Downloaded from gsabulletin.gsapubs.org on July 16, 2014 Geological Society of America Bulletin

Cenozoic normal faulting and the shallow structure of the rift near Socorro, New Mexico

CHERYL D. CAPE, SUSAN McGEARY and GEORGE A. THOMPSON

Geological Society of America Bulletin 1983;94, no. 1;3-14 doi: 10.1130/0016-7606(1983)94<3:CNFATS>2.0.CO;2

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Cenozoic normal faulting and the shallow structure of the Rio Grande rift near Socorro, New Mexico

CHERYL D. CAPE* SUSAN McGEARY \ Department of Geophysics, Stanford University, Stanford, California 94305 GEORGE A. THOMPSON I

ABSTRACT INTRODUCTION BACKGROUND GEOLOGY

Migrated versions of a deep seismic sec- Deep seismic-reflection profiles from the The Rio Grande rift (RGR) extends from tion across the Albuquerque basin in the COCORP (Consortium for Continental Re- central Colorado to northern Mexico, with southern Rio Grande rift (COCORP Abo flection Profiling) project span the Rio the northern part separating the cratonic Pass line 1 and Socorro line 1 A) are used to Grande rift between Albuquerque and So- Great Plains from the Colorado Plateau develop a detailed interpretation of the shal- corro, New Mexico. Study and interpreta- and the southern part merging into the low structure of the rift. Our interpretation, tion of the data have dramatically verified Basin and Range province (Chapin, 1971). which is consistent with nearby drill-hole the presence of a sill-like magma body pre- The rift trends north-northeast and is char- and gravity data, suggests that listric fault- viously inferred from microearthquake and acterized by a belt of en echelon basins and ing has been the dominant extensional style other geophysical data by Sanford and oth- fault-block ranges that individually trend of faulting in this part of the rift. During the ers (1977). The reflection profiles have also more nearly north-south. The Albuquerque early stages of rifting, blocks of Paleozoic yielded much information about the inter- basin, in the central portion of the rift, is the and Mesozoic prerift sedimentary strata nal structure of the rift itself, which is a southernmost discrete basin, and it is overlying crystalline basement complex graben of late Cenozoic age, and bounded by the Colorado Plateau to the were offset and rotated along listric normal have given insights into the structure of the west and the Great Plains to the east. Ap- faults rooted in the basement. Syntectonic crystalline crust below the graben (Oliver proximately 130 km of COCORP deep deposition of thick sections of mid-Tertiary and Kaufman, 1976; Brown and others, seismic lines were run transverse and paral- sedimentary and volcanic rocks filled the 1980; Jurdy and Brocher, 1980; Brocher, lel to the rift in the southern Albuquerque developing fault-bounded basins. These 1980, 1981a, 1981b). basin, 40 km north of Sorocco, New Mex- units were continuously rotated during on- This study focuses on the shallow struc- ico (Fig. 1) (Oliver and Kaufman, 1976). going listric faulting and subsequently bur- ture of the rift (upper 5 km) as determined The rocks of this area range in age from ied by later Tertiary and Quaternary basin from migrated COCORP seismic sections. Precambrian to Holocene (Fig. I) and have fill. Present surface fault traces may reflect Migrated sections have been used for our recently been described by Kelley (1977), continued movement on the deeper listric interpretation because migration is needed Condie and Budding (1979), and Chapin faults or a more recent minor episode of to place dipping seismic events in their and others (1978). Precambrian crystalline shallow normal faulting. In contrast to proper position and to collapse diffractions basement consists of a thick sequence of models of crustal extension proposed for that obscure the important signals. We pro- metasedimentary rocks (especially quart- nearby areas in the Rio Grande rift and pose an interpretation of the COCORP zite) and metavolcanic rocks intruded by Basin and Range province, no clear evi- lines that documents the style of normal granitic to gabbroic plutons. Overlying dence is seen for two time-distinct stages of faulting seen at shallow crustal levels across Paleozoic and Mesozoic strata include faulting characterized by early closely the rift and to test this interpretation we Mississippian to marine limestone, spaced faulting with large stratal tilts fol- have compared it to nearby drill-hole and sandstone, and shale; Permian evaporites lowed by wide-spaced high-angle faulting. gravity data. and terrestrial sandstones and mudstones; The large listric faults flatten into basement One of the major questions about this and continental deposits; or converge upon an apparent detachment and other rifts concerns what happens to and marine and nonmarine sed- surface at a depth of about 5 km, posing the normal faults at depth. Are the faults imentary rocks. The combined thickness for intriguing questions about the connection listric? That is, do they flatten and become the Paleozoic and Mesozoic rocks averages between shallow structure and deeper ex- subhorizontal at moderate depths of 5 or about 2.8 km (Kelley, 1977). tension within the crust. 10 km? Or do they maintain dips of 45° or Cenozoic deposits include the Eocene more into the crystalline crust? If the faults Baca Formation, a nonmarine sequence of are listric, how is extension accommodated conglomerate, sandstone, and mudstone, •Present address: Conoco, Inc., 290 Maple in the deep crust? Our structural analysis of and the Datil volcanics, a thick Court, Suite 284, Ventura, California 93003. the seismic data addresses these questions. sequence of pyroclastic rocks and interme-

Geological Society of America Bulletin, v. 94, p. 3-14, 9 figs., 2 tables, January 1983.

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Figure 1. Location map of the COCORP seismic lines and the geology of the southern Albuquerque basin from Jurdy and Brocher (1980) and Kelley (1977). MZF = Manzano fault, PF = Paloma fault, MF = Montosa fault, LPF = Los Pinos fault, EJF = East Joyita fault, WJF = West Joyita fault, HSF = Hubbell Springs fault, BF = Belen fault, PCF = Puerco fault, GF = Gabaldon fault, CYF = Coyote fault, CF = Comanche fault, JF = Jeter fault, CCF = Cerro Colorado fault.

