Evolution of the central Rio Grande rift, New Mexico: New potassium-argon ages

Earth and Planetary Science Letters, 51 (1980) 309-321 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

309

[21

EVOLUTION OF THE CENTRAL RIO GRANDE RIFT, NEW MEXICO: NEW POTASSIUM-ARGON AGES W.S. B A L D R I D G E 1, P.E. D A M O N 2, M. S H A F I Q U L L A H 2 and R.J. B R I D W E L L 1 1 Los Alamos Scientific Laboratory, Geosciences Division, MS 9 78, Los Alamos, NM 87545 (U.S.A.) 2 University o f Arizona, Department o f Geosciences, Laboratory o f Isotope Geochemistry, Tucson, A Z 85 721 (U.S.A.)

Received February 6, 1980 Revised version received August 4, 1980

New K-At age determinations on mid-Oligocene to Pleistocene volcanic and shallow intrusive rocks from the central Rio Grande rift permit a more detailed understanding of the tectonic and magmatic history of the rift. Initial extension in the region of the central rift may have begun prior to 27 m.y. ago. By 25 m.y. ago broad basins existed and were filling with volcaniclastic sediments derived mainly from volcanic centers in the San Juan and Questa areas. Continued tectonic activity narrowed these basins by 21-19 m.y. ago, indicated in the Santa Fe area by tilting and faulting that immediately postdate 20-m.y.-old latite. Uplift of the Sangre de Cristo, Sandia, and Nacimiento Mountains shed clastic debris of the Santa Fe Group into these basins. Early rift magmatism is characterized by an overlap of mid-Tertiary intermediate intrusive and extrusive activity, extending to 20 m.y. ago, with mafic and ultramafic volcanism, ranging from 25 to 19 m.y. Both volcanism and tectonic activity were minimal during the middle Miocene. About 13 m.y. ago renewed volcanic activity began. Tectonism commenced in the late Miocene, resulting in the present, narrow grabens. The term "Rio Grande rift" should be restricted to these grabens formed during post-mid-Miocene deformation. Widespread eruption of tholeiitic and alkali olivine basalts occurred 3 - 2 m.y. ago. The Rio Grande drainage system was integrated 4.5 - 3 m,y. ago, leading to the present erosional regime. These intervals of deformation and magmatism correspond generally with a similar sequence of events in the Basin and Range province south of the Colorado Plateau. This similarity indicates that the Rio Grande rift is not a unique structure in the southwestern U.S., and must be related to the larger context of the entire Basin and Range province.

1. Introduction The Rio Grande rift consists o f a series o f interc o n n e c t e d grabens e x t e n d i n g from central C o l o r a d o across N e w M e x i c o to Chihuahua and West Texas (e.g. [1,2]). L i p m a n and Mehnert [3], Chapin and Seager [4], Baltz [5], and Chapin [6] present the e v o l u t i o n o f the n o r t h e r n and s o u t h e r n rift in some detail. Inability to correlate disconnected o u t c r o p s o f major sedimentary and igneous units and a shortage o f precise dates in the main graben-filling sediments have h a m p e r e d a detailed understanding o f the central rift. We present new K-Ar dates on igneous rocks mainly from the Espafiola ba'sin in the central Rio Grande rift (Fig. 1) in order to clarify the t e c t o n i c history o f the central rift, to relate episodes o f magmatism to t e c t o n i s m , and to address the question o f

m a g m a t i c e v o l u t i o n during the rifting process. The Espafiola basin is o f key importance in understanding the e v o l u t i o n o f the rift because it is the type locality o f the Santa Fe Group, the main rift-filling unit in the central rift, and because it has been magmatically active since the late Oligocene. While the J e m e z Mountains volcanic field, located on the western margin o f this basin, constitutes the major occurrence of volcanic rocks associated with the rift and was active f r o m Middle Miocene to Pleistocene, its stratigraphic relationship w i t h the sediments o f the Espafiola basin is n o t well defined. Hence this paper focusses primarily on igneous occurrences within the graben-filling sediments. The depth o f Tertiary sediment fill in the Espafiola basin probably exceeds 2 k m [7]. H o w e v e r , exposure o f these sediments is m o r e c o m p l e t e in the Espagiola

