Sub-andean Thrust and Fold Belt (STFB) in the west (Pardo, 1982; MCgard, 1984). This is bordered to the east by subsiding basins, in northeastern (Maranon Basin) and southeastern Peru (Madre de Dios Basin). Between these two basins, the STFB is bordered in Central Peru by low foothills, which belong to the Sub-andean Tilted Block Zone (STBZ) (Dumont et al., 1991) and the Acre Basin area, which is an example of incised topography. The STBZ overlies a flat slab of oceanic crust, which probably has been subducting since the early Pliocene (Jordan et al., 1983). The Maraiiibn Basin constitutes the transition zone between the extensive cratonic region of the Brazilian Shield, moving in a westward direction,

:>. ..... Fig. 1. Structural scheme of Peruvian Sub-andes. 1 = STFB: Sub-andean Thrust and Fold Belt; 2 = STBZ: Sub-andean Tilted Block Zone; 3 = Iquitos Arch (upland border); 4 = ria lakes; 5 = Lake Puinahua; 6 = Lake Punga; 7 = Tapiche Fault; PD = Pastaza Depression; UD = Ucamara Depression.

71

RELATION BETWEEN LAKE PATTERNS AND NEOTECCONICS

and the Andean Foredeep or Piedmont, which is being pushed eastward by the subsiding Nazca plate. The basin began subsiding during the Early Cretaceous and accumulated up to 5,000 m of post-Jurassic sediments in the central part (Sam, 1974). The MaraiiBn Basin includes the northtrending Pastaza Depression to the north (Laurent and Pardo, 1975) and the ~-trending Ucayali-Maraiiitn (Ucamara, in brief) Depression to the south (Laurent, 1985). The Ucamara Depression is bordered eastward by the Iquitos Arch (Fig. l), which was uplifted between the early Mesozoic and Early Tertiary (Sanz, 1974; Laurent, 1985). Subsidence during the late Tertiary ended, as a result of early Quatemary tectonic events, which may be correlated with 2 Ma tectonic events Wbrier and Soler, 1991). High fluvial terraces (Dumont et al., 1988) suggest that the rising of the Iquitos geanticline was still continuing during the late Pleistocene.

The Ucamara Depression The Ucamara Depression (Fig. 1, UD) is a 25,000 km2 flood plain, covered by wetlands and alluvial ridges. It is bounded to the north by the Maranon River and to the west by low Sub-andean foothills. The south and east borders are sharp morphost~ctural ~undaries formed, by the Tapiche Fault and the bluff line at the margin of the uplands of the Iquitos Arch respectively. The area is covered by rain forest and outcrops are restricted to road scarps, bluff lines in the uplands and river banks in the depression. Floral system

The drainage area of the MarafGn Basin extends from just south of the Equator to the regions near lake Titicaca (1431, providing one tenth of total water supplies of the Amazon Basin. Three “white water” rivers bearing a high sedi-

J Fig. 2. Morphostructural scheme of the Lake Puinahua area (for location see Fig. 1). I and 2 = late Hercynian uplift of crystalline rocks (basement structures); 2 = Santa Lucia uplift; .? = swamps; 4 = lakes; 5-B = main structures from Paleozoic and basement units; 5 = faults, 6 = faulted border of the Santa Lucia Uplift, 7 = synclines, 8 = anticline.

J.F.DUM0N-T

ment load, cross the Ucamara Depression, from north to south these are: the Maranon, the Ucayali and the Tapiche rivers (Fig. 1). During high water stages, the depression is flooded extensively and fluvial connections form between the rivers. Small black water streams which contain much suspended organic matter, drain these swampy areas. The gradient through the Ucamara Depression is about 4 x NITS, close to the mean slope of the Amazon River downstream from Iquitos to the sea (3 x lo-?. Elongated lakes

Fig. 3. Morphost~ctural scheme of the Lake Punga area (for location see Fig. 1). I= limit of the late Hercynian uplift of Santa Eiena (basement structure); 2 = low foothills border; 3 = Iquitos upiand border; 4 = floating vegetation; 5 = open water lake; 6 = Tapiche Fault; 7 = lineaments on LANDSAT MSS imagery; V= village of Vicungo; AC = location of an abandoned course of the Tapiche River, no precise age.

Two elongated lakes occur in the Ucamara Depression: Lake Puinahua in the central part, and Lake Punga, to the south (Fig. 1). Lake Puinahua (Fig. 2) covers an area of 1,324 km2 (about 25 x 60 km) and is drained by the Puinahua channel, a secondary channel of the Ucayali River. This is composite lake, comprising several swamps and lakes; the main part trends N60” and it is wider at the southwestern end. The northeastern part turns northward, trending N40”. The southwestern limit of the lake is the more distinct and trends approximately NNW. Two

Fig. 4. Lake Punga near Vicungo (see Fig. 3) during a low water stage (September 14th, 1989). The lake is 2 m deep; tree trunks (up to 70 cm in diameter) can be seen rising above the water level.