diate to felsic breccias and flows. Overlying querque basin has revealed a variable dle to Holocene interval of rela- these units is the Miocene Popotosa Forma- thickness for the Santa Fe and younger tively slow extension, wide-spaced normal tion, which consists mainly of volcanic fan- basin fill, ranging from 0.6 to more than 6 faulting, and gently tilted blocks (Chamber- glomerates with interbedded volcanic flows km (Foster, 1978; C. E. Chapin, 1981, writ- lin, 1981). Shafiqullah and others (1980) and playa deposits. All of these formations ten commun.). have documented a similar tectonic evolu- vary laterally in terms of both facies and Rifting to the south in the Socorro and tion for southern Arizona, as have Zoback thickness, with reported thicknesses ranging Las Cruces areas has been estimated by and others (1981) for the Basin and Range from several hundred to several thousand Chapin and Seager (1975) to have started province. The present topography of tilted metres (Spradlin, 1976; Chapin and others, approximately 25 to 29 m.y. B.P., while fault-block intrabasin horsts, including the 1978; Machette, 1978). Woodward (1977) prefers an age of 26 m.y. Ladron Mountains (Fig. 1) and the Socor- The extensive cover of younger basin fill B.P. for the northern Albuquerque basin. ro-Lemitar, Chupadera, Magdalena, and seen in Figure 1 contains both the Miocene- These estimates are based on the initiation Bear uplifts located farther to the south and Pleistocene , consisting of of extensional faulting, bolson sedimenta- west, was formed during the second stage of conglomerate, sandstone, mudstone, and tion, and major basaltic andesite volcanism. rifting (Chapin, 1979). interbedded volcanic rocks, and Quaternary The rift is thought to be superimposed upon Volcanism occurred concurrently with alluvial sand and gravels. Although to the a major tectonic belt that was previously the rifting, with the first phase of basaltic south the Popotosa Formation has been deformed during the Paleozoic (ancestral andesite volcanism ceasing about 20 m.y. described as the basal member of the Santa Rocky Mountains) and again during the B.P. A new phase of bimodal basalt- Fe Group (Bruning and Chapin, 1974; Laramide orogeny (Chapin and Seager, rhyolite volcanism began about 14 m.y. Machette, 1978), its extent within the Albu- 1975). Rifting in the Socorro area is thought B.P. (Chapin and Seager, 1975) and has querque basin is unknown; therefore, in to have occurred in two styles: (1) a late continued to the present, accelerating about Figure 1 it has been included in the category Oligocene-early Miocene period of rapid 5 m.y. B.P. Thick sections of bolson sedi- of pre-Santa Fe Tertiary volcanic and sed- extension, close-spaced normal faulting, ments were deposited during the rifting, imentary rocks. Drilling in the Albu- and strong rotation of beds; and (2) a mid- including the Popotosa Formation between Downloaded from gsabulletin.gsapubs.org on July 16, 2014 I 1 RIO GRANDE RIFT, NEW MEXICO

26 and 7 m.y. B.P. (Chapin and others, vibrators operating simultaneously with a and was determined using first arrivals of 1978; Chamberlin, 1981) and the more 10-32 Hz sweep. A sweep length of 20 sec the COCORP data. recent upper Santa Fe Group. Extension and a total recording time of 40 sec were Figures 3a, 4a, and 5 show Socorro line still continues today as shown by recent used for Abo Pass lines 1 and 2, along with 1A in both two-way traveltime and depth. fault scarps, high heat flow, modern eleva- a station spacing of 100 m. The Socorro An approximate conversion to kilometres tion changes, and geophysical evidence of surveys (lines 1A and 2A) used a 134-m sta- of depth for the time sections can be done modern magma bodies and anomalously tion interval and a sweep length and record- by multiplying the time by 1.5 to 2, thus thin crust beneath the rift (Chapin, 1979; ing time of 25 sec and 50 sec, respectively. corresponding to velocities of 3 to 4 km/sec. Cordell, 1978). The recorded data were processed using This means that there is a vertical exaggera- One of the questions we address in this standard industry programs as described in tion of 1.2 to 1.7 for the first few seconds. report is the nature of normal faulting at Brown and others (1980). Additional proc- Abo Pass line 1 is shown in Figure 6a and in depth as seen on the COCORP seismic essing, including migration and depth con- this case is output in kilometres of depth lines. Three general models for extensional version, was done on Socorro line 1A by with no vertical exaggeration. Surface faults normal faulting have been proposed, mainly Digicon, Inc., for demonstration purposes. as shown by Kelley (1977) (Fig. 1) have been on the basis of work in the Basin and Range Due to the much better quality of the Dig- labeled on the seismic sections where they province (Stewart, 1980). These models are icon sections, especially in the shallow could be traced into the subsurface. In some illustrated in Figure 2: horst and graben structure, we used the Digicon migrated line instances, surface traces of the faults do not block faulting, tilted fault-block, and listric 1A for our final interpretation. The mi- correspond exactly to the apparent loca- fault. No general agreement exists as to grated version of Abo Pass line 1 was pro- tions as seen on the seismic sections. The which style is the most prevalent, and duced at Stanford using a 45° finite-dif- following discussion of the two lines high- examples of all three styles have been pre- ference wave equation program developed lights our proposed interpretation of the sented in the Basin and Range province and by the Stanford Exploration Project (Lynn, seismic data. Rio Grande rift, ranging in scale from 1979). This program is capable of allowing hundreds of metres to individual mountain for laterally varying velocities, as well as the Socorro Line 1A ranges. For example, Kelley (1977) prefers a normal velocity variation with depth. The predominantly high-angle horst and graben velocity function used to migrate the data The most prominent shallow feature of structure for the Albuquerque basin, and was taken from Jurdy and Brocher (1980) Socorro line 1A (Figs. 3a and 4a) is a series Brown and others (1980) show high-angle of reflectors which continue across the faults in their interpretation of the RGR Albuquerque basin from VP (vibrating COCORP lines. The tilted fault-block point) 420 to VP 160 at depths ranging from model of Morton and Black (1974) was used 1.5 to 5 km (events A1-A4, Fig. 3b; A5-A6, by Chamberlin (1978) to explain structures Fig. 4b). These reflectors define the western seen within the Lemitar Mountains to the boundary of the rift at VP 420 and a major south of the seismic lines, and a listric fault intragraben horst centered under VP 240, model was presented by Woodward (1977) believed to be the subsurface extension of for rifting near Albuquerque. Our interpre- the Ladron Mountains located a few kilo- tation of the migrated COCORP lines sug- metres to the south (Fig. 1). Several alter- gests that a listric fault model best explains nate interpretations have been presented as the shallow structure of the Rio Grande rift. to what these reflectors represent, including (1) a Cenozoic rift volcanic unit, perhaps SEISMIC INTERPRETATION correlative with the La Jara Peak Basaltic Andesite of the Datil volcanics; or (2) an An approximate east-west seismic cross Upper Cretaceous sedimentary layer; or section of the RGR within the southern possibly (3) the top of the Precambrian Albuquerque basin is obtained by combin- basement, or a basal Paleozoic horizon ing COCORP Abo Pass line 1 and Socorro (Brown and others, 1980). We believe that line 1A (Fig. 1). Line 1 starts within the these reflectors represent both tectonic and Precambrian and Paleozoic rocks of the depositional contacts, and we attribute sig- Manzano and Los Pinos Mountains that nificance to the large changes in amplitude define the eastern boundary of the basin of the reflector across the basin (for exam- and then crosses over the shallowly covered ple, A5 versus A6, Fig. 4b). Our interpreta- Paleozoic to Cenozoic rocks of the Hubbell- tion is that the high-amplitude events AI, Joyita bench. Line 1A extends west-north- A2, and A5 represent major listric faults or décollement zones of listric faulting, and west from the Hubbell-Joyita bench into the Figure 2. Illustration showing three gen- that the much weaker events A3 and A6 deeper rift sediments of the main basin, eral models of normal faulting: (a) horst represent depositional contacts between the eventually crossing the western border into and graben block faulting, (b) tilted fault- Precambrian basement and the overlying the westward-dipping Paleozoic rocks of block, (c) listric faulting. Note upward de- Paleozoic and Mesozoic sediments. The fol- the Lucero uplift. The 24-fold VIBROSEIS crease in dip of section in the tilted lowing discussion details the upper struc- (Trademark Conoco, Inc.) data were shot fault-block and listric models due to deposi- ture of the rift from west to east as by Petty Ray Geophysical Co. using five tion during faulting. Downloaded from gsabulletin.gsapubs.org on July 16, 2014