0012-821X/80/0000-0000/$02.50 © 1980 Elsevier Scientific Publishing Company

310 basin than in any other basin of the rift. This exposure is due to (1) uplift of the Espa~ola basin relative to the Albuquerque-Belen basin to the south, resulting in deep dissection of the Espa~ola basin fill, and (2) a general synclinal dip of the Tertiary fill, exposing progressively younger beds toward the center of the basin [8]. Hence, it is unlikely that large volumes of volcanic rocks are concealed in the basinfilling sediments. We have conducted limited field checking of stratigraphic relationships. However, in general we have relied on the literature for lithologlc descriptions and stratigraphic assignments.

2. Analytical procedures Whole-rock samples are prepared for K-Ar dating as follows: After initial crushing, phenocrysts are removed by handpicking. Samples are then ground to 100-150/~m. Feldspar is concentrated by heavy liquid and magnetic separation methods. Glass and altered minerals are separated by flotation on a liquid of specific gravity 2.50-2.55, and carbonate is removed by leaching with 0.5 N phosphoric acid. Potassium is analyzed on a Perkin-Elmer model 403 atomic absorption spectrophotometer. Three splits of each sample are taken into solution with HF, buffered with NaC1, and brought to standard volume. A standard rock is run concurrently with each sample. Analyses are repeated if the spread exceeds 1.5% or the potassium content of the standard differs from the accepted value by more than 2%. For argon analyses, samples are mounted in induction-heated molybdenum crucibles suspended in 90-mm air-cooled pyrex fusion envelopes, evacuated, and baked for two days at 260°C. After fusion the released argon is purified using a Ti-foil getter, CuCuO furnace, and Zr-A1 appendage pump. The Ar is divided into two or more atiquots for separate analysis. Analysis are performed by the static method on a Nier-type 15.24-cm-radius, 60 ° sector field mass spectrometer with magnetic sweeping. For young (i.e., Pleistocene) samples, the standard error is determined primarily by atmospheric argon contamination. The standard deviation is respectively 2.2%, 15%, and 100% for negligible, 90%, and 98.4% atmospheric argon.

Whole-rock chemical analyses were obtained by electron microprobe analysis of glass beads fused from whole-rock powders on a resistance furnace utilizing a 5-cm-long strip of Ir [9].

3. K-At dates: stratigraphic and tectonic setting K-Ar dates on volcanic and intrusive rocks from the central Rio Grande rift fall into two age groups: late Oligocene to early Miocene and late Miocene to Pleistocene. These groups, which are separated by a non-magrnatic interval of about 6 m.y., correlate generally with tectonic activity. The older K-Ar ages permit correlation of the sediments filling the early rift basins (Abiquiu, Picuris, and Los Pifios Formations) and show the contemporaneity of magmatism of intermediate composition. The younger ages date the renewed tectonism that resulted in the present rift basins and the volcanism that continued to the Pleistocene. 3.1. Espinaso Formation and related intrusive rocks

Porphyritic monzonite and volcanic and volcaniclastic units of intermediate composition (Espinaso Formation-Espinaso Volcanics of Stearns [ 10]) crop out at the southern margin of the Espafiola basin (Fig. 1) [ 10-12]. The close spatial and stratigraphic association of the flows and intrusive monzonitic rocks and their compositional similarity suggest that these rocks are genetically related. These monzonitic intrusive bodies (Fig. 1) are northern outliers Of the more extensive intrusions of the Cerrillos Hills and Ortiz Mountains. Bachman and Mehnert [ 13] obtained ages of 47 and 34 m.y. on latitic intrusive rocks from the southern Cerrillos Hills and Ortiz Mountains, respectively. They suggest, however, that the Cerrillos Hills sample may have been conta: minated by inclusions and that the 34-m.y. date may be more representative of the age of the intrusions. If this date is valid, the older of these rocks may not belong to the same magmatic event as the younger intrusive rocks. For example, Damon and co-workers [14,15] showed that in the nearby Basin and Range province a gap in magmatism exists from about 50 to 38 m.y. ago, separating Laramide magmatism from later mid-Tertiary magmatism. Our date of 30 m.y.