RELATION

BETWEEN

LAKE

PATTERNS

AND

73

NEOTECTONICS

belts of non-pe~anently flooded terrains, interpreted as levees, separate the white water of the Puinahua channel from the lake and swamp areas. Lake Punga (Fig. 3) trends N45”, is 35 km long and arrow-shaped, with a width of 13 km in the southwest and 4.5 km in the northeast. Lake Punga is the product of subsidence which occurred in historical times (Dumont and Garcia, 1991). Previous to 1927, the area was only occasionally flooded, but between 1927 and 1929 it was flooded annually and later became a permanent lake. As a result, the Tapiche River shifted towards the southeast and at present follows the northern border of the lake, entering it at some points. In these areas the channel of the river is separated from the black water of the lake by a levee only l-l.5 m high and 5-50 m wide. A flooded forest is visible in the lake and tree

trunks rise just above the low water level. They are preserved from decomposition by the acid black water (Fig. 4). During periods of low flow the bases of the dead trees lie under 2 m of water; high flows pass over the levees by 50 cm, suggesting a subsidence of more than 4 m. 3ase~ent st~~t~res and elongated lakes Laurent and Pardo (1975) and Laurent (1985) studied the basement structures of the Maranon basin based on the interpretation of seismic reflection profile sections. They reported few deformations from the Mesozoic and Cenozoic de. posits, but the pre-Mesozoic basement was divided into several faulted and uplifted blocks (Fig. 5). The Ucamara Depression extends mainly over an area characterized by NNE to W-E trending, late Paleozoic strike-slip faults, which

Fig. 5. Basement structures of the Ucamara Depression, from Laurent (lY85). See Fig. 1 tar location. 1 = low foothills; 2 = late Hercynian uplifts of crystalline rocks; 3 = Iquitos upland; 4 = elongated lakes; 5-7 = pre-Cretaceous structures: 5 = basement faults, 6 = synclines, 7 = anticlines; 8 = lineaments observed on LANDSAT MSS imagery (006-064, 4th August, 1975); 9 = Tapiche Fault.

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are related to the Maranon Fault Zone (Laurent, 1985), a belt of high density. The main faults shown in Figure 5 trend ENE. The morphology of Lake Puinahua suggests that the occurrence of pre-Cretaceous faults in the basement determined the boundaries of the subsiding blocks (Fig. 2). This lake is located within depressions of Paleozoic rocks limited to the north, east and south by strike-slip faults in the basement. Some discrepancies between lake boundaries and basement faults may be due to non-vertical transmission of the fault movement through the overlying sediments. The western limit of the lake is superimposed over a fold belt parallel to the eastern border of the Santa Lucia, Paleozoic uplift (Laurent, 1985). This fold belt represents a northern extension of the Sub-andean structures of Central Peru, with an eastern margin in the continuation of the Tapiche Fault (Fig. 5). The former courses of the Ucayali River during the Quaternary (Pacaya and Samiria Rivers; Dumont, 1993) trend north along this belt and the uplift. The fold belt, as well as

IXJMONT

the Santa Lucia uplift, have no topographical expression on the surface of the depression This suggests a control of the river trend by reactivated structures. The northeast orientation of Lake Puinahua is sub-paralle1 to the “en Cchelon” system of the Maranon Fault Zone reported for the late Paleozoic by Laurent (1985). The structure of the Santa Elena uplift (Fig. 3) is interpreted as a crystalline horst surrounded by PaIeozoic sedimentary strata (Laurent, 1985). The southwestern limit of Lake Punga is superimposed over the southwestern border of the Santa Elena uplift and trends parallel to the Tapiche Fault. The northeastern elongation of the lake is parahel to a few of the structural features northward and northwestward from the Santa Elena high zone, according to Laurent (1985). S~ismot~to~ic and n~ot~toni~ data Focal mechanisms from the Sub-andes are compressional: P axes are horizontal and trend statistically E-W, and T axes are nearly vertical

Fig. 6. Neotectonic scheme of northeastern Peru. I = Iquitos upland border; 2 = Ucamara Depression; 3 = Pastaza (north) and Ucayali (south) depressions; 4 = Sub-andean Tilted Block Zone; 5 = Sub-andean Thrust and Fold Belt; 6 = ria lakes; 7 = efongated lakes; 8 = reactivation of the Tapiche Fault during the Quaternary; 9 = statistical trends of lineaments in upland; 10 = minimum principal stress direction of faults in Quaternary fluvial deposits (Dumont et al., 1988). Stereonets: SU= Suarez et al. (1983); AS = Assump~ao and Suarez (1988); A = Assump&o (19921, number are those used by the authors.

