Cenozoic palaeocanyon evolution, Ancestral Cascades arc volcanism, and structure of the Hope Valley–Carson Pass region, Sierra Nevada, California

Abstract

We use new geologic mapping, geochronological, and geochemical data on Tertiary volcanic, volcaniclastic, and intrusive rocks to investigate the volcanic, stratigraphic, and structural evolution of the Carson Pass region south of Lake Tahoe in the central Sierra Nevada. Volcanic and volcaniclastic rocks were deposited in east–west-trending palaeocanyons carved into Mesozoic granitic and metamorphic basement rocks; sediments were transported westwards towards the present-day Central Valley of California. In the Carson Pass–Hope Valley area, two palaeotributaries are preserved in faulted terrane east of the present-day Sierran crest (Hope Valley area); these merge at the crest to form one large, >7 km wide palaeocanyon that is undisrupted by faults west of the crest (Carson Pass–Kirkwood area). This single, large palaeocanyon roughly coincides with the present-day Mokelumne River drainage. New ⁴⁰Ar/³⁹Ar dates and stratigraphic data east of the crest, integrated with previously published data west of the crest, constrain the ages of strata and unconformities within the Hope Valley–Carson Pass–Kirkwood palaeocanyon system. We interpret three major erosional unconformities to record uplift events at ca. 23–16, 13.5–11, and 10–7 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 mapped 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 – herein named the Red Lake Fault on the west and the Hope Valley Fault on the east. However, 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 largest volcanic centres in the Sierra, the 6.34 ± 0.14–6.18 ± 0.14 Ma Markleeville Peak Center; (2) andesite lava flows erupted at 6.22 ± 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 altered andesite, indicating that the fault started slipping before magmatism ceased. Our stratigraphic and geochronologic data do not permit an estimate of the amount of pre-6 Ma displacement in the Hope Valley graben. The geometry of the palaeocanyon system indicates that a dextral component of slip demonstrated for transtensional faults in other parts of the region did not operate in the Hope Valley graben.

Keywords:

• Sierra Nevada
• Ancestral Cascades arc
• Walker Lane Belt

Acknowledgements

This work was supported by NSF grant EAR-0711181 (to Busby), NSF grant EAR-0711276 (to Busby and Putirka), and USGS EDMAP grants 05HQAG0010 (to Busby) and 06HQAG061 (to Busby), and a GSA graduate student research grant (to Hagan). We would like to thank our field assistants, including Dana Vukajlovich, Liz Lovelock, Adrian Almanza, Megan Gambs, Chip Hagan, and Senator Kay Hagan. We thank Joyce at the Hope Valley Cafe, for storing gear and providing delicious pie. We thank Steve DeOreo for helping to introduce Hagan to the region, and Duane DeVecchio for visiting the field area and giving geologic advice. Thanks also to Marlon Jean in Putirka's laboratory at CSU Fresno, and Abed Jaouni at the Berkeley Geochronology Center for assistance in the laboratory.