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Richard Chamberlin

The Rio Grande rift of central New Mexico is dominated by NNW- to NE-striking normal faults of Late Oligocene to Quaternary age. Crustal extension is distributed across many subparallel fault blocks. Fault traces commonly show abrupt changes in strike, often bifurcate, and occasionally step over to adjacent faults along transverse ramps (e.g. Chamberlin et.al, 1994a; Chamberlin and Harrison, 1996). Based on observations of striated fault surfaces most rift faults are apparently dip-slip to slightly oblique-slip normal faults. There is generally little or no evidence of significant strike slip faulting in areas of high-angle normal faulting. However, minor lateral slip may occur where a north or south propagating fault tip is deflected by a preexisting basement shear zone oriented at a high angle to the propagating rift fault (Chamberlin, 2000). Broad zones of lateral shear may be accommodated by a combination of minor vertical axis rotations and distributed oblique-slip normal faulting in which the dip-slip component is dominant.
Strong horizontal axis rotation and dip-slip separation of numerous subparallel fault blocks characterize a highly extended domain in the Socorro-Magdalena region. In late Oligocene time, closely spaced, originally high-angle normal faults and intervening blocks were progressively rotated like falling dominoes (Chamberlin, 1976,1978,1983). The process of domino-style extension was recorded in the Lemitar Mountains by wedge-shaped prisms of basaltic lavas that occur between westerly tilted ignimbrite sheets that range in age from 28.7 to 27.4 Ma. As much as 150% extensional strain in the Lemitar Mountains has caused originally high-angle down-to the-east domino faults to be rotated to horizontal orientations and locally to the geometry of east-vergent thrust faults (younger over older). Minor down-to-the-west antithetic normal faults have been rotated to the orientation of high-angle reverse faults and NE-striking transverse segments of corrugated fault traces now exhibit strike-slip striations and distinct lateral offsets. In domains of steeply dipping strata and sub-horizontal normal faults it seems appropriate to label these rotated structures as pseudo-thrust faults, pseudo-reverse faults and pseudo strike-slip faults, depending on their present geometry.

The ENE-trending Socorro accomodation zone (SAZ of Chapin, 1989; "transverse-shear zone" of Chapin, Chamberlin, Osburn and White, 1978) is a regional tilt-block domain boundary that separates west-tilted domino blocks on the north from east-tilted blocks on the south. Periodic magmatism along the SAZ has presumably allowed domino fault blocks to rotate in opposing directions but without the faults propagating laterally across the zone such that opposing faults would interlock at depth (i.e soft zones around intrusions permit highly distributed shear at depth). The SAZ has changed its character with evolving thermal regimes over the last 29 Ma. It has become shorter and more diffuse in late Miocene to Pliocene time (Chamberlin and Eggleston, 1996).
Northeast striking en echelon basins of Neogene age in the Reserve-Datil-Magdalena area (San Agustin arm of the rift) and a few observations of sub-horizontal striations on NE striking high-angle faults suggest a regional component of sinistral shear along southeastern margin of the Colorado Plateau (Chamberlin, 1993; Chamberlin et.al, 1994b). Westward migration of late Oligocene magmatism (Socorro-Magdalena caldera cluster), however, suggests a dextral component of shear may have occurred along the SE margin of the Colorado Plateau at this time.

Photo: Domino-style microfaults offset heavy mineral laminations in poorly indurated sands of the upper Santa Fe Group about 1 km east of the Grefco Perlite mine. Microfaults are associated with erosion and westward slumping in the footwall of the Socorro Canyon fault.

• Chamberlin, R. M., 2000, Partitioning of Dextral Slip in an Incipient Transverse Shear Zone of Neogene Age, Northwestern Albuquerque Basin, Rio Grande Rift, New Mexico: in Cole J.C. ed., U.S. Geological Survey Middle Rio Grande Basin Study— Proceedings of the Forth Annual Workshop, Albuquerque, New Mexico, February 15-16, 2000: U.S. Geological Survey Open-File Report 00-488, p. 51.
• Chamberlin, R. M., and Harrison, B., 1996, Pliocene and Pleistocene displacement history of the Socorro Canyon Fault, Central Rio Grande rift, New Mexico (abs.), New Mexico Geology, v. 18, no. 2, p. 56.
• Chamberlin, R. M. and Eggleston, T. L., 1996, Geologic map of the Luis Lopez 7.5 minute quadrangle, Socorro County, New Mexico: New Mexico Bureau of Mines and Mineral Resources Open-file report 421, 152 p. (map scale 1:24,000).
• Chamberlin, R. M., Cather, S. M., Anderson, O. J., and Jones, G. E., 1994a, Reconnaissance geologic map of the Quemado 30x60 minute quadrangle, Catron County, New Mexico: New Mexico Bureau of Mines and Mineral Resources Open-File Report 406, 30 p., (map scale 1:100,000).
• Chamberlin, R. M., Cather, S. M., McIntosh, W. C., Anderson, O. J., and Rattè, J. C., 1994b, First-day road log from Socorro to Magdalena, Datil, western Crosby Mountains, Sawtooth Mountains, Pie Town, Quemado and Quemado Lake: New Mexico Geological Society, Guidebook 45, p. 1–45 (see p.8, mileage 27.6).
• Chamberlin, R. M., 1993, Neogene sinistral transtension along the Hickman fault zone, southeastern Colorado Plateau, New Mexico [abs.]: Geological Society of America Abstracts with Programs, v. 25, no. 5, p. 19.
• Chamberlin, R. M., and Osburn, G. R., 1986, Tectonic framework, character, and evolution of upper crustal extension domains in the Socorro area of the Rio Grande rift [abs.]; in Beatty, B., and Wilkinson, P. (eds.), Frontiers in geology and ore deposits of Arizona and the southwest: Arizona Geological Society Digest, v. XCI, p. 464.
• Chamberlin, R. M., and Osburn, G. R., 1984, Character and evolution of extensional domains in the Socorro Area of the Rio Grande rift, central New Mexico [abs.]: Geological Society of America, Abstracts with Programs, v. 16, no. 6, p. 476.
• Chamberlin, R. M., 1983, Cenozoic domino-style crustal extension in the Lemitar Mountains, New Mexico —A summary: New Mexico Geological Society, Guidebook 34, p. 111–118.
• Chapin, C. E., Chamberlin, R. M., Osburn, G. R., White, D. W., and Sanford, A. R., 1978, Exploration framework of the Socorro geothermal area, New Mexico: New Mexico Geological Society, Special Publication 7, p. 115–129
• Chamberlin, R. M., 1978, Structural development of the Lemitar Mountains, an intrarift tilted-fault-block uplift, central New Mexico [abs.]: Program and abstracts for International Symposium on the Rio Grande rift, Los Alamos Scientific Laboratory Pub. LA-7487-C, p. 22–24.
• Chamberlin, R. M., 1976, Rotated early-rift faults and fault blocks, Lemitar Mountains, Socorro County, New Mexico [abs.]: Geological Society of America, Abstracts with Programs, v. 8, no. 6, p. 807.
• Chapin, C. E., 1989, Volcanism along the Socorro accommodation zone, Rio Grade rift, New Mexico; in Field excursions to volcanic terranes in the western United States, V. I., Southern Rocky Mountain Region, C. E. Chapin and J. Zidek (eds.), p. 46-57.
  

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