IO CAPE AND OTHERS 0 3 km 1 • • • w

VP 400 350 300 250 _! I I 0.0

4.0

Figure 3a. Western half of Oigicon migrated Socorro line 1 A. Time is in two-way traveltime.

determined using the western (Fig. 3a) and j3 km eastern (Fig. 4a) halves of line IA and the W E hypothesized identity for reflector A as given above. —0.0 As described by Brown and others (1980), the western boundary of the basin was TIME found to correlate with the edge of the (SEC ) Lucero uplift at VP 420, and not the Coyote — 2.0 fault to the east as proposed by Kelley (1977). On the seismic section, the rift boundary is shown by the strong east- dipping reflector A1 (Fig. 3b), which has an apparent dip of 40°. As are all the dips pre- 4.0 sented in this discussion, this angle is only approximate as it was determined from the unmigrated depth section. Brown and oth- Figure 3b. Line drawing of Figure 3a showing labeled events discussed in text. ers (1980) give several alternate interpreta- tions for this reflector, including: (1) that it A3), forming the west flank of the subsur- The top of the inferred Paleozoic-Meso- represents downwarped prerift strata, (2) face Ladron horst. This reflector has a zoic section appears to be truncated by an that it is a low-angle listric normal fault, much weaker amplitude than Al and A2 and overlain by many high- and (3) that the reflector results from a ser- and is therefore interpreted as representing amplitude subhorizontal reflectors such as ies of closely spaced, high-angle normal the contact between the Paleozoic section those seen at C (Fig. 3b). We interpret these faults. The amplitude and continuity of this and the underlying Precambrian basement. events as representing Cenozoic sedimen- reflector and the presence of a set of weak A set of very faint reflectors overlying A3 tary basin fill, correlative with the Santa Fe westward-dipping reflectors at B support a can be seen dipping westward approxi- Group, which was deposited after the fault- listric fault interpretation. We propose that mately 30° on the west side of the horst. ing and rotation of the underlying older these B reflectors represent the Paleozoic- These reflectors appear to be the continua- beds. This basin fill deepens to about 2 km Mesozoic sedimentary section (similar to tion of the Paleozoic-Mesozoic section seen 1.5 sec) above another 2 km or more of that seen to the west in the Lucero uplift) at B, which fits in well with surface observa- Paleozoic-Mesozoic section, which com- that has been downdropped and rotated tions of Paleozoic sediments dipping west- pares well to the thicknesses described in along a listric normal fault (reflector Al). ward 25° to 40° on the west flank of the the background geology section. The inter- The reflectors at B appear to be cut by sev- Ladron Mountains (Kelley, 1977). The preted Santa Fe reflectors are very discon- eral high-angle normal faults, all of which reflector D, against which these dipping tinuous, due mainly to the poor data flatten and converge upon reflector A2. reflectors seem to truncate, perhaps repre- quality, but it is possible that some of the Reflector A2 is continuous to approxi- sents another listric fault plane that has discontinuity can be attributed to recent mately VP 300, but at this point the A downdropped and rotated the prerift normal faulting. horizon begins to shallow eastward (event section. Farther to the east, reflector A4 defines Downloaded from gsabulletin.gsapubs.org on July 16, 2014 RIO GRANDE RIFT, NEW MEXICO I 1

3 km GABALDON PUERCO FAULT FAULT VP 200 150 100 50 1 I 1 i 1 „ J 1 , , 1 • i 1 i i i i

TIME (SEC )

2.0

— 4.0

Figure 4a. Eastern half of Digicon migrated Socorro line 1 A. Time is in two-way traveltime.

the top of the Ladron horst under VP 240 and then starts to deepen near VP 220, turn- VP 150 ing into reflector A5 (Fig. 4). Unfortu- .0.0 nately, due to the high amplitude of these events, an overlying shadow zone of no data TIME ^^ E • s was created during automatic gain process- (SEC ) F A6 ^ ' ing. This zone has considerably reduced the — 2.0 data quality, especially above the horst, and A5 N _ ' it is not clear what happens to the Paleo- zoic-Mesozoic reflectors. However, on the eastern side of the horst, the dipping reflec- — 4.0 tors located at F and the more horizontal beds at E truncate against event A5. We think this high-amplitude arcuate event Figure 4b. Line drawing of Figure 4a showing labeled events discussed in text. represents another major listric fault, with

0 2 km

VP 0.0

DEPTH (KM)

6.0

Figure 5. Digicon unmigrated Socorro line 1A depth section showing decrease in dip upward in thick unit of dipping reflectors. Downloaded from gsabulletin.gsapubs.org on July 16, 2014 IO CAPE AND OTHERS

2 km EAST JOYITA LOS PINOS MONTOSA w FAULT FAULT THRUST I 200 LL

Figure 6a. Stanford migrated Abo Pass line 1 in kilometres of depth.

