PLIOCENE-PLEISTOCENE KINEMATIC EVOLUTION OF A RIO GRANDE RIFT SUB-BASIN, SUNSHINE VALLEY-COSTILLA PLAIN, COLORADO AND NEW MEXICO

The Sunshine Valley-Costilla Plain sub-basin in northern New Mexico and southern Colorado consists of approximately 1.2 km of basin fill capped by Pliocene Servilleta Basalt and three pre-Rio Grande Gorge gravel deposits. Mapping of Pleistocene fault scarps in this region primarily shows north-trending fault zones comprising the Southern Sangre de Cristo, Sunshine Valley, and Mesita fault zones, which all display post-middle Pleistocene displacement. Paleoseismic investigations and well data correlations reveal changes in slip rates an order of magnitude over 100 k.y. intervals, but m.y. intervals slip rates range from 0.1 to 0.2 mm/yr. Analysis of aeromagnetic data reveals a rhombohedral fault pattern preserved in the sub-surface and on the Servilleta Basalt consisting of both northeast- and northwest-trending faults. With the inception of extension, an initial pattern of faults, similar to conjugate pairs, began to form. Oblique displacement on each fault strand is indeterminate due to the faults being in the subsurface. The overprinting of north-trending faults on this older rhombohedral fault pattern is an analog for fault orientations becoming more perpendicular to the region stress field as extension progresses within a region.