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314 on a biotite-augite monzonite porphyry at La Cienega (sample UAKA-77-90, Table 1) is the youngest yet obtained on intermediate intrusive rocks from the central rift. The oldest rocks of the Espinaso Formation occur at the type locality, south of the area shown in Fig. 1. Paleontological data indicate that the lower contact of this formation is approximately at the Eocene/ Oligocene boundary (38 m.y.) [10]. Also, hornblende from cobbles of latite porphyry in volcaniclastic rocks in the lower part of the type section yields a K-Ar age of 37 m.y. [16]. However, the age of the upper contact, with sediments of the Abiquiu(?) Formation, is unknown. In the southern Espafiola basin (Fig. 1), rocks of the Espinaso Formation are slightly younger than the associated monzonitic intrusive bodies (samples UAKA-77-88, -91, and -92, Table 1). The oldest volcanic unit (29 m.y.) is an andesite which crops out along the western margin of the Sangre de Cristo uplift. Bachman and Mehnert [13] obtained an age of 25 m.y. on this same unit. This discrepancy of about 15% may be due to the fact that they do not remove groundmass glass and mesostasis minerals, which are highly susceptible to alteration and argon loss, from their samples. The youngest volcanic unit (20 m.y.) is a glassy latite occurring adjacent to a monzonite intrusion (Fig. 1). These rocks are younger than the lower part of the Espinaso Formation in the type locality. If they are younger than the entire type section, then they should perhaps be redefined as a separate, local formation. At present, however, the age range represented by the type Espinaso section is so uncertain that we think it best to retain the original usage of Stearns [ 10,11], who called these intermediate volcaniclastic rocks of the southern Espafiola basin Espinaso Formation. Based on the general lithologic similarity of units comprising the Espinaso Formation and the fact that some of them are clearly overlain by the Cieneguilla Limburgite (Fig. 2), Stearns [10,11] and Sun and Baldwin [12] inferred that the flows of the Cieneguilla Limburgite postdate all units of the Espinaso Formation. However, the contact of the uppermost unit of the Espinaso Formation (glassy latite) with the Cieneguilla Limburgite is nowhere exposed, and field relations are ambiguous. Our date of 25 m.y. on the Cieneguilla Limburgite (sample UAKA-77-89,

