LATE OLIGOCENE UPLIFT, INVERSION AND EMERGENCE OF THE CASCADIA FOREARC BASIN, SOUTHERN OREGON COAST RANGE, USA

The Cascadia margin is an archetypal active arc-trench system where Eocene to Miocene pre-arc and forearc sedimentary and volcanic rocks are exposed in the slowly (~0.1 mm/yr) uplifting and eroding Oregon Coast Range (OCR). Despite this general knowledge, the timing and spatial patterns of uplift and relief generation, and the geodynamic mechanisms responsible for them, are not well understood. The age of onset of forearc uplift is particularly difficult to address in the OCR where erosion has almost completely removed late Eocene to Miocene marine deposits from the axis of the range. Isolated inboard and coastal outcrops of far-travelled Columbia River basalt flows hint at an emergent and likely dissected topographic barrier in the OCR by ~16 Ma, but the initiation age of uplift and basin inversion remains undocumented.

Here we present a large detrital zircon U-Pb data set (n = 6,324 ages) from the Eocene-Miocene Coos Bay Basin and from modern rivers draining the ancestral Cascades arc (ACA) in southwestern Oregon that constrains the depositional ages of Coos Bay strata, the onset of ACA magmatism, and the initiation of uplift and inversion of the Cascadia forearc basin in the southern OCR. Arc magmatism initiated at ~40 Ma along the entire southern Oregon segment of the ACA, with high flux events at around 34 and 24 Ma. In the Coos Bay Basin, we document a major change in provenance during middle Miocene (~18–15 Ma) deposition of the Tarheel formation that is marked by (1) disappearance of ACA zircon (40–12 Ma), and (2) onset of recycling of pre-ACA Challis-Clarno zircon (52–40 Ma) from underlying strata. This provenance change corresponds precisely with a regional angular unconformity between the Oligocene Tunnel Point Sandstone (~33.5–30 Ma) and the overlying Tarheel formation.

The data indicate that uplift and inversion of the forearc basin to emergence of the southern OCR initiated in late Oligocene to early Miocene time (~30–18 Ma). Structural and stratigraphic relations record subsequent pulses of middle to late Miocene deformation and/or erosion in the Coos Bay area. The 15–20 Myr delay between the initiation of Cascadia subduction at ca. 49–46 Ma and the onset of forearc uplift is consistent with modelling studies that show how both tectonic underplating and thermally-activated lower crustal flow (driven by thick sedimentation) can each promote forearc deformation and uplift over similar spatial and temporal scales.