CENOZOIC PALEOCANYON EVOLUTION, ANCESTRAL CASCADES ARC VOLCANISM, AND STRUCTURE OF THE CARSON PASS REGION, SIERRA NEVADA CALIFORNIA

We present new petrographic, geochronological, and geochemical data on Tertiary volcanic, volcaniclastic and intrusive rocks of the Carson Pass region in the central Sierra Nevada. Volcanic-volcaniclastic rocks were deposited in E-W trending canyons carved into Mesozoic granitic and metamorphic basement; these paleocanyons transported material westward toward the present-day Central Valley. We map two paleotributaries in faulted terrane east of the present-day Sierran crest (Hope Valley – Carson Pass area) that merge at the crest to form one large (6 km wide) paleocanyon that continues westward, undisrupted by faults (Carson Pass – Kirkwood area). We integrate new ⁴⁰Ar/³⁹Ar dates and stratigraphic data east of the crest with our previously published data west of the crest, in order to constrain the ages of strata and unconformities within the Hope Valley – Carson Pass - Kirkwood paleocanyon system. We interpret three major erosional unconformities to record uplift events at about 18 – 16 Ma, 13 – 11 Ma, and 8 – 6 Ma. In other parts of the central Sierra, these uplift events are inferred to correspond to range-front faulting events.

We propose the term “Hope Valley graben” for the structural feature we map immediately east of the Sierran crest at Carson Pass. It is a nearly symmetrical full graben that offsets volcanic rocks as young as 6 Ma at least 400 m (1300 ft) on each of its bounding faults (The Red Lake fault on the west and the Hope Valley fault on the east). We infer that faulting began before eruption of the 6 Ma volcanic rocks for three reasons: (1) the graben localized emplacement of one of the biggest volcanic centers in the Sierra, the 6.3 ± 0.14 Ma to 6.14 ± 0.14 Ma Markleeville Peak center; (2) andesite lava flows were erupted at 6.18 ± 0.14 Ma from the Red Lake fault and abut it within the graben; and (3) brecciated granite along the Red Lake fault is intruded by andesite, indicating that the fault started slipping before arc magmatism ceased, although the andesite is too altered to date directly. Our data do not permit us to estimate the amount of pre-6 Ma displacement in the Hope Valley graben. We use the geometry of the paleocanyon system to show that unlike other faults in the region, the faults in Hope Valley graben had no dextral component of slip.