Abstract

Subduction erosion is an important process at convergent margins but evidence in the geologic record is scarce because the involved materials are typically lost into the mantle. Detrital zircon dating of accretionary complex sediment allows us to document episodic growth that can be linked to possible triggers. Detrital zircons (n = 2508) from the Mesozoic Chugach accretionary complex, southern Alaska, have U-Pb ages that record progressive subduction accretion punctuated by two periods of tectonic erosion. Lithology and maximum depositional ages permit division of the Chugach accretionary complex into four main units. The oldest, the blueschist-greenschist unit, represents partially subducted sediment associated with a Jurassic oceanic arc, with subduction erosion from 180 to 170 Ma. The end of this erosion period is dated by the oldest maximum depositional age of the Potter Creek assemblage, a <156–169 Ma unit consisting of chert, argillite, and volcanic rocks. A trondhjemite pluton that intrudes the forearc was caused by ridge subduction at 125 Ma and resulted in a second period of subduction erosion lasting until 104 Ma.

The end of Aptian–Albian erosion was marked by deposition of massive sandstone and conglomerate of the 101–91 Ma McHugh Creek assemblage. This influx of clastic sediment is interpreted to have occurred in response to the collision of Wrangellia with North America. This event and the erosional event preceding it steepened the forearc region, allowing mass wasting of forearc crust into the trench, filling it by 89 Ma, the oldest maximum depositional age of the Valdez Group flysch. Clasts in conglomerate include granodiorite from the basement of the Talkeetna arc dated from 199 ± 3 Ma to 179 ± 3 Ma, and sandstone clasts with maximum depositional ages of 100 Ma. From 89 Ma to at least 72 Ma, the Valdez Group flysch was deposited via turbidite fans onto the oceanic crust beyond the trench and accreted as imbricate thrust slices that retained coherent bedding.

The source for detrital zircons in the Potter Creek assemblage is likely a Middle–Late Jurassic oceanic arc, possibly the Talkeetna arc. The abundance of zircons from ash fall tuffs is consistent with easterly winds and suggests the Chugach accretionary complex was south of latitude 25°N in the Late Jurassic. The dominant source for the Albian McHugh Creek assemblage and the Upper Cretaceous Valdez Group flysch was likely the arc associated with the Coast Mountains batholith. Jurassic (ca. 165 Ma) zircons in the McHugh Creek assemblage could have been derived from exhumed plutons, or may be second-cycle zircons derived from the Potter Creek assemblage. The first appearance of Proterozoic and Archean zircons in the Valdez Group records the breakdown of topographic barriers formed by the accreted arc terranes as rivers encroached into continental North America by the middle Late Cretaceous.

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