The Reactivation of the Lost Lakes Fault in Yosemite National Park (USA): When Did it Happen and What Does it Mean?
Richard Becker, UW-Madison
Basil Tikoff, UW-Madison
David Greene, Denison University
The Lost Lakes Fault is an 8 km long NW-striking structure that parallels the Sierra Crest in Yosemite National Park. The northernmost 3 km is exposed in a pair of SW-facing cirques while the southern portion lies in a SE-draining glacial basin. In the cirques, the fault is marked by a prominent scarp that dips 80 degrees northeast and ranges in height from 0-8 m, with 2-4 m being more typical. In the basin to the south, the topographic expression of the fault is minor.
Both sub-horizontal and sub-vertical lineations are present along the scarp and field evidence indicates multiple periods of fault activity: The presence of pseudotachylite, cataclasite, chlorite, quartz, and epidote along the fault surface suggests deformation at substantially greater pressures and temperatures than those provided by the near-surface environment. We interpret these materials to be the result of an early period of dextral strike-slip deformation, possibly Cretaceous in timing. A later period of normal faulting is indicated by the sub-vertical lineations and an offset of the Eocene erosion surface along the ridgeline between the two cirques. This displacement might have happened as recently as the last deglaciation. Efforts to further constrain the timing and nature of deformation along the Lost Lakes Fault continue.