Abstract

Tertiary strata of the central Sierra Nevada are dominated by widespread, voluminous volcanic breccias that are largely undivided and undated, the origin of which is poorly understood. These dominantly andesitic strata are interpreted to be eruptive products of the ancestral Cascades arc, deposited and preserved within paleocanyons that crossed the present-day Sierra Nevada before Basin and Range faulting began there. These strata are thus important not only for understanding the paleogeography of the Ancestral Cascades arc, but also for reconstructing the evolution of the Sierra Nevada landscape.

Our regional-scale mapping shows that paleocanyon fills of the central Sierra Nevada are dominated by intrusions and vent-proximal facies along the present-day Sierran crest, with more distal facies extending westward down the paleocanyons. Vent-proximal facies consist of lava domes that collapsed to generate block and ash flow tuffs, which in turn were remobilized down-canyon to produce coarse-grained volcanic mudflow and dilute flow (fluvial) deposits. Lava flows are rare; instead, magmas invaded wet volcaniclastic sediment to form in situ peperite piles that were partly remobilized to form debris flows or block and ash flows with peperite domains.

Detailed mapping and dating of previously undifferentiated Tertiary strata in the Carson Pass–Kirkwood area of the central Sierra Nevada has allowed us to identify 6 unconformity-bounded sequences preserved within a paleocanyon as deep as 650 m cut into Mesozoic granitic basement. The Carson Pass–Kirkwood paleocanyon trends NE-SW, with a paleo-transport direction roughly parallel to the modern Mokelumne River drainage (toward the SW). Sequence 1 consists of Oligocene silicic ignimbrites sourced from Nevada. Sequences 2 through 6 consist of dominantly andesitic rocks of the Miocene Ancestral Cascades arc, with 40Ar/39Ar ages ranging from 14.69 ± 0.06 Ma to 6.05 ± 0.12 Ma. These new dates provide constraints on the ages of unconformities.

Vertical relief on the unconformities within the paleocanyon ranges from 12 to 303 meters; with paleoslope gradients range from 3° to 48°. No evidence exists for widening or deepening of the paleocanyon into Mesozoic basement during Tertiary time, so we infer that unconformity 1 (the paleocanyon floor and walls) was inherited from Cretaceous time. The deepest unconformities within the paleocanyon fill reincised into granitic basement in the early Miocene (unconformity 2), between 14 and 10 Ma (unconformity 5), and between 10 and 6 Ma (unconformity 6). The paleocanyon was also beheaded (cut off from sources to the east) by ca. 10 Ma. We suggest that the early Miocene reincision records tectonism related to the onset of arc magmatism in the Sierra Nevada. The middle Miocene reincision may record the onset of Basin and Range faulting in the central Sierra. The late Miocene reincision may correspond to uplift attendant with the northward sweep of the triple junction through the latitude of the central Sierra. Paleocanyons of the central Sierra differ from those of the northern and southern Sierra by showing steeper local paleorelief, and the bouldery stream deposits attest to higher axial gradients than envisioned for other parts of the range. This may indicate that the uplift history of the range is not uniform from segment to segment.

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