EXTENT, STYLE AND AGE OF BASIN AND RANGE FAULTING EAST OF PYRAMID LAKE

Interpretations of global positioning system and earthquake focal mechanism data suggest strain partitioning to accommodate a broad transition zone between strike slip and extensional faulting in the northwestern Basin and Range. To address the nature of this transition zone, we've begun investigations of the history of faulting across an area to the east of the Pyramid Lake strike-slip fault zone. Along this ~35 km transect are a series of north-south trending extensional fault blocks (the Lake, Shawave, Nightingale, and Selenite ranges) of Mesozoic granite unconformably overlain by Tertiary volcanic and sedimentary sections.

Stratigraphic and structural investigations of the Tertiary volcanic and sedimentary sequences provide preliminary constraints on the timing and amount of extension across these four ranges. At the western end of the transect are the east-dipping (~20 degrees) Tertiary volcanics of the Lake range that are tilted due to slip across the unnamed west-dipping fault on the west side of the Selenite-Shawave block. Between the northern Nightingale and southern Selenite ranges is a thick (>1 km) section of interbedded pyroclastic ash fall, vesicular basalt, flow breccia, and andesite units dipping 33 degrees to the southeast which are cut by a rhyolite dome. Rocks of this section may correlate with the ~13.8-15.0 Ma Pyramid Sequence mapped to the west in the Virginia Mtns and Lake Range. These strongly tilted units are overlain by a section of gently dipping (8 degrees) Miocene diatomite, ash fall sediments, and debris flows capped by a basaltic layer. At the eastern end of the transect are west-dipping interbedded Tertiary sediments tilted ~21 degrees by motion on the Granite Springs fault, the range bounding fault on the east side of the Shawave Mtns. Preliminary apatite fission track thermochronometry and geomorphology show uplift along the Granite Springs fault began no later than 12 Ma and continued into the Quaternary Period.

Constraints on tilted Miocene volcanic and sedimentary sections across this area show dip slip motion along N-S trending, range bounding faults initiated no later than 12Ma. Geomorphic interpretation of non-range bounding, Quaternary faults imply dip-slip offset and a shift in fault orientation from ~N-S to ~N40E which may be related to the onset of strike slip faulting within the adjacent Walker Lane belt.