New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/26 Evolution of the Rio Grande rift in the Socorro and Las Cruces areas Charles E. Chapin and William R. Seager, 1975, pp. 297-321 in: Las Cruces Country, Seager, W. R.; Clemons, R. E.; Callender, J. F.; [eds.], New Mexico Geological Society 26th Annual Fall Field Conference Guidebook, 376 p. This is one of many related papers that were included in the 1975 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. New Mexico Geol. Soc. Guidebook, 26th Field Conf., Las Cruces Country, 1975 297 EVOLUTION OF THE RIO GRANDE RIFT IN THE SOCORRO AND LAS CRUCES AREAS by CHARLES E. CHAPIN New Mexico Bureau of Mines and Mineral Resources Socorro, New Mexico and WILLIAM R. SEAGER Department of Earth Sciences, New Mexico State University Las Cruces, New Mexico INTRODUCTION unpublished chemical and isotopic data. To all of these people Much controversy in recent years surrounding the Rio we express our gratitude; any errors in interpretation, how- Grande rift has been caused by failure to look beyond its ever, are solely the responsibility of the authors. present geomorphic expression at its past history. Specifically, when did rifting begin? ... What were the early basins and THE PIONEERING WORK OF KIRK BRYAN uplifts like? ... Where were they? ... How big? ... How Any discussion of the Rio Grande rift should begin with the much volcanism and of what kind? ... What were the main work of Kirk Bryan, his students, and the masterful paper source areas for the basin-fill sediments? ... Under what con- published by Bryan in 1938. This paper, buried in an obscure ditions were the sediments transported and deposited? ... Was hydrologic report, has not received the attention it merits. there a culmination of uplift and faulting? ... If so, when? ... Bryan coined the term "Rio Grande depression" for the series Has the type or rates of sedimentation and volcanism changed of structural basins through which the Rio Grande flows from with time? ... When did integrated drainage begin and how the San Luis Valley of Colorado to El Paso, Texas (Fig. 1). has it evolved? ... If the rift was different in the past, what Recognizing the similarities in age and origin of the basin-fill might it be like in the future? The answers to these questions, deposits, Bryan assigned them to the Santa Fe Formation and to others unperceived, are recorded in the basin-fill de- (since raised to group status by Baldwin, 1963). posits. To read this record requires knowledge of the sequence and distribution of rock types on the uplifts in order to work Bryan recognized that the Rio Grande rift had evolved over a considerable period of geologic time as is evident from the out the sequence and facies of deposits in the basins which, in following excerpts from his 1938 paper: turn, is necessary to unravel the structural history of the up- lifts. In other words, a circuitous and often repetitious path of 1) "The end of the great volcanic period ... [middle Ter- investigation, from basin margin to basin fill and back again, is tiary] ... merged into the beginning of the series of move- ments by which the Rio Grande depression was formed, ..." necessary to understand the whole. (p. 205) The purpose of this paper is to document the sequential 2) "It appears, however, that initial basins were formed, into development of the Rio Grande rift in two areas where the which streams carried gravel and sand derived from the ero- basin-fill deposits are well-exposed and where extensive field sion of the previously extruded lavas and into which tuffs studies have provided the necessary stratigraphic detail. The from the still lingering volcanoes were blown or washed by similar geologic histories of these two areas (interpreted the streams, and that these deposits were deeply eroded and in places faulted before the succeeding Santa Fe formation independently until our collaboration on this paper) gives us was deposited." (p. 205) confidence that the data may be of wider applicability. We 3) "... deformation is evident and has become one of the have used time subdivisions for the Cenozoic as given by Har- principal criteria for distinguishing the formation ... [lower land and others (1964); they are plotted on Figure 9. Santa Fe Group] ... from the Quaternary sand and gravel." We wish to acknowledge the work of our colleagues without (p. 209) whom this paper could not have been written. Russell E. 4) "Most of the existing mountains and highland areas were Clemons, John W. Hawley, and Frank E. Kottlowski have pro- also mountainous in Santa Fe time. They were reduced in vided major contributions to our understanding of the Las Pliocene time ... [now known to be Miocene] ... and were rejuvenated to form the present ranges. Other mountains Cruces area. Thesis investigations by R. B. Blakestad, D. M. appear to have been new-born ... [very late Cenozoic, but Brown, J. E. Bruning, C. Burton, R. M. Chamberlin, E. G. pre-Pleistocene] ... as, for example, the Socorro-Lemitar Deal, D. A. Krewedl, D. B. Simon, E. J. Spradlin, W. H. Wilkin- uplift. So far as present information goes, all the ranges, with son, and T. M. Woodward aided greatly in unraveling the late the exception of the Magdalena Mountains and the part of the Sangre de Cristo Range north of Sierra Blanca, owe their Cenozoic history of the Socorro-Magdalena area. G. 0. Bach- present position to the post ... [lower] ... Santa Fe uplift." man and J. W. Hawley provided stimulating discussions of the (p. 209) late Cenozoic sedimentary and geomorphic features and 5) "The Rio Grande depression is a structural depression improved the manuscript. W. E. Elston read parts of the manu- consisting of minor basins and reentrants and having a border script and provided data and helpful advice on the Datil- of diverse types that merges to the south into a number of Mogollon volcanic field. P. E. Damon was a catalyst for the similar basins. The structural depression is only in part coin- cident with the present course of the river." (p. 215). section on Transitional Volcanism from a discussion at the 1974 Geological Society of America Meeting in Flagstaff and Some of Bryan's statements need to be modified in light of through recent discussions of unpublished data. We are espe cially indebted to J. W. Stinnett, Jr., for permission to use RIO GRANDE RIFT 299 recent data (as we have done in brackets), but they clearly the roof of an Oligocene batholith and allowed the intrusion demonstrate that 37 years ago his perception of the Rio of numerous stocks and dikes (Chapin and others, 1974). Grande rift was that of a dynamic structure, changing with Radiometric dating of plutons and intruded ash-flow sheets time. has bracketed the extensional faulting as occurring after 30 to 31 m.y. but prior to 28 m.y. The critical dates are: PRE-RIFT GEOLOGIC SETTING Potato Canyon Tuff 30.3 ± 1.6 m.y. The youngest major ash-flow unit involved in intrusion- The "Rio Grande depression" is an intracratonic continental alteration-mineralization events in the Magdalena district rift superimposed during the late Cenozoic upon a north-trend- and dated by E. I. Smith (in press) by the fission-track ing tectonic belt that was intensely deformed during the late method. This date is in good agreement with a 31.8 ± 1.7 Paleozoic (Ancestral Rocky Mountains) and again during the m.y. fission-track date on the underlying A-L Peak Tuff Laramide. Figure 2 shows how well these three periods of (Smith, op. cit.) and a 31.7 m.y. average of three K-Ar dates (Burke and others, 1963; Weber and Bassett, 1963; deformation are superimposed and how previous deformations Weber, 1971) on the Hells Mesa Tuff (restricted sense, influenced later ones. For example, the change in trend from Chapin, 1974b) whose eruption preceded the A-L Peak.