Abstract

A discontinuous zone of subdued, west-facing fault scarps offset Quaternary alluvium along 58 km of the western piedmont of the Santa Rita Mountains, south of Tucson, Arizona. Scarps trend NE to N-S and are 1 to 6 km basinward from the deeply embayed mountain front. Scarp heights range from 1 to 7 m, and maximum scarp slope angles range from 3° to 9.5°. A 6-m-deep trench excavated in middle Pleistocene and younger deposits exposed a high-angle fault zone with about 3.5 m of total displacement.

Estimated ages of faulted and unfaulted surfaces, extent of degradation of piedmont scarps, and stratigraphic relations exposed in the trench suggest an age of about 100 ka for the most recent surface-rupture event. Units having well-developed relict soils of late Pleistocene age and older are faulted, whereas units having weakly to moderately developed soils, of latest Pleistocene and Holocene age, are not offset. Morphologic fault scarp analyses suggest an age of 60 to 100 ka. Earthquake magnitude estimates derived using minimum and maximum seismic source moment calculations range from 6.4 to 7.3

One or more earlier surface-rupture events are implied by significantly high scarp heights on mid-Pleistocene than on late Pleistocene surfaces. Scarp heights on middle and early (?) Pleistocene surfaces are similar, restricting faulting to post-middle Pleistocene time (<200 to 300 ka). Stratigraphic relationships observed in the trench also suggest two fault-displacement events. Tectonic landform analysis of the mountain front-piedmont area implies several m.y. of inactivity along the range-bounding fault system prior to these late Quaternary displacements. Extremely long recurrent intervals between displacement events and reactivation of faulting after an interval of tectonic quiescence are typical of late Quaternary fault behavior in southeastern Arizona.

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