75

(Suarez et al., 1983). Nonetheless, the orientation of P axes around the Wcamara Depression is near NE (Fig. 6; Assump$o, 1992). A focal mechanism, located 300 km east from the Ucamara Depression, shows a NE P axis orientation, which is nearly horizontal, and the mechanism corresponds to a ~-stri~ng fault (Fig. 6, stereonet AS.9; Assump@o and Suarez, 1988). Two focal mech~isms from ~surnp~~o (1992), located 100 km and 250 km west of the Ucamara Depression, show P axes with an orientation/dip of 57/12 (Fig. 6, A.890504) and 73/24 (Fig. 6, A.900530) respectively. All data are of C quality (k25”), which indicates P axes trend from NNE to ENE. The Tapiche Fault is a deep fault, reactivated with reverse motion during the late Tertiary (Quechua) phases (Laurent 1985; and H. Laurent personal communication, 1990). Along the main segment of the fault we have observed folded Tertiary beds that have been overlaid discordantly by a fluvial terrace, 3 m above the high water level of the Tapiche River (Fig. 6, no. 8). The Iithology of this terrace is similar to the Pleistocene deposits of the Iquitos area (Dumont et al., 1988). The present position of the terrace suggests an uplift of the south hanging block of the main fault during the Quaternary. Neotectonic data from the Iquitos Upland include lineaments and fault plane analyses. Eighty lineaments have been observed on LANDSAT imagery (Fig. 5) in areas covered by late Pliocene and Quaternary deposits. Most are morphologic boundaries but some are river trends. Two classes of trends have been observed (Fig. 6, no. 9): the main class, related to the most visible morphological scarps, is sharply defined between N135” and N150” (maximum N145”), in total 816 km of lineaments. A second class ranges from N35” to N68” (maximum between N55” and N65”), with a cumulate length of 614 km. Normal fault movements have been reported from the early Quaterna~ deposits of the Iquitos Upland (Dumont et al., 1988), 60 km northeast of Lake Puinahua (Fig. 6, no. 10). The principal stress direction from thirteen striated normal fault planes, computed using the Carey-Gailhardis (Carey, 1976, 1979) inverse method give a least horizontal stress (aj) oriented at N330”, and a

15 , 0

20

,

10

,

,

60

,

,

80

,

,

100

,

I6

,fj

,t

+

120

140

160

Ima Fig. 7. Normal fault data from the fquitos upland (location in Fig. 6, no. IO). Arrows attached to fault traces correspond to the measured slip vector, S, (Wulf stereonet, lower hemisphere). Large black arrows give ~imuths of the minimum rr, computed principal stress direction. Histogram shows deviation between measured, S, and predicted t slip vector for each fault plane. Fault planes 15 and 16 (same number on stereonet and histogram) show a large deviation, related to near-vertical fault plane dips. Data from two stations, 14 km and 15.5 km, on the Jenaro Herrera to Angamos road (see Dumont et al., 1988, for more detail).

greatest principal stress (al) that is roughly vertical (Fig. 7). Discussion

Focal mechanisms from the nearest events located east and west of the Ucamara Depression are compressional, with P axes trending NNEENE. This is close to the trend of the lakes, N45 (Lake Punga) and N40*-N60” (Lake Puinahua). This suggests that the elongated lakes may be the surface expression of tensional stress. Superimposition of elongated lakes over basement structures suggests that tensional faults are reactivated basement faults, and this may explain the deviations from the direction of m~mum stress.

J.F. DUMONT

76

The arrow shape of the lakes suggests they are not simple tensional structures. Downwarping of blocks in the basement on the southwestern broad end of the lakes, was probably induced by the reactivation of faults trending roughly NW-NNW, which are parallel to the Sub-andes front. Downwarping movements extend away from the Subandes towards the sharp end of the lakes, which more or less parallels the direction of maximum stress. The main class of lineaments (N145”), parallels the Tapiche Fault (Fig. 5, no. 8 and Fig. 6, no. 9); such lineaments limit the length of Lake Punga (Fig. 3). A secondary class is represented by tiny lines scattered between N55” and N65” and are interpreted as the surface expression of tension fractures.

the Tapiche-Punga case, the invaded areas contain tree trunks of hardwood (F. Kahn, personal communication, 19891, that cannot represent previous marshes, but are typical of seasonnally inundated or well drained forest formations. This observation is critical to understanding the difference between sedimentary and subsidence-controlled avulsions. The dead flooded forest of Lake Punga shows similarities to the Brownwood Subdivision in the Houston-Galveston area, which has undergone 2.7 m of subsidence since the early 1940s (Holzschuh, 1991). The limited extension of Lake Punga disallows the hypothesis of flooding due a rise in the base level downstream. Due to the flatness of the area this would have flooded all the Ucamara Depression, as occurs during exceptionally long floods.

