ON THE UTILITY OF INTEGRATING SOIL CHARACTERISTICS WITH RADIOMETRIC DATING IN THE STUDY OF ACTIVE FAULTS: AN EXAMPLE FROM EASTERN CALIFORNIA

Widespread application of novel chronologic techniques for dating alluvial fan surfaces and deposits has begun to reveal intriguing temporal patterns in fault slip over timescales ranging from 10⁴–10⁵ yr. However, epistemic uncertainties can severely limit a technique’s utility; cosmogenic nuclide (TCN) ages, for instance, are strongly influenced by degradation of geomorphic surfaces, a process whose effects are often difficult to recognize and even harder to quantify. Moreover, the difficulty and expense of obtaining ages often limits their application to a single site along a given structure. Soil profile development indices (PDI) can be acquired for a fraction of time and cost of absolute ages, and calibrated soil chronosequences can provide a reliable chronologic framework across a region of similar climate.

We illustrate this approach with two examples drawn from recent work along active faults in eastern California. Our soil chronosequence utilizes well-preserved and dated surfaces from Panamint Valley and the western Mojave Desert and covers a range in age from ~10³-10⁵ ka; surfaces are dated using a combination of radiocarbon, OSL, ¹⁰Be and ³⁶Cl. Morphologic descriptions of soil development were conducted at all sites to provide data for PDI values, to establish local soil stratigraphy, and to ascertain local surface and landscape history critical for interpretation of TCN results. Along the Garlock fault, displacement of a ca. 30ka fan has blocked drainage along the southern flank of the Slate Range; soils developed within an alluvial fan unit truncated by the shutter ridge are ~2-4 ka, suggesting extremely rapid slip along the fault in the late Holocene. In southern Panamint Valley, right-lateral displacement of a debris-flow levee provides a precise estimate of fault slip. However, exposure ages of unweathered boulders scatter from 14–28 ka. The degree of soil development and local relationships with dated shorelines indicate that this flow is ~9-12 ka. In each example, soil development provides critical information not available via direct dating. Thus, with the recent improvements in methods for dating geomorphic surfaces, the time is ripe to rejuvenate the concept of establishing regional soil chronosequences where age control is required across large tectonically active regions.