Table 1) indicates that extrusion of this unit occurred while the Espinaso Formation was being erupted. 3.2. Abiquiu, Picuris, and Los Pfhos Formations The Abiquiu Formation (Abiquiu Tuff of Smith [17]), cropping out in the northern'and northwestern Espa~ola basin (Fig. 1), is a tuffaceous sand and gravel containing numerous volcanic clasts of intermediate composition. This formation unconformably overlies the E1 Rito Formation and underlies the Tesuque Formation (Fig. 2). On the northern margin of the Espa~ola basin (southern Tusas uplift), Abiquiu Formation interfingers with gravels of the lowermost Los Pi~os Formation [18]. Basalt flows interbedded in the Los Pi~os Formation were termed the Jarita Basalt Member by Barker [19]. These flows extend in isolated outcrops from the Tusas uplift to near Ojo Caliente (Fig. 1). Recent mapping by May [ 18] shows that the southernmost exposure of this basalt (22 m.y. old, sample UAKA-77-83) interfingers with sediments of the medial Abiquiu Formation. These sediments also include a small flow of olivine nephelinite (Cerro Negro, sample UAKA-77-82, Table 1), whose age is 19 m.y. Near Ojo Caliente, therefore, the Abiquiu Formation ranges at least from 22 to 19 m.y. However, May [20] suggests that the base of the Abiquiu Formation in the type area near Abiquiu is probably correlative with the Los Pitios Formation near Petaca, which is older there than in the Ojo Caliente area (approximately 25-26 m.y.B.P.; e.g., UAKA-77-84). Tuffaceous sediments similar to those of the Abiquiu Formation crop out in the northeastern Espahola basin (Fig. 1). These sediments constitute the Picuris Formation (Picuris Tuff of Cabot [21]). Tuffaceous sediments with interbedded basalt flows which crop out in isolated occurrences along the west side of the Sangre de Cristo uplift were mapped as Picuris Formation by Galusha and Blick [22], although Kelley [8] considered these sediments to belong to the lower Tesuque Formation. Because of their lithologic similarity to the Abiquiu Formation and their similar stratigraphic position (Fig. 2), it has been assumed (e.g. [8,11,22]) that the Picuris Formation is equivalent in age to the Abiquiu Formation. Our date of 25 m.y. (sample UAKA-77-80, Table 1) on a basalt flow interbedded in these sediments southeast of

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316 Espafiola confirms that these tuffaceous sediments correlate with the Abiquiu Formation.

3.3. Mid-Miocene to Pleistocene volcanism Renewed volcanism began about 13 m.y. ago with formation of small basaltic tuff rings and associated flows and dikes in the northern Espafiola basin [18, 23]. Also, siliceous pumice interbedded in the upper Tesuque Formation, dated at 1 1 - 1 3 m.y. ago [24], probably records earliest eruptive activity in the Jemez Mountains area [20]. Activity in the Jemez Mountains volcanic field continued intermittently until late Pleistocene time (e.g. [25]). About 10 m.y. ago, magmatic activity in the northwestern Espa~ola

basin resulted in intrusion of the Cation del Cobre dike 2.5 km north of Abiquiu [13] and of the Rio del Oso dike swarm and the basalt flow at Chili (samples UAKA-77-87 and -81, respectively, Table 1). The Cation del Cobre dike was intruded along a fault zone which offsets the Abiquiu against the Eocene E1 Rito Formation (see Fig. 2). The Rio del Oso dikes intrude the Ojo Caliente Sandstone, the upper member of the Tesuque Formation. The Chili flow, 4 km east of the Rio del Oso dikes, is interbedded in the lower Chamita Formation [22]. This flow and the enclosing sediments are tilted 20 ° to the south. In the type locality the Chamita Formation is 4 . 5 - 6 . 0 m.y. old [23,26]. Our age on the Chili flow, located about 3 km northwest of the type locality of the Chamita,

TABLE 2 Compositions (wt.%) and Barth-Nigglication norms (cation %)

SiO2 TiO2 A1203 Fe20 a * FeO MgO CaO Na20 K20 P2Os

UAKA77-81

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49.36 1.58 16.63 1.90 8.67 7.63 9.73 3.26 0.62 0.34

42.14 2.60 12.78 2.36 10.77 11.00 11.87 3.55 0.73 1.47

53.29 1.99 15.51 1.88 8.57 5.43 8.97 3.47 0.32 0.27

52.25 1.48 16.80 1.54 7.02 5.84 8.45 4.46 1.53 0.64

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48.50 1.50 16.75 1.95 8.88 8.08 9.65 3.42 0.76 0.33

42.46 1.88 11.40 1.98 9.03 15.54 12.98 3.09 0.81 1.01

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0.00 4.47 28.40 27.99 1.30 0.00 14.02 0.00 19.05 0.00 2.02 2.08 0.69

0.00 0.00 0.00 14.41 16.15 3.71 32.60 0.00 26.26 0.28 2.01 2.54 2.05

* Fe203 calculated from Fe3+/Fe2+ = 0.I. Mg-value = 100 (Mg/Mg + Fe2+). Analyses UAKA-77-85 and -86 are respectively F212 and 281 from Baldridge [9]; analysis UAKA-77-89 is A271 from Baldridge [41].