0 2 km the unmigrated depth section (Fig. 5). This W change in dip is taken as an indication of VP 250 0.0 deposition during listric faulting. Therefore, these upper reflectors might be correlative with the synrift Popotosa Formation and upper Datil volcanics, overlying the prerift -4.0 lower Datil and Baca Formations. The jux- DEPTH taposition of volcanic and sedimentary (KM ) rocks within these formations is a possible explanation for the observed high ampli- -8 0 tudes. The subhorizontal reflectors proposed earlier as representing Santa Fe basin fill 12.0 are present above the thick set of dipping Figure 6b. Line drawing of Figure 6a showing labeled events discussed in text. reflectors at F and are also seen to form the fill of a complex graben located at E accompanying westward tilting and rota- thickness (>4 km), dip angles (10° to 15°), (Fig. 4b). This graben, bounded by the tion of the eastern beds. This fault has sev- and the decrease in dip as one goes upward listric fault A5 and the higher-angle Gabal- eral kilometres of dip-slip displacement and in the section, we believe these reflectors don and Puerco faults, has an approximate possibly can be related to the surface faults, represent a unit of prerift and synrift depth of 2.5 km (1.8 sec), well within the such as the Jeter and Cerro Colorado faults, Cenozoic sediments and volcanics that was limits of Santa Fe fill presented above. that form the north and eastern sides of the deposited upon the Cretaceous-Paleozoic Although it is impossible to determine the Ladron Mountains (Fig. 1). The reflector sedimentary section, both units being exact stratigraphic relations from the seis- A5 flattens out starting at VP 190 and even faulted and rotated by the listric fault (event mic data alone, our interpretation that the appears to be rising slightly to the east. This A5) to the west. The lower boundary of this upper layer of unit F is most likely com- rise can be explained by velocity pull-up thick set of reflectors has been dashed in as posed of Popotosa Formation and that the that was not entirely removed during migra- event A6 and again shows the amplitude Santa Fe Group, along with some down- tion. The velocity anomaly is due to the contrast between what we have interpreted dropped Popotosa section, forms the gra- older, higher-velocity dipping section (loca- as the Paleozoic-Precambrian contact and ben fill at E is consistent with the following tion F) which replaces the younger, lower- the fault plane to the west. Note that the surface observations: (1) extensive outcrops velocity subhorizontal fill present to the overlying interpreted Paleozoic-Mesozoic of the Popotosa are seen on the east side of west of the Puerco fault (location E). reflectors have a higher amplitude than the Ladron Mountains (Fig. 1) dipping 20° The high-amplitude set of westward- those seen to the west of the horst, possibly to 60° to the west and faulted against the dipping reflectors seen at F was previously due to the thinner cover of basin fill. Precambrian (Kelley, 1977), and (2) while interpreted as representing alluvial fan de- The decrease in dip within the upper third the Popotosa and the underlying volcanics posits and/or tilted older basin fill (Brown of the set of reflectors can be seen in the tend to have conformable dips (Chapin and and others, 1980). However, due to their migrated time section but is most evident in Seager, 1975), there is an angular uncon- Downloaded from gsabulletin.gsapubs.org on July 16, 2014 I 1 RIO GRANDE RIFT, NEW MEXICO

formity present between the Popotosa and large normal faults. The most prominent including that it represents a major bound- the younger Santa Fe beds, which generally fault in the section, which again shows a ing fault or possible zone of intrusion. It is dip less than 10° (Brüning and Chapin, listric style of deformation, corresponds to also possible that this zone is due to disrup- 1974). the East Joyita fault and can be followed as tion within the Precambrian basement re- reflector G (Fig. 6b). The downward curva- lated to the Laramide thrusting. This idea is Abo Pass Line 1 ture of the rotated reflectors to the east of reinforced by the interpretation that the dis- this fault is perhaps an indication of reverse continuous reflector J represents the sub- The migrated Abo Pass line 1 shown in drag, the presence of which also argues for a surface expression of the Montosa or Figure 6a crosses the shallow Hubbell- listric style of faulting (Hamblin, 1965). Paloma thrust (Brown and others, 1980). Joyita bench, which is known to exist from There are several other major faults, includ- surface outcrops of pre-Santa Fe section ing H and I (Fig. 6b), that can be inferred WELL AND GRAVITY DATA (Kelley, 1977) (Fig. 1). A set of high- from the seismic section. However, unlike amplitude faulted reflectors ~2 km thick Socorro line 1 A, these normal faults tend to In order to test the validity of our seismic can be seen in the upper part of the seismic have fairly high angles and, except for the interpretation of the subsurface structure, section. These reflectors are thought to East Joyita fault, are not seen as distinct we have compared this interpretation with represent Paleozoic-Mesozoic sedimentary high-amplitude reflectors. Also, unlike the two independent data sources—well and rocks because of their thickness and from previous line, there are no clear reflectors gravity data. We will first look at the results correlation of similar reflectors seen on that can be correlated with the later Ceno- from four wells drilled in the southern Albu- Socorro line 2A with outcrops to the south zoic basin fill sediments. querque basin, all within 25 km of the seis- (Brown and others, 1980). The Precambrian The section crosses the eastern boundary mic lines (Table 1; Fig. 1). The information basement contact cannot be clearly distin- of the rift at the Los Pinos fault located at from these wells is compared to the strati- guished beneath the Paleozoic section, the base of the Los Pinos Mountains (Fig. graphy as interpreted from the seismic sec- which is comparable to the weak deposi- I). This fault is interpreted from the seismic tions. Then we will look at the results of tional reflector interpreted from Socorro section as a high-angle, west-dipping nor- gravity modeling across the rift, using the line 1A. However, it is curious that the mal fault located approximately at VP 188 gravity map of Cordell and others (1978) for Paleozoic-Mesozoic section is composed of (Fig. 6a). A little farther to the east, the the observed anomaly profile and our seis- much higher-amplitude reflectors than Laramide Montosa and Paloma thrusts are mic interpretation as the basis for the grav- those seen in line 1 A. One possible explana- seen within the Precambrian and Paleozoic ity model. Both the well information and tion for this amplitude difference can be rocks of the Los Pinos and Manzano gravity information are found to be consis- found after looking at the overlap area of Mountains. On the seismic section, these tent with our interpretation. the two lines (that is, line 1, VPs 1-80; line thrusts are located above a major west- The first two wells looked at, the Humble I A, VPs 1-60). This comparison shows that dipping shadow zone (location K) that 1 Santa Fe and the Shell 2 Santa Fe, have line IA contains much less distinct reflec- separates the faulted reflectors of the been correlated with the basin to the east of tors, indicating that the different field Hubbell-Joyita bench and the underlying the Ladron horst because they are located parameters of line 1 have caused a higher basement reflector J from a thick set of deep east of the Gabaldon fault (Fig. I). Both seismic resolution. high-amplitude basement reflectors located wells bottomed in Mesozoic rocks, and The Paleozoic-Mesozoic section has been at L (Fig. 6b). Brown and others (1980) about 2.8 km of Paleozoic and Mesozoic extensively disrupted and downdropped on offer several explanations for this zone, rocks were estimated above Precambrian

