RESOLVING “DEEP TIME” FAULT SLIP RECORDS USING MORPHOCHRONOLOGY AND TERRESTRIAL LASER SCANNING

“Deep time” fault records, spanning tens to hundreds of thousands of years, can be resolved using morphochronology. In this approach, fault displacement history is reconstructed by measuring offsets and ages of faulted landforms, such as fluvial terrace risers, stream channels, alluvial fans, and shorelines. However, a fundamental problem in morphochronology is the differentiation of the records of cumulative faulting from the geomorphic processes that create and modify landforms. For example, differential lateral stream erosion of faulted terrace risers has been shown to lead to erroneous displacement measurements. Terrestrial laser scanning (TLS) datasets provide high-resolution (cm-scale) measurements of landform geometry, which can be used to identify and mitigate these problems. Here, we use TLS in the analysis of two different slip-rate sites. At Tuzidun (37.7N, 86.7E) along the active, left-lateral Altyn Tagh fault in NW China (N. Tibet), a faulted terrace riser is presently offset 54±3 m. Analysis of riser topography reveals significant differences in riser slope steepness on opposite sides of the fault. These results, in addition to terrace chronology constrained by ¹⁴C and cosmogenic radionuclide dating, indicate that the displaced riser segments are diachronous. Further topographic analysis limits cumulative downstream riser offset since its formation (6.0±0.8 ka to 5.7±0.4 ka) to 54-89 m, based on the width of the upstream channel. These observations bracket the Altyn Tagh fault slip rate to 9-16 mm/yr since the mid-Holocene. At the second site, an offset alluvial fan (ca. 50-300 ka) at Fort Sage (40.1N, 120.0W) along the active, right-lateral Warm Springs Valley fault in W. Nevada provides constraints on the slip record along this fault system. Retro-deformation of the fan constrained from topographic swath profiles constrains the displacement to 85-125 m. Cosmogenic ¹⁰Be and ³⁶Cl radionuclide analyses will constrain the age of the fan surface and allow us to determine the fault’s slip rate. In summary, morphologic analysis of TLS datasets, such as riser slope steepness and topographic profiles, provide essential insights regarding the convolved records of faulting and geomorphic processes and provide critical constraints needed to reconstruct “deep time” fault records.