Cenozoic domino-style crustal extension in the Lemitar Mountains, New Mexico--A summary
— Richard M. Chamberlin

The Lemitar Mountains are located in central New Mexico near the axis of a north-trending zone of crustal extension known as the Rio Grande rift (Chapin and Seager, 1975; Cordell, 1978). Detailed geologic mapping of the Lemitar Mountains (Chamberlin, 1976, 1978, 1982) has revealed a complex structural pattern dominated by subhorizontal to moderately east dipping, dip-slip, normal faults that repeat over and over a strongly west-tilted sequence of Oligocene volcanic strata (fig. 1, A-A' , B-B'). Abrupt changes in thickness of basaltic-andesite lavas and angular unconformities between adjacent ash-flow tuffs locally oc-cur across these low-angle normal faults. Restoration of the ash-flow sheets to their original horizontal positions allows only one reasonable kinematic interpretation—the subhorizontal faults were high-angle nor-mal faults bounding simultaneously rotating blocks in middle to late Oligocene time (31-27 m.y. ago). 

I have previously referred to this simultaneous rotation and dip-slip displacement of subparallel fault blocks as domino style rifting because in cross section the subequally tilted blocks look similar to a train of fallen dominoes (Chamberlin, 1978). The term domino-style extension may(?) be relatively new but the concept is not, and can be found in several discussions of extended terranes in the Basin and Range province (Emmons and Garrey, 1910, fig. 15; Thompson, 1960, fig. 3).

Based on detailed mapping and 30,000 m of drill holes in the Yer-ington district of Nevada, Profett (1977, fig. 15) has elegantly docu-mented simultaneous rotation and displacement of originally high-angle, domino-like (gently curved) normal faults in an area that has undergone more than 100 percent extension. Although the fault planes are gently curved (3-7°/km), Profett's palinspastic cross sections demonstrate that the faults have been rotated (domino-style) to their present subhorizontal attitude. The recognition of rotated fault surfaces, either planar or gently curved, is a key aspect of domino-style extension, which is not found in most discussions of imbricate, listric normal fauting that tacitly assume the fault surfaces are fixed in space (see Anderson, 1971).

Figure 2 illustrates another important aspect of domino-style normal faulting, which is that after about 30 degrees of rotation the original domino faults are no longer in a favorable orientation for displacement as gravity shears. Continuing crustal extension must then form a second set of high-angle domino faults, which is coaxially superimposed on the first set. Buried low-angle faults in the Yerington district (Profett, 1977) and the Lemitar Mountains strongly support this aspect of Morton and Black's (1975) hypothetical model.

It must be emphasized that the concept of domino-style faulting is at best only a first-order approximation of the crustal extension process. In the real crust, vertical and lateral changes in ductilty, preexisting structural grain or lack thereof, isostatic adjustments, magmatic intru-sions, normal and reverse drag adjacent to faults, and other factors can, and do, create departures from the simple model of domino blocks (fig. 1).

The purpose of this summary article is to briefly describe structural features observed in the Lemitar Mountains and to interpret from them the late Cenozoic structural evolution of the area. In addition, palin-spastically restored cross sections are presented which allow a reason-able estimate of the amount and rate of extension in the Lemitar Mountains area.