317 indicates that here the Chamita Formation is at least 9.6 m.y. old. The Cation del Cobre and Rio del Oso dikes and the Chili flow may have been part of a single small magmatic event, though lava compositions range from alkali olivine basalt to olivine tholeiite (Table 2). The voluminous flows comprising the Lobato Basalt, which form high mesas on the northeast side of the Jemez volcanic field (Fig. 1), were extruded 9.6-7.4 m.y. ago [27,28]. This volcanism was followed at 5 m.y. by extrusion of the flows capping Sierra Negra (sample UAKA-77-93, Table 1). These flows spread across faults which offset the Abiquiu Formation against the Chama-E1 Rito Member of the Tesuque Formation (e.g. [8]), and are in turn broken by younger faults. These flows and dikes were approximately contemporaneous with renewed tectonism in the Espafiola basin. A series of parallel, northeast-trending normal faults, which brings successively younger rocks down to the east, delineates the western margin of this basin (Fig. 1). Downfaulting of the Abiquiu Formation against the E1 Rito Formation occurred between 19 and 10 m.y. ago; the Chama-E1 Rito Member of the Tesuque Formation was downfaulted against the Abiquiu prior to 5 m.y. ago. Tilting within the Espatola basin postdates the 9.6-m.y.-old Chili flow. At least some activity on these marginal faults postdates the 5-m.y.-old Sierra Negra basalt. This deformation corresponds in age and style with post12-m.y.-old Basin and Range tectonism in southeastern Arizona (see next section). Basaltic volcanism became widespread in the central rift only after 5 m.y. ago. Earliest volcanism in the Cerros del Rio volcanic field (4.4. m.y. ago) is represented by Black Mesa at San Ildefonso (sample UAKA-77-86, Table 1), a deeply eroded vent con> plex capped with river gravels of the Pliocene Puye Conglomerate [22]. Most of the volcanism in this field occurred 2 - 3 m.y. ago [13,29,30]. The youngest volcanism of the Cerros field consists of flows overlying the older (1.4 m.y.) Bandelier Tuff Member and overlain by the younger (1.1 m.y.) Bandelier Tuff Member erupted from the Jemez volcanic field (sample PED-51-60, Table 1 ; [27,28,31 ]). Basaltic rocks of the small E1 Alto volcanic field, 13 km south of Abiquiu (Fig. 1), yield an age of 3.2 m.y. (sample UAKA-77-85, Table 1). Similarly, lavas of Santa Ana

Mesa and other small volcanic fields further south in the Albuquerque-Belen basin are dominantly late Pliocene or Pleistocene, mainly 2.6 m.y. to 140,000 years old [13,32]. The voluminous Taos Plateau volcanic rocks range in age from 4.7 to 1.8 m.y. (sample UAKA-72-65, Table 1 ; [33,34]). Our date of 4.6 m.y. on the highest flow at the Rio Grande Gorge is nearly 30% older than that of Ozima et al. [33]. Their range in ages of 4.4-3.6 m.y. for flows exposed in the gorge is not in agreement with their paleomagnetic results, which show a normal polarity at the top and bottom of the sequence and a reversed polarity in the middle. Recent results from Iceland [35] indicate that six magnetic events occurred during this interval. An 4°Ar/36Ar vs. 4°K/36Ar reference "isochron" for the data of Ozima et al. [33] yields an average age of 4.4 m.y. and an intercept of 292.6. They did not correct (about 1%) for mass discrimination. Using their intercept of 292.6 we obtain, after correcting their data, a narrow range in ages of only 4.5-5 m.y. This range agrees well with their paleomagnetic data indicating three magnetic events and with our age of 4.6 m.y. on the highest flow at the Rio Grande Gorge.