TABLE 1. WELLS IN THE SOUTHERN ALBUQUERQUE BASIN

Well/location/county Formation Velocity* (km/sec) Depth, km (sec)f Thickness, km (sec)

Central New Mexico 1, Santa Fe 2.1 surface 0.64 (0.61) Livingston; Cretaceous? 3.7 0.64 (0.61) I6-3N-1E; Socorro

Grober 1, Fuqua; Qal/Santa Fe 2.1 surface 1.387(1.32) 19-5N-3E; Tertiary, Baca? 2.8 1.387(1.32) Valencia

Humble 1, Santa Fe; Santa Fe 2.1 surface 1.873(1.78) I8-6N-1W; Baca 2.8 1.873(1.78) 1.146(0.82) Valencia Cretaceous-Paleozoic 3.7-4.5 3.019(2.60) 2.864(1.40) Precambrian (est.) 6.1 5.883(4.00)

Shell 2, Santa Fe; Santa Fe 2.1 surface 2.371(2.26) 29-6N-IW; Baca 2.8 2.371(2.26) 0.984(0.70) Valencia Cretaceous-Paleozoic 3.7-4.5 3.356(2.96) 2.801(1.37) Precambrian (est.) 6.1 6.157(4.33)

Note: data from Foster (1978). •Formation velocities as derived from refracted waves by Jurdy and Brocher (1980). fTwo-way time in seconds for depth and thickness determined using given velocities. Downloaded from gsabulletin.gsapubs.org on July 16, 2014 IO CAPE AND OTHERS

basement (Table I) (Foster, 1978). Above of the subsurface thicknesses of the pre- lines of intersection parallel to the northeast the Mesozoic section, the wells penetrated Santa Fe section would have a much trends of the surface geology. Since we were ~ I km of Baca Formation and then about thinner unit of Paleozoic and Mesozoic concerned only with detail within the rift, it 1.9 km and 2.4 km of Santa Fe sediments, rocks and a corresponding increase in was decided to apply a correction of -0.046 respectively. On the seismic depth section thickness of the Cenozoic volcanics and mgal/km (west to east) to the profile in (Fig. 5), we have interpreted ~4 km of related sediments. order to arbitrarily force the reference grav- downdropped pre-Santa Fe section (loca- The Grober 1 Fuqua well and the Central ity values to be equal over the uplifts bound- tion F in Fig. 4b), and from 1 to 2.5 km of New Mexico 1 well (Fig. 1) do not offer ing the rift. Even though errors were Santa Fe basin fill. These thicknesses com- much useful information about the pre- introduced in determining the anomaly pro- pare well to those described in the Santa Fe section, because the identity of the file due to the large scale and large contour drill holes. units beneath the basin fill has not been interval (5 mgal) of the Cordell map, we feel We have interpreted the upper portion of accurately determined. However, these two confident that, within the scale of our the reflectors at F (Fig. 4b) as representing wells do give depths for the Santa Fe fill of model, we have an adequate estimate of the the Baca Formation plus the Popotosa about 1.4 km and 0.6 km, respectively. A gravity anomaly across the rift. We also Formation and Datil volcanics because comparison of these depths with Socorro looked at a profile determined from a these younger sediments and volcanics are line 1A VPs 1-160 (Fig. 4a) shows that they detailed gravity study (1 mgal contour exposed near the seismic lines (Fig. 1). It is agree fairly well with our interpretation of a interval) over the eastern half of the basin possible that these units were not differen- thin cover of basin sediments overlying the (Sanford, 1978), and there were only slight tiated in the wells but were grouped to- downdropped Paleozoic-Mesozoic section differences in the general shape of the gether with the Baca Formation and/or the and Tertiary volcanics. anomaly between the two profiles. Santa Fe fill. Alternatively, as the Socorro For a second test of our interpretation, Our preferred model for the density dis- area is on the northeastern periphery of the two-dimensional gravity modeling was done tribution within the rift is shown in Figure Datil-Mogollon volcanic field (Weber, along the seismic lines using the complete 7, along with its computed profile and the 1963), the volcanics and related sedimentary Bouguer anomaly map of the RGR by Cor- observed profile. This model is based on the rocks may not have been deposited near the dell and others (1978) for the observed subsurface interpretation described in the well sites. Also, C. E. Chapin (1981, written anomaly profile. This profile was obtained previous section, with slight modifications commun.) cautioned that a Laramide uplift by plotting Abo Pass line 1 and Socorro line to keep the unit thicknesses and distribu- beneath the eastern half of this part of the IA on the gravity map and reading off the tions consistent within the model. Even RGR resulted in irregular erosion of the gravity values along the lines. The junction though the thickness of the Paleozoic- Mesozoic and Paleozoic rocks. It appears, between the two lines was taken at line I VP Mesozoic section might be highly variable therefore, that a more exact interpretation 10 and line 1A VP 58 in order to make the due to Laramide erosion, for simplicity we

--I80

--I90 MGAL --200 j ¡~i n •? t n -210 Calculated from model

SOCORRO LINE l"A ABO PASS LINE I

DEPTH

(KM)

