Late Middle Miocene Emergence of the Olympic Peninsula Shown by Sedimentary Provenance

Samuel Shekut,Alexis Licht,Tamer S Abu-Alam

doi:10.2113/2020/7040598

Abstract

AbstractThe Olympic Peninsula is the uplifted portion of the Cascadia accretionary wedge and forms the core of a 200 km scale oroclinal bend on the west coast of Washington State. The accretionary wedge started forming 45 million years ago following the accretion of the Siletzia igneous province along the Cascadia subduction margin. Low-temperature thermochronology studies have shown that the core of the peninsula has been continuously exhumed for the last 14 million years. The earlier onset of oroclinal bending, uplift, and emergence remains poorly documented. Here, we explore the Cenozoic drainage history of the Cascadia forearc and accretionary wedge to reconstruct the deformation history of the Olympic Peninsula. We use detrital zircon provenance and grain petrography data from modern rivers draining the Cascades, the Cascadia forearc and accretionary wedge, as well as from Eocene to late middle Miocene sedimentary units from the same areas. We first show a clear difference of sedimentary provenance between sedimentary units in the accretionary wedge, with older units reflecting mélange and imbricated strata that began as part of Siletzia, and younger units reflecting trench-fill material sourced from the Cascades and accreted to the wedge. We show that the accretionary wedge was directly fed from the Cascade arc until at least 16.5±0.5 Ma, providing a maximum age for the emergence of the Olympic Peninsula. Fluvial deposits in the Cascadia forearc basin dated at 13.3±1.3 Ma display zircon age spectra and sedimentary grain petrography features typical of recycled accretionary wedge material. Although these deposits may also reflect local input, middle Miocene exhumation rates suggest the Olympic Peninsula was an active sediment source. Our results bracket the timing of emergence of the Olympic Peninsula to a narrow window in the late middle Miocene. We suggest that the initial onset of accretionary wedge deformation and oroclinal bending predates this by at least 10 million years, in the upper Oligocene, and is marked by flexural subsidence and high sedimentation rates recorded in strata of the Seattle Basin. Our results support a composite history for the development of the Cascadia accretionary wedge rather than models predicting a gradual and steady build-up.

Highlights

The Olympic Peninsula of Western Washington State is the subaerially exposed portion of the Cascadia accretionary wedge [1, 2], which developed following the Eocene accretion of the Siletzia igneous province along the west coast of North America [3, 4]
Age distributions and sedimentary grain petrography of the Miocene coastal Olympic subduction complex (OSC) and modern Olympic rivers indicate that they are derived from the Cascades; this is consistent with deposition and incorporation in a typical accretionary wedge, with Cascade material transported through the forearc and into the subduction trench [27]
These data support the interpretation of Brandon et al [7] who proposed that the upper OSC was deposited as the western continuation of the Siletzia terrane, and was later imbricated and underthrust beneath the eastern strata of Siletzia following the initiation of the Cascadia subduction zone (Figure 9: middle Eocene to middle Miocene)

Summary

Introduction

The Olympic Peninsula of Western Washington State is the subaerially exposed portion of the Cascadia accretionary wedge [1, 2], which developed following the Eocene accretion of the Siletzia igneous province along the west coast of North America [3, 4]. Substantial deformation and uplift of the Cascadia accretionary wedge have resulted in a large oroclinal bend with the Olympic Mountain range at its core [5]. These mountains are unique in that they are larger and higher than all other coast mountain ranges formed by accretionary wedges on the Pacific Coast of North America.

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