4. Evolution of the central Rio Grande rift

Callender and Zilinski [36] describe vertical faults along the west side of the Albuquerque-Belen basin which were intruded 27.1 m.y. ago by mafic to intermediate magmas. These faults may represent the earliest mid-Tertiary crustal extension in this region. Broad basins, whose axes corresponded generally with the axes of the modern basins, certainly existed in the northern Espaffola basin region 25 m.y. ago, suggesting that initial extension occurred prior to that time. A minimum age of 25 27 m.y. for the initiation of crustal extension in the central rift agrees with beginning of extension 2 8 - 3 i m.y. ago in the southern rift [4] and 26 27 m.y. ago in the northern rift [3]. Detailed stratigraphic and structural work by inany investigators, especially Stearns [ 11 ] and Galusha and Blick [22], has documented that the evolution of the region occupied by the present Rio Grande rift occurred in two tectonic intervals, an early "pre-Abiquiu" and a later "post-Santa Fe"

318

deformation. The earlier deformation, to which we assign an age of 2 5 - 1 9 m.y. ago (Fig. 3), deformed lower units of the Espinaso Formation and older rocks and resulted in formation of broad basins whose axes correspond generally with the axes of the modern rift grabens. This deformation probably continued until the beginning of Santa Fe time, resulting in the slightly narrower Tesuque basins and in uplift of the Sangre de Cristo, Nacimiento, and Sandia Mountains. The later deformation is marked primarily by renewed uplift of these mountains in the late Miocene and Pliocene [ 11 ]. Faulting such as is present in the northern Espa?lola basin may have begun as early as middle Miocene (Fig. 3). Possibly downwarp of the Tesuque basins was continuous, in which case these two intervals of deformation only represented accelerated tectonism. However, evidence for downwarp during deposition of Santa Fe Group strata has never been substantiated [11,22].

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30 20 106 YEARS BEFORE PRESENT

.J ?= IO

Fig. 3. Magma production in the Espafiola basin during Cenozoic. Units with less than 0.1 km 3 omitted. For Jarita basalts, only those falling within area of Fig. 1 are represented. Dashed lines indicate intermediate rocks of the midTertiary magmatic event. Distribution function f6r, and timing of early activity in, the CerriUos Hills are unknown; total volume includes intrusive rocks extending from 47 to 30 m.y.B.P. Distribution function for Cerros del Rio is hypothetical; time interval and total volume are correct. Horizontal shaded bars represent time intervals of most intense deformation in central Rio Grande rift (see text).

The broad basins which existed in the central rift 25 m.y. ago were filled with clastic and tuffaceous sediments of the Abiquiu and Picuris Formations. These sediments were probably derived mainly from volcanic highlands to the north in the San Juan Mountains and near Questa in the Sangre de Cristo Mountains [11 ]. Similar tuffaceous sediments (Abiquiu(?) Formation of Stearns [ 11 ]) southeast of Santa Fe were probably also derived from these sources as well as from the Espinaso Formation. Deposition of Abiquiu-type sediments continued to at least 19 m.y. ago. The basins in which these sediments were deposited were much more extensive than the modern basins of the rift (e.g. [11]). Magmatism associated with the earliest extension along the present Rio Grande rift was dominated by the widespread mid-Tertiary calc-alkalic magmatic event. These magmas consist mainly of calc-alkalic rhyolite, andesite, and doreite (high-potassium basaltic andesite or calcic latite) [37]. In the northern and southern rift, volcanic rocks of intermediate composition, associated with the San Juan, Questa, and Mogollon volcanic fields, were erupted as late as 20 m.y. ago [4,38]. In the central rift, intrusion of monzonitic magma in the Ortiz Mountains and Cerrillos Hills occurred as recently as 30 m.y. ago, with associated volcanism extending to 20 m.y. ago. Earliest mafic and ultramafic volcanism in the central rift began 25 m.y. ago and continued to 19 m.y. ago. This volcanism, which occurred along the margins of the present Espafiola basin, included a wide range of rock types. Olivine nephelinite was erupted in the northern and southern Espa~ola basin. Quartz tholeiitic lavas of the Jarita Member of the Los Pi~os Formation were also erupted along the northern margin of the basin and were widespread throughout the Tusas uplift. Along the east side of the Espa~ola basin lavas of both alkali olivine basalt and olivine tholeiite (Baldridge, unpublished analyses) are interbedded in tuffaceous sediments of the Picuris Formation. All of these basalt occurrences are volumetrically minor (<1 km3; Fig. 3). The basins in which the Tesuque sediments were deposited were more restricted than those of the Abiquiu-type sediments, although still broader than the present basins [11]. These basins were formed after about 2 1 - 1 9 m.y. ago. For example, tilting and faulting in the Santa Fe area, representing breakup of