2 X VERTICAL EXAGGERATION

Figure 7. Two-dimensional gravity model of southern Albuquerque basin along COCORP seismic lines. Layers of density 2.3 to 2.56 g/cc approximate the Cenozoic section, while those of 2.6 g/cc correspond to the Paleozoic-Mesozoic section. Observed gravity anomaly taken from Cordell and others (1978). Downloaded from gsabulletin.gsapubs.org on July 16, 2014 RIO GRANDE RIFT, NEW MEXICO I 1 have taken the thickness to be fairly con- presence of the high-density volcanic and depths of fill, and approximate contacts stant across the rift. The density contrast volcaniclastic rocks of the Datil volcanics between units have been taken directly from used between the Paleozoic-Mesozoic sec- and Popotosa Formation within the subsur- the interpreted seismic sections. Therefore, tion and pre-Santa Fe Cenozoic volcanics face Cenozoic section. It is interesting to the structure as drawn reflects the apparent and sediments is so slight (0.04 g/cc) that note that the density of 2.56 g/cc used for dips along the strike of the seismic lines. considerable variations in the thicknesses of this unit is the mean density determined by Some of the faults and contacts may be the two units will still fit the gravity model Ramberg and others (1978) for several sam- steeper than what is pictured, particularly in reasonably well. The depths for the different ples of Datil volcanics. the western half of the section where the line units were taken directly from the Abo Pass The 2.3 g/cc used for the flat-lying Santa trends obliquely to the regional structure. line I migrated depth section, and for Fe basin fill is comparable to values used in The stratigraphy of the faulted sedimentary Socorro line IA depths were taken from the previous studies, whereas the value of 2.52 units is inferred from the seismic sections unmigrated depth section after estimating g/cc needed for the graben immediately to and based on known regional thicknesses the necessary migration corrections. The the east of the Ladron horst is very large and nearby drill-hole data. Although exact densities in the model were determined by (Fig. 7). However, this high density most stratigraphic contacts cannot be determined combining previously published density in- likely indicates the presence of down- from the seismic sections, we think that this formation from the RGR with the density dropped Popotosa Formation and Cenoz- picture of the rock units beneath the basin values required when using the depth con- oic volcanic rocks within the graben. Also, fill is consistent with all available informa- straints given by the seismic data. The den- the high density might be reflecting the tion. This interpretation agrees with that of sity distribution is as follows: Precambrian presence of slivers of pre-Cenozoic high Jurdy and Brocher (1980), who deduced the crystalline basement, 2.67 g/cc; Paleozoic density rocks within the complex zone of presence of the Paleozoic-Mesozoic section and Mesozoic sedimentary section, 2.60 faulting above and on the eastern side of the above Precambrian basement from refrac- g/cc; Tertiary prerift and early rift sedi- horst. Patches of these prebasin rocks are tion velocities calculated from the CO- ments and volcanics, 2.56 g/cc; and Tertiary seen faulted against the Precambrian on the CO RP data. However, we do recognize that and Quaternary basin fill, 2.3 and 2.52 g/cc. east side of the Ladron Mountains (Kelley, due to Laramide erosion, the Paleozoic- Mesozoic section could be substantially When these density values are compared 1977). Therefore, even though high densities thinner than shown, especially in the eastern to densities from well data and previous are required for our model, we think that half of the basin, with a corresponding studies in the RGR (Table 2), our values, they are reasonable and that the gravity increase in thickness of the pre-Santa Fe especially for the Cenozoic section, tend to model is consistent with our interpretation. Cenozoic volcanics and sediments. be substantially higher. As shown in Table It is very important to point out that pre- vious studies that have used gravity model- 2, the density used for the Paleozoic- Our interpretation of migrated versions ing for structural interpretation did not Mesozoic section is comparable to densities of the COCORP lines suggests the impor- have detailed depth constraints and there- used in previous studies but is high when tance of major listric faulting in the forma- fore have a non-uniqueness problem be- considering the low values reported for the tion of this part of the Rio Grande rift. We tween the density values and model depths. Mesozoic section. However, Ramberg and feel that we have recognized listric faulting others (1978) describe average densities in the subsurface by the seismic expression from well and sample data of 2.61 and 2.64 DISCUSSION of the East Joyita fault on Abo Pass line I g/cc for Paleozoic rocks, suggesting that the (Fig. 6a). This in turn reinforces our inter- value we have used may be fairly accurate. Our generalized geologic model of the pretation of deeper listric faults farther to The pre-Santa Fe Cenozoic density values shallow structure of the Rio Grande rift is the west. Although a completely unambigu- are also high, but we think this indicates the shown in Figure 8. The locations of faults, ous interpretation cannot be made of this

TABLE 2. COMPARISON OF DENSITY VALUES IN g cc

Joesting Mattick Sanford Decker Shell* This and others (1967) (1968) and others Santa F'e study (1961) (1975) Pacific 1

2.15 2.3 Santa Fe 2.2 2.2 (2.52) 2.3 2.17 2.19 c pre- ci Santa Fe 2.4 2.56

2.36 Mesozoic 2.4 2.36 2.45 2.55 2.6 Paleozoic 2.56 2.57 2.6

Precambrian 2.7 2.67 2.67 2.67 2.67 2.67

»Taken from Brocher (1980) Downloaded from gsabulletin.gsapubs.org on July 16, 2014 IO CAPE AND OTHERS

LUCERO LADRON HUBBELL-JOYITA MANZANO-LOS PINOS UPLIFT HORST BENCH MTS

W

DEPTH

(KM)

| | QUATERNARY-TERTIARY BASIN FILL f~] MESOZOIC-PALEOZOIC SEDIMENTARY ROCKS

IJ77I TERTIARY VOLCANIC AND SEDIMENTARY ROCKS [7^1 PRECAMBRIAN CRYSTALLINE ROCKS

Figure 8. Generalized geologic cross section of the southern Albuquerque basin from interpretation of the COCORP seismic lines. In the eastern half of the basin, the thickness of the Paleozoic-Mesozoic section could be considerably thinner than shown due to prerift Laramide erosion, with a corresponding increase in the thickness of the Tertiary volcanics and sediments. This figure is drawn at a 2:1 vertical exaggeration to match the gravity model scale (Fig. 7). Attitudes of fault planes and bedding in the sedimentary rocks would be flattened proportionately in a true-scale section.

