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Downloaded from gsabulletin.gsapubs.org on July 16, 2014 Geological Society of America Bulletin Cenozoic normal faulting and the shallow structure of the Rio Grande 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 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes Geological Society of America Downloaded from gsabulletin.gsapubs.org on July 16, 2014 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 Precambrian 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 Permian 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 Triassic and Jurassic continental deposits; or converge upon an apparent detachment and other rifts concerns what happens to and Cretaceous 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 Oligocene 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. 3 Downloaded from gsabulletin.gsapubs.org on July 16, 2014 IO CAPE AND OTHERS 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 Miocene 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 Santa Fe Group, 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.