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i <br />' ' the Basin and Range physiographic province to the west <br />th <br />t <br />l <br />4 <br />(M <br />1986) <br />B <br />S <br />illi <br /> e <br />organ e <br />a <br />., <br />on years ago, <br />y <br />to <br />m <br />. <br /> formerly separate basins of the ancestral Rio Grande Rift <br /> became integrated into the present day Rio Grande drainage <br /> (Machette, 1986). Tectonic activity continues today as <br /> evinced by a plethora of Quaternary and Holocene fault <br /> scarps distributed ubiquitously throughout the Length and <br /> breadth of the rift. <br /> The structural style is remarkably consistent through- <br />; out the length of the Rio Grande Rift, although deformation <br /> in the northern portion of the rift appears to be chxiracter- <br /> ized more by uplift with only a minor component of extension, <br />' whereas extension dominates the central and southern <br /> portions of the rift with only relatively minor components <br /> of uplift. The ratio of extension to uplift in the San Luis <br /> Valley area of the rift, for example is approximately 0.1 <br />' (Cordell, 1982). As one moves south along the Rio Grande <br /> Rift extension progressively increases yielding a ratio of <br />I <br />~ 3.0 across the Albuquerque Basin, and 11.0 at Las Cruces. <br />if <br />i <br /> t <br />ng <br />These ratios suggest a hinge-type behavior to r <br /> which may have an influence on the type and severity of <br /> tectonic activity to be expected in the future. The under- <br /> lying mechanism that drives the rifting may, for example,help <br /> explain the lower fault segmentation observed for Sari Luis <br /> Valley faults relative to faults farther south along the <br />' rift. <br /> Disagreement exists among workers as to the pret:ise <br /> structural style of faults forming the rift. Some im~estigators <br />f <br />bl <br /> e o <br />believe that rift faults are basement structures capa <br /> large earthquakes. Opposing this view, however, is a large <br /> and increasing body of evidence suggesting that most rift <br /> faults are rotational in nature (listric) and as such are <br /> constrained to relatively shallow depths and hence small <br /> magnitude earthquakes. This latter view is supported by recent <br /> COCORP data in the Socorro area (Cape et al., 1983) snd <br /> the surprisingly low historical seismicity of the entire Rio <br /> <br /> 2 <br /> <br />