data, wc think that a model of listric fault- offset the deeper faults within the limits of shallower sections of prerift and synrift ing is consistent with the known surface seismic resolution. Perhaps some evidence strata. We have not found clear evidence in geology, drill-hole data, and gravity data. It for this type of faulting is given by the dis- the seismic sections for an early stage of is interesting to speculate that we might also continuity of the interpreted Santa Fe faulting characterized by large stratal tilts be seeing some sort of detachment faulting reflectors (location C in Fig. 3b). (2) Some and close-spaced faults followed by high- in the west side of the rift, perhaps similar to of the high-angle faults at the surface may angle wide-spaced faulting. Either the data that observed by Davis and others ( 1980) in merge downward into the pre-existing are not clear enough to distinguish between the Whipple Mountains of southeastern major listric faults, indicating recent move- these two stages, or extension in this area California. The presence of offset blocks of ment on these faults. The coincidence of our cannot be separated into two time-distinct reflcctors (location B in Fig. 3b) over a sub- interpreted subsurface faults with those seen styles of faulting. horizontal high-amplitude reflector suggests at the surface can be seen by a comparison The listric faults seen on the seismic sec- that the faults which have downdropped of Figure 8 with Figure I. For example, the tions can account for observed extension to these sediments may flatten into a detach- fault interpreted under Socorro line IA VP a depth of - 5 km, at which the faults flatten ment or décollement surface. However, it is 100 (Fig. 8) could correspond to the Belen into the crystalline basement. No faults, list- not at all clear from the seismic section fault (Fig. 1). and the fault below VP 330 ric or otherwise, were recognized below this whether these faults curve into the detach- could be the subsurface trace of the Coyote depth. Either deeper faults cannot be recog- ment fault or, alternatively, remain nearly fault. nized on these seismic sections, or the planar in a tilted fault-block style of faulting Most of the extension across this part of basement has already begun to extend by until they flatten abruptly into, or are trun- the rift thus seems to have been accom- some other proccss at a depth of about 5 to cated by, the detachment surface. plished by listric faulting. A geologic history 6 km. The relationship between some of the reconstructed from the cross section in- The connection between known surface known surface faults seen offsetting the cludes four main features. (1) During the faulting and deeper extension is an in- Santa Fc section in the Albuquerque basin early stages of rifting, blocks of prerift triguing problem, highlighted by the occur- (Fig. I) and our model of the subsurface strata bounded by listric normal faults rence of earthquakes at hypocentral depths structure is not entirely clear due to the began to be rotated. (2) Syntectonic deposi- as great as 10 to 15 km (Woodward, 1977) poor data quality. However, the fact that tion of mid-Tertiary sediments and volcan- and by basalt eruptions that suggest fractur- the set of A reflectors (Figs. 3b and 4b) ics filled the developing fault-bounded ba- ing to mantle depths. Among the possibili- remains continuous across the rift below sins. These units were continuously rotated ties that need to be tested in future research known surface faults limits these faults to during the ongoing listric faulting. (3) The are whether (1) a few unseen master faults two possibilities. ( 1 ) The surface faults may prerift and synrift deposits were buried by that extend to great depth may underlie correspond to a late event of small-scale, later Tertiary and Quaternary basin fill. (4) zones of shallow listric faults; (2) "ductile" high-angle faulting that is a minor feature Young high-angle normal faults and or or "plastic" extension and thinning of the within the rift. These faults are confined to continuing movement along the pre-existing crust becomes dominant below the listric the shallow section and do not appear to listric faults cut this later basin fill and the faults; (3) decoupling along subhorizontal Downloaded from gsabulletin.gsapubs.org on July 16, 2014 I 1 RIO GRANDE RIFT, NEW MEXICO

data processing programs. C. E. Chapin and W. R. Seager provided very thoughtful reviews of the manuscript. We again extend appreciation to C. E. Chapin and his col- leagues at the New Mexico Bureau of Mines and Mineral Resources for the enlightening field trip held in March 1980. This work was supported by National Science Foundation Grant EAR 78-22762.

REFERENCES CITED

Brocher, T. M., 1980, Reflectivity analysis of multichannel seismic reflection data with application to the COCORP Rio Grande rift survey [Ph.D. thesis]: Princeton, New Jer- sey, Princeton University, 168 p. 1981a, Shallow velocity structure of the Rio Grande rift: A reinterpretation: Journal of detachment faults may step deep exten- from the middle and lower crust. A model Geophysical Research, v. 86, p. 4960-4970. sion laterally away from shallow extension; linking deeper crustal extension with the 1981b, Geometry and physical properties (4) similar decoupling may occur in mag- shallow structure is discussed below. of the Socorro, New Mexico, magma bodies: matic intrusions; and (5) the deeper crust Journal of Geophysical Research, v. 86, p. 9420-9432. may be progressively distended by dikes and Speculative Model Brown, L. D., and others, 1980, Deep structure of other intrusions in a manner analogous to the Rio Grande rift from seismic reflection processes at spreading ocean ridges. Seismic At the end of the discussion section, we profiling: Journal of Geophysical Research, investigations, especially earthquake stud- proposed five possible connections between v. 85, p. 4773-4800. ies, are needed to help resolve the important deep extension and the shallow structure as Brüning, J. E„ and Chapin, C. E„ 1974, The Popotosa Formation—A Miocene record of question of deep extension. seen on the seismic-reflection profiles. Basin and Range deformation, Socorro Among these five, we tentatively regard it County, New Mexico: Geological Society of SUMMARY AND SPECULATIONS most likely that unseen master faults under- America Abstracts with Programs, v. 6, lie the zones of shallow listric faulting (Fig. p. 430. Chamberlin, R. M., 1978, Structural develop- 1. Our interpretation of migrated CO- 9). We think that ductile deformation is not ment of the Lemitar Mountains, an intrarift CORP seismic lines over the Rio Grande likely at a depth of only about 5 km, and, tilted fault-block uplift, central New Mexico rift suggests that listric faulting, where the moreover, that interpretation is inconsistent [abs.], in Proceedings, International Sym- fault plane decreases in dip with depth, has with the occurrence of earthquakes at mid- posium on the Rio Grande Rift, Santa Fe, been the most dominant style of faulting. crustal depths. Our model suggests that the New Mexico, October 1978: Los Alamos Scientific Laboratory, New Mexico, On the western side of the rift, we also collapse of high topography toward a p. 22-24. deeper trough, landslide fashion, along list- appear to see evidence for a detachment sur- 1981, Cenozoic stratigraphy and structure of face into which the faults abruptly flatten. ric faults and detachment faults conceals the the Socorro Peak volcanic center, central 2. Continuous reflectors seen below deeper structure. Magma bodies at greater New Mexico: A summary: New Mexico known surface faults imply that the surface depth may play an important role in decou- Geology, v. 3, p. 22-24. pling various elements of the structure. Chapin, C. E„ 1971, The Rio Grande rift. Part 1: faults are either small-scale minor features Modifications and additions: New Mexico or that they merge downward into the Geological Society, 22nd Field Conference, major subsurface faults. No clear evidence ACKNOWLEDGMENTS Guidebook, p. 191-201. is seen on the seismic sections for a history 1979, Evolution of the Rio Grande rift, in Riecker, R. E., ed., Rio Grande rift: Tecton- of rifting characterized by early closely This work was undertaken as part of a ics and magmatism: Washington, D.C., spaced faults and large stratal tilts followed tectonics research seminar in the Geophys- American Geophysical Union, p. 1-5. by wide-spaced, high-angle faulting. ics Department at Stanford University. Chapin, C. E„ and Seager, W. R„ 1975, Evolu- 3. Our model of the shallow structure of Special appreciation goes to R. Bracken tion of the Rio Grande rift in the Socorro the Rio Grande rift can account for exten- and L. Hale for their work on the gravity and Las Cruces areas: New Mexico Geologi- cal Society, 26th Field Conference, Guide- sion only to a depth of approximately 5 km, modeling and data migration, and to D. book, p. 297-321. at which the faults flatten into crystalline May for his helpful comments on the var- Chapin, C. E., and others, 1978, Exploration basement. No high-angle faults offset the ious drafts. We also acknowledge the initial framework of the Socorro geothermal area, prominent reflectors seen at this depth, and work done by M. Wilson, S. O'Hare, M. New Mexico: New Mexico Geological So- ciety Special Publication 7, p. 115-129. no deep faults are recognized below these Schnapp, and M. Yeaman. We thank J. Condie, K. C., and Budding, A. J., 1979, Geology reflectors. This suggests that the uppermost Claerbout and the Stanford Exploration and geochemistry of Precambrian rocks, crust is extending in a style much different Project for the use of their computer and central and south-central New Mexico: New Downloaded from gsabulletin.gsapubs.org on July 16, 2014 IO CAPE AND OTHERS