319 the Abiquiu-age basins, occurred after extrusion of the glassy latite 20 m.y. ago [11,12]. Renewed uplift of the Sangre de Cristo, Nacimiento, and Sandia Mountains, reflected partly by the change from tuffaceous sediments of the Abiquiu and Picuris Formations to fanglomerates of the lower Santa Fe Group (Fig. 2), probably began at that time [11 ]. These fanglomerates (Nambe Member of the Tesuque Formation) contain the first influx into the Espafiola basin of granitic and gneissic clasts derived from the Sangre de Cristo Mountains. Baltz [5] indicates that the basins in which the Tesuque Formation was deposited were broad and shallow, and primarily downwarped rather than downfaulted. While downwarping undoubtedly occurred, the presence of the upper Oligocene to lower Miocene ( 2 5 - 1 9 m.y.) volcanic rocks along fault zones at the margins of the present Espa~ola basin implies that these marginal fault zones existed at this time and served as conduits for magmas. After formation of the basins of Tesuque age, significant tectonism did not occur again until the late Miocene or early Pliocene, at which time uplift of the Sangre de Cristo, Sandia, and Nacimiento Mountains was renewed (e.g. [11 ]). This renewed uplift may be reflected in the change in lithologic character from the Zia Sand to the Cochiti Formation southwest of the study area [ 11 ], and possibly in the termination of deposition of the Chamita Formation [22]. Volcanism was absent from the central rift from 19 m.y. until about 13 m.y. ago. This mid-Miocene lull corresponds to a similar hiatus in volcanism between about 20 and 13 m.y. ago in the southern rift [4,6]. Volcanism began again in the central rift about 13 m.y. ago with emplacement of small basaltic tuff rings in the northern Espafiola basin. Earliest activity in the Jemez Mountains volcanic field may have begun 11--13 m.y. ago. An initial basalt-rhyolite sequence was followed by extensive eruptions of mainly andesite and rhyodacite. Volcanic activity culminated 1.4-1.1 m.y. ago with eruption of the widespread rhyolitic Bandelier Tuff and concomitant caldera formation. Eruption of minor ash-flow deposits and extrusion of silicic domes, controlled primarily by ring fractures, continued to 0.1 m.y. ago. Basaltic volcanism 10 m.y. ago in the northwestern Espafiola basin resulted in eruption of both olivine tholeiite and alkali olivine basalt. Flows and dikes

emplaced at this time were approximately contemporaneous with renewed tectonism. In the southern rift, Chapin and Seager [4] have summarized evidence that faulting and uplift 7 4 m.y. ago disrupted the early broad basins and formed the narrow, steep-sided present basins. From a fossil flora occurring in the lower to middle Tesuque Formation north of Santa Fe, Axelrod and Bailey [39] concluded that epeirogenic uplift in the Rio Grande rift area exceeded 1 km in the last 10-15 m.y. We agree that outcrops in which this flora occurs have been elevated > I km since deposition of these sediments, but suggest that this uplift represents breakup of the Tesuque-age basin and uplift on the flanks of the Sangre de Cristo Mountains rather than epeirogenic uplift. Except for tire voluminous Lobato Basalt, consisting at least in part of olivine tholeiite (Baldridge, unpublished analyses), volcanism was sparse and intermittent until after 5 m.y. ago (Fig. 3). Widespread eruption of basaltic lavas, mainly between 3 and 2 m.y. ago, occurred throughout the central (and northern) rift, and built the extensive volcanic fields of the Cerros del Rio and Taos Plateau and smaller volcanic fields of the central rift. These lavas consisted predominantly of olivine tholeiite, basaltic andesite, alkali olivine basalt, and hawaiite, but included lesser volumes of more silicic compositions (e.g. [9,30,34, 40]). Except for the presence of highly silica-undersaturated olivine nephelinite among the early (25-19 m.y. old) volcanic rocks, no systematic petrographic or chemical difference between the early mafic lavas and the uppe~ Miocene/Pleistocene basalts exists (Table 2). Continued tectonic activity resulted in formation of the Velarde graben between 5 and 3 m.y. ago [23]. Mafic volcanism in the central rift ceased almost completely by about 1 m.y. ago. The Rio Grande became a major through-flowing stream 3-4.5 m.y. ago [ 13,23], resulting in partial excavation of the basin-filling sediments. Our data on the elevations and ages of basalts from the Rio Grande Gorge, Mesa de Abiquiu, and Black Mesa (samples UAKA-72-65, UAKA-77-85, and UAKA-77-86, respectively) indicate that erosion in the Espa/Iola basin of the central rift has proceeded at an average rate of 3 5 - 4 0 m/m.y, over the last 4.5 m.y. and 54 m/m.y, over the last 3.2 m.y. Since the Rio Grande may not yet have existed 4.5 m.y. ago, the latter figure is probably a more accurate estimate of the present erosion rate.

320 In summary, these intervals of deformation and magmatism which culminated in the present-day Rio Grande rift correspond generally with a similar sequence of events in the Basin and Range province of Arizona. For example, Damon and coworkers [371 distinguish three stages of tectonism and magmatism in southeastern Arizona: Stage 1 was an episode of intermediate to silicic magmatism extending from about 38 to 24 m.y. ago (mid-Tertiary magmatic event). Stage 2 was a transition period extending from about 24 to 12 m.y. ago. Crustal extension characterized by curved fault planes and rotation of blocks began during stage 1 and extended to about 15 m.y. ago, resulting in the formation of broad basins. Stage 3 (Basin and Range disturbance) encompasses the last 12 m.y. and resulted in the narrow, steepside basins of the modern Basin and Range topography. The term "Rio Grande rift" should be restricted to the narrow, interconnected grabens which formed during this post-mid-Miocene stage of deformation. At present not enough data are available to indicate whether the styles of faulting of the two tectonic intervals in the central Rio Grande rift correspond to these of the Basin and Range province. However, the overall similarity of tectonic and magmatic events with those in the Basin and Range indicates that the rift is not a unique structure in the western U.S. The origin of the Rio Grande rift must be viewed in the larger context of the entire Basin and Range province.

Acknowledgements We thank R.E. Riecker and A.W. Laughlin of the Geosciences Division of Los Alamos Scientific Laboratory for their interest in and support of this work. We are grateful, too, to R. Ingersoll and P. Kautz (University of New Mexico) for valuable discussions and aid in field checking the Espinaso Formation. C.E. Stearns read an early version of this manuscript and made many helpful comments. Critical reviews by C. Chapin, S.J. May, R.E. Riecker, and two anonymous reviewers significantly improved this manuscript. The Division of Geothermal Energy of the U.S. Department of Energy (DOE) provided partial support for this work. Additional support was provided by the Office of Basic Energy Sciences of DOE, by National Science Foundation grant EAR78-11535, and by the State of Arizona.

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