Abstract
The 45 km-long, north-trending East Franklin Mountains Fault lies in the southern Rio Grande rift (Texas). Late Quaternary normal fault scarps up to 20 m high traverse the eastern range front, but also bulge up to 2 km out onto the piedmont in lobes as much as 3.5 km wide. In the lobe areas the range-front fault strand displays no fault scarps, as if displacement has shifted to the piedmont strand since ca. 400 ka. We divide the faulted piedmont alluvial fans into seven age classes (from Middle Pleistocene to Holocene), based on erosional dissection and soil profile development. Piedmont scarps in Qf1 deposits (250–400 ka) range from 18 to 20 m high; in Qf2-3 deposits (65–100 ka), 9–10 m high; and in Qf3-4 deposits (20 ka), 2.7–4 m high (the latest displacement event). Scarp heights underestimate true vertical displacement of the footwall surface, which lies buried beneath younger fan deposits of unknown thickness on the hanging wall. Hanging-wall aggradation also causes variable scarp heights along strike on single-age landforms, with burial thickness a second-degree polynomial function (parabolic) of distance to the nearest active stream. Our paleoseismic trench on a 9.8 m-high scarp exposed evidence for five displacement events in the past 44 ka, the latest three of which had vertical displacements of 3–4 m. Such displacements imply surface ruptures longer than the 26.5 km fault length previously mapped in the USA, indicating rupture extended onto adjacent faults. The minimum long-term slip rate (>11.2 m in 44 ka) is 0.25 mm/yr; over last full seismic cycle (ca. 11–29 ka) it is 0.20–0.21 mm/yr. Optically-stimulated luminescence (OSL) dating shows that fine sand-silt grains in colluvial deposits were not all zeroed during deposition. Single-aliquot dating of quartz grains yielded multiple OSL age peaks, the youngest peaks of which yield a chronology compatible with stratigraphy and soil development in the colluvial wedge sequence.
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