Mexico Bureau of Mines and Mineral Kelley, V. C., 1977, Geology of the Albuquerque the vicinity of Socorro, New Mexico, in Resources Memoir 35, 58 p. basin: New Mexico Bureau of Mines and Heacock, J. G., ed.. The Earth's crust: Its Cordcll, L., 1978, Regional geophysical setting of Mineral Resources Memoir 33, 60 p. nature and physical properties: Washington, the Rio Grande rift: Geological Society of Lynn, H. B., 1979, Migration and interpretation D.C., American Geophysical Union Geo- America Bulletin, v. 89, p. 1073-1090. of deep crustal seismic reflection data [Ph.D. physical Monograph 20, p. 385-403. Cordell, L„ Keller, G. R.. and Hildenbrand, thesis]: Stanford. California, Stanford Uni- Shafiqullah, M„ and others, 1980, K-Ar geo- T. G.. 1978, Complete Bouguer gravity versity, 159 p. chronology and geologic history of south- anomaly map of the Rio Grande rift: U.S. Machette, M. N., 1978, Late Cenozoic geology of western Arizona and adjacent areas, in Geological Survey Open-File Report 78-958, the San Acacia-Bernardo area, in Hawley, Jenney, J. P., and Stone, C., eds.. Studies in scale 1:1.000,000. J. W., ed., Guidebook to Rio Grande rift in western Arizona: Arizona Geological So- Davis. G. A., and others, 1980, Mylonitization New Mexico and Colorado: New Mexico ciety Digest, v. 12. p. 201-260. and detachment faulting in the Whipple- Bureau of Mines and Mineral Resources Spradlin. E. J., 1976, Stratigraphy of Tertiary Buckskin-Rawhide Mountains terrane, Circular 163, p. 135-137. volcanic rocks, Joyita Hills area, Socorro southeastern California and western Ari- Mattick, R. E., 1967, A seismic and gravity pro- County, New Mexico [M.S. thesis]: Albu- zona, in Crittenden, M. D., Coney, P. J., file across the Hueco bolson, Texas: U.S. querque, New Mexico, University of New and Davis, G. H., eds., Cordilleran meta- Geological Survey Professional Paper 575- Mexico, 73 p. morphic core complexes: Geological Society D, p. 85-91. Stewart, J. H., 1980. Regional tilt patterns of late of America Memoir 153, p. 79-129. Morton, W. H„ and Black, R„ 1974, Crustal Cenozoic basin-range fault blocks, western Decker, E. R., and others, 1975, Significance of attenuation in Afar, in Pilger, A., and United States: Geological Society of Amer- geothermal and gravity studies in the Las Rosier, A., eds.. Afar depression of Ethio- ica Bulletin, v. 91, p. 460-464. Cruces area: New Mexico Geological So- pia: Inter-Union Commission on Geody- Weber, R. H., 1963, Cenozoic volcanic rocks of ciety, 26th Field Conference, Guidebook, namics Scientific Report 14, p. 55-65. Socorro County: New Mexico Geological p. 251-259. Oliver, J., and Kaufman, S., 1976, Profiling the Society, 14th Field Conference, Guidebook, Foster. R. W„ 1978. Selected data for deep drill Rio Grande rift: Geotimes, v. 21, p. 20-23. p. 132-143. holes along Rio Grande rift in New Mexico, Ramberg, I. B„ Cook, F. A., and Smithson, S. Woodward, L. A., 1977, Rate of crustal exten- in Hawley. J. W.. ed.. Guidebook to Rio B., 1978, Structure of the Rio Grande rift in sion across the Rio Grande rift near Albu- Grande rift in New Mexico and Colorado: southern New Mexico and West Texas based querque, New Mexico: Geology, v. 5, New Mexico Bureau of Mines and Mineral on gravity interpretation: Geological Society p. 269-272. Resources Circular 163, p. 236-237. of America Bulletin, v. 89, p. 107- 123. Zoback, M. L., Anderson, R. E., and Thompson, Hamblin. W. K., 1965, Origin of "reverse drag" Sanford, A. R., 1968, Gravity survey in central G. A., 1981, Cainozoic evolution of the on the downthrown side of normal faults: Socorro County, New Mexico: New Mexico state of stress and style of tectonism of the Geological Society of America Bulletin, Bureau of Mines and Mineral Resources Basin and Range province of the western v. 76, p. 1145-1 164. Circular 91, 14 p. United States: Royal Society of London Joesting, H. R., Case, J. E., and Cordell, L. E., • 1978, Characteristics of Rio Grande rift in Philosophical Transactions, ser. A, v. 300, 1961. The Rio Grande trough near Albu- vicinity of Socorro, New Mexico, from geo- p. 407-434. querque, New Mexico: New Mexico Geolog- physical studies, in Hawley, J. W., ed., ical Society, 12th Field Conference, Guide- Guidebook to Rio Grande rift in New Mex- book, p. 148-150. ico and Colorado: New Mexico Bureau of MANUSCRIPT RECEIVED BY THE SOCIETY Jurdy. D. M.. and Brocher, T. M„ 1980, Shallow Mines and Mineral Resources Circular 163, AUGUST 17, 1981 velocity model of the Rio Grande rift near p. 116-121. REVISED MANUSCRIPT RECEIVED Socorro, New Mexico: Geology, v. 8, Sanford, A. R., and others, 1977, Geophysical JANUARY 18, 1982 p. 185-189. evidence for a magma body in the crust in MANUSCRIPT ACCEPTED FEBRUARY 25, 1982

Primed in U.S.A.