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Implications for Timing and Continuity of Sediment Source Rock Exhumation along the Upper Alaska Peninsula-Cook Inlet Forearc Basin Corridor, Alaska, from Apatite and Zircon Fission-Track Thermochronology

Gillis, Robert J.1; Lepain, David L.1; Reifenstuhl, Rocky R.1; Decker, Paul 2
1 Division of Geological & Geophysical Surveys, State of Alaska, Fairbanks, AK.
2 Division of Oil & Gas, State of Alaska, Anchorage, AK.

Extending ~530 km from the upper Alaska Peninsula (AKP) northeastward along the western margin of the prospective Cook Inlet (CI) forearc basