EMPLACEMENT AND SUBSEQUENT MODIFICATION OF THE QUATERNARY EUREKA VALLEY LANDSLIDE, EASTERN CALIFORNIA

The Eureka Valley landslide (EVL) is in the SE Eureka Valley, west of the Last Chance Range (LCR). The EVL deposit is an elongated, lobate, 1700 m long, 400 m wide sheet that trends ~N60W, nearly orthogonal to the LCR front trend. The source region is at ~1800 m at the top of the LCR western margin. It is a translational landslide because it was translated as a sheet and lacks a curved surface of rupture and distinct rotational blocks. The EVL likely initiated along a paleolandslide scarp, which was part of the closely spaced, west-dipping regional range-bounding normal faults of the LCR west margin. The triggering mechanism may be a seismic event and/ or a very wet season. The EVL is monolithologic limestone breccia and the original source region stratigraphy is mostly preserved. The edges of the EVL are ~16 m high cliffs, which expose west-dipping normal faults that offset the layered breccia for 5-7 m. Some normal faults terminate upward at a flat detachment surface below a thin layer of sediment. This geometric relationship is characteristic of an extensional duplex system. At the surface the normal faults strike obliquely from the edges, forming a series of minor en echelon ridges and shallow troughs oblique to the main lateral ridge. The west-dipping normal faults, their en echelon surficial pattern and a superposed mechanical stratigraphy contribute to an interpreted 3D simple shear model in which the normal faults in cross section view are Riedel shears that accommodate top-west EVL transport via layer-parallel simple shear. The en echelon fault pattern in map view is likely generated by a Poiseuille-like flow, characterized by two parallel shear zones with opposite senses of shear accommodating faster motion along the long axis of the EVL sheet. The relationship of the EVL to alluvial fan channels in the SE means the EVL was likely emplaced in the Quaternary. After emplacement, part of the EVL deposit underwent erosive fluvial modification, mainly by development of alluvial channels, which removed most of the SE half of the EVL and deposited it on older alluvial fans. Also, the block underneath the EVL rotated back towards the LCR and the resulting valley was filled by erosional alluvium, producing a Buttress unconformity. The EVL is also used as an analogue for planetary work to ground truth and evaluate remotely sensed observations.