RETHINKING THE COLLUVIAL WEDGE: A REVISED MODEL FOR COLLUVIAL EVENT STRATIGRAPHY IN SLOPED ENVIRONMENTS

Recent work in California, New Mexico, and Taiwan suggests the classical model of the ‘colluvial wedge’ may not adequately describe colluvial event stratigraphy observed from fault ruptures with relatively small vertical displacements and/or long recurrence intervals. Colluvial event stratigraphy exposed in paleoseismic trenches often does not replicate the morphological or textural characteristics of archetypal colluvial wedges: the observed long downslope attenuation of colluvial packages in fault contact are unlikely to have formed from free-face degradation alone and are often texturally indistinct from the modern colluvia generated upslope of their causative faults. We postulate that these deposits are created and preserved by a combination of continued active surface colluvial transport and vertical displacements generated by surface rupture. This sequence of events forms the basis of our new colluvial event model that emphasizes the faulting and preservation of existing slope colluvia rather than the generation of scarp-derived colluvial wedges.

Where faults traverse sloped geomorphic surfaces, bioturbation, freeze-thaw, seasonal cycling, water infiltration, and tree throw combine to produce downslope translation of colluvium. We term this tabular near-surface region of colluvial accumulation and mobilization the ‘colluvial transport zone’ (CTZ). When offset by surface fault rupture, the CTZ is displaced along the fault with offset equivalents on both the hanging wall and footwall. Following an earthquake, the CTZ continues to generate and transport colluvium downslope; muting or removing the fault scarp, burying the former CTZ material in the normal fault hanging wall or reverse fault footwall, and ultimately reestablishing a uniform CTZ across the fault. Burial of the older (down-dropped) CTZ preserves a colluvial record of the event.

In summary, our work indicates that a model describing the transport and preservation of colluvial bodies may more accurately depict colluvial event stratigraphy in areas where downslope colluviation dominates the geomorphic signal of surface fault rupture and scarp formation. In many instances, both the CTZ and the classical colluvial wedge models should be considered holistically to fully evaluate a fault’s paleoseismic surface rupture history.