ROCK UPLIFT RATES FOR THE INTERIOR OF THE WESTERN TRANSVERSE RANGES, CALIFORNIA, FROM FLUVIAL TERRACES OF THE SESPE RIVER

The Western Transverse Ranges (WTR) are an active East-West trending thrust and fold belt that experiences a high rate of crustal shortening. Rock uplift rates ranging from 0.5 to 7 mm/yr have been measured around the edges of the WTR in lower elevation areas. Interior areas ascend to elevations well over 2,000 meters. However, a lack of sufficient uplift data in these higher elevations, make it difficult to determine how or when the higher topography was established.

To expand our understanding of the rock uplift in these higher elevations of the WTR, we evaluated fluvial strath terraces along the Sespe River. We use a combination of field mapping, elevation surveys, GIS analyses, and infra-red stimulated luminescence dating to determine the uplift rate at these terraces.

Three terrace levels were dated; the lowest terrace level (T1) was measured to be 78.3 to 80.7 ka. The second terrace (T2) was measured to be 106 to 141.6 ka. The third terrace (T3) dates were too young to fit this model and are considered inaccurate due to post-depositional mixing or accidental sampling of colluvium rather than a terrace deposit. We calculated the incision rates to be 0.175 ± 0.015 mm/yr for T1, and 0.155 ± 0.025 mm/yr for T2.

Due to a strong convexity in the Sespe River’s profile, we know the river is not in equilibrium. As a result, incision rates do not reflect the full extent of rock uplift in the region. To remedy this, we try a new method of creating a hypothetical profile to estimate the elevation of the active channel if incision had kept pace with regional uplift. The resulting uplift is 1.825 ± 0.455 mm/yr for T1, and 2.345 ± 0.035 mm/yr for T3.

Uplift calculations using the Sespe’s hypothetical river profile yield rates that are similar to those at lower elevations in the WTR. This implies there is still ongoing uplift within the interior of the WTR, but most of the shortening is likely accommodated at the southern front of the mountain belt. While local active faults such as the San Cayetano Fault have high displacement rates and are driving uplift, the convexity of the Sespe River shows that erosion and rock strength play a significant role in the interior of the WTR’s elevation.