Interpretation of lake boundaries

Elongated lakes versus the location of ria lakes

One problem is that lake boundaries are less clear than a fault crossing the surface of exposed landforms. An analogy is suggested with artificial subsidence due to mining activities or withdrawal of oil or water from wells. Downward movement, caused by the collapse of cavities, is transmitted to the surface by a high-angle fracturing in the lower part, with progressive adjustment in the upper part, so that no scarps are visible on the surface (Saxena, 1991). Removal of groundwater has led to subsidence of up to 2.7 m in the Houston-Galveston area, while fault creep and tilting have been observed at distances up to 500 m away from the fault scarp (Holzer and Gabrysch, 1987). These faults probably connect in the sub-surface to pre-existing faults of natural origin (Van Siclen, 1967). Similar processes may explain the discrepancy between the location of basement faults and lake boundaries.

Ria lakes are formed by quick aggradation of the river belts and a subsequent damming of the tributaries close to the junction. Two areas containing numerous ria lakes can be observed. Most of them are located along the Pastaza Depression, with others along the Ucayali Basin of Central Peru, on the western border of the STBZ (Fig. 1 and Fig. 6, to the north and south, respectively). No similar ria lakes occurs in the Ucamara Depression. We suggest that the different shape and location of lakes may be related to different subsidence styles. In front of the upper foothills, which are characterized by overthrusting and folding, subsidence by overloading has probably been more effective, and has generated high aggradation and ria lakes. In contrast, tectonic subsidence in the Ucamara Depression, in front of low foothills, involves basement block tectonics that have generated elongated lakes.

Implication for the jluuial network

Comparison to the Beni Basin

The shifting of the Tapiche River as a result of the formation of Lake Punga is different from the previously described examples of avulsions. In a sedimentary-controlled avulsion, the new river channel invades marshes of the flood plain that are lower than the levees (Smith et al., 1989). In

The Beni Basin is the only region where the elongated lakes in a similar geodynamic setting have been previously described (Plafker, 1964; Allenby, 1988). Nevertheless, important differences in the number, shape, scale and geomorphic setting of the rectangular lakes can be ob-

RELATION BETWEEN LAKE PitTTERNS AND NEWTECTONICS

served in both regions. According to Allenby (1988), lakes in the Beni Basin were formed by tension, induced by upward flexing of the crystalline basement, and are aligned along the watershed divide. In the Ucamara Depression, lakes are related to basement-controlied subsiding blocks and tend to capture the river channels. Conclusions Elongated lakes in the Ucamara Depression appear to be controlled by basement structures which have been reactivated due to stress, along the Sub-andes/ Brazilian craton bounda~. Blockcontrolied subsidence occurs in front of the low foothills, influencing the trend of rivers by capturing them. Ria lakes are located in front of the upper foothihs and are due to subsidence related to more efficient overloading. Analysis of lake patterns may help to define neotectonic provinces in subsiding basins.

This work was completed as part of the Cooperative Research Program between the Institut Fraqais de Rechercbe Scientifique pour le ~~veloppement en C~p~ration ~~RST~~) and the Institute Geogfisico de1 Peru (I.P.G.). This study was also supported by the Instituto para la Investigaci~n de la ~azonia Peruana (I.I.A.P.1. We specially thank H, Laurent and A. Pardo for comments on the basement structures, M. Sebrier and 0. Bellier for helpful discussions and comments, and l&3. Ass~rnp~~o for &omments and complementary data on focal mechanisms. T, Killeen and an anonymous reviewer helped to improve the English for which we are grateful. References Ahenby, R.J., 1988. Origin af reetanguIar and aligned lakes in the Beni Basin of Bolivia. Tectonophysics, 145: l-20. Assump$o, M., 1992. The regional intraplate stress field in South America. J. Geaphys. Res., 97 (B8): 11,889-D, 903. Assump$o, M. and Suarez, G., 1988. Source mechanisms of moderate size earthquakes and stress orientation in midplate South America. Geophys. J., 92: 253-267.

77 Carey, E., 1976. Analyse num6rique dune methode m&an&e &?mentaire appliqu& B I’&ude d’une population de f&es: caleul d’un tenseur moyen des contraintes, g I’aide des stries de glissement. Thesis, Univ. Paris XI, Centre d&say, Paris, 138 pp. Carey, E., 1979. Recherche des directions principafes de contraintrs associ&s au jeu d”une population de failles Rev. G&l. Dyuam. G6ogr. Phys., 21(l): 57-46. Dumont, J.F., 1993. The Upper Amazon river system. In: S.A. Schumm and B. WinkIey (Editors), Engineering Problems Associated with the Natural Variability of Large alluviai Rivers. Am. Sot. Civ. Eng., New York, NY, in press. Dumont, J.F. and Garcia, F., 1991. Active subsidence controlled by basement structures in the Mar&on Basin of northeastern Peru. In: Land Subsidence. Proc. 4th Int, Symp. Land Subsidence May 1991. Int. Assoc. Hydrol. Sci., 200: 343-350. Dumont, J.F., Lamotte S. and Fournier, M., 1988. Neotectonica de1 Arco de Iquitos (Jenaro Herrera, Per%). Bol. Sot. Geol. Peru, 77: 7-71. Dumont, J.F., Deza, E. and Garcia, F., 1991. Morphostructural provinces and neotectonics in Amazonian lowlands of Peru. J. S. Am. Earth Sci., 4: 287-29.5. Holzer, T.L, and Gabrysch, RX.., 1987. Effect of water-level recoveries on Fault Creep, Huston, Texas. Ground Water, 25: 392-397. Ho&huh, Y.C., 1991. Land subsidence in Houston, Texas USA. In: 4th Int. Symp. on Land Subsidence Field Trip Guidebook, pp. I-21. Jordan, T.E., Isacks, B.L., Ahmendinger, R.W., Brewer, J.A., Ramos, V.A. and Ando, C.J., 1983. Andean tectonics reIated to geometry of subducted Nazca plate. Geol. Sot. Am. Bull., 94: 341-361. Laurent, H,, 1985. El pre-Cretaceo en el Oriente peruano, su distribucicin y sus rasgos estructurales. Bol. Sot. Geol. Per+ 74: 33-59. Laurent, H. and Pardo, A., 1975. Ensayo de interpretaci~n de1 basamento de1 Nororiente Peruano. Bol. Sot. Geol. Peru, 45: 25-48. Migard, F., 1984. The Andean erogenic period and its major st~ctures in central and northern Peru. J. Geol. Sot. London, 141: 893-900. Pardo, A,, 1982. Caracteristicas estructurales de la faja subandina de1 norte de1 Peru. In Proc. Symp. Expforacion Petrolera en las Cuencas Subandinas de Venezuela, Colombia, Ecuador y Peru (Bogotd, Colombia), Asociaci6n Colombiana de Geologos y Geofisicos de Petroleo (unpub0 Plafker, G., 1964. Oriented lakes and lineaments of northeastern Bolivia. Bull. Geol. Sot. Am., 75: 503-522. R%inen, M., Sale, J., Kalliola, R., 1987, Fluvial perturbance in the western Amazon Basin; reguIation by long term Sub-Andean tectonics. Science, 238: 1398-1401. Sanz, V.P., 1974. Geologia preliminar de1 rio Tigre-Corrientes en ef Nororiente peruano. Bol. Sot. Geol. Peru, 44: 106127.

78 Saxena, N.C., 1991. Subsidence management in Jharia Coalfield, India; a concept. In: Land Subsidence. Proc. 4th Int. Symp. on Land Subsidence, May 1991. Int. Assoc. Hydrol. Sci., 200: 181-193. SCbrier, M. and Soler, P., 1991. Tectonics and magmatism in the Peruvian Andes from late Oligocene time to the present. In: Andean Magmatism and its Tectonic Setting. Geol. Sot. Am. Spec. Pap., 26.5: 259-278. Smith, N.D., Cross, T.A., Dufficy, J.P. and Clough, S.R., 1989. Anatomy of an avulsion. Sedimentology, 36: 1-23.

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Stemberg, H.O’.R., 1950. Vales tectonicas na planicie amazonicas? Rev. Bras. Geogr., 4 (12): 511-534. Suarez, G., Molnar, P. and Burchfield, B.C., 1983. Seismic&y, fault plane solutions, depth of faulting, and active tectonics of the Andes of Peru, Ecuador and southern Colombia. J. Geophys. Res., 88: 10403-10428. Van Sicken, DC., 1967. The Houston fault problem. In: Proc. 3rd Annu. Meet. Am. Inst. Prof. Geol., Texas Section (Dallas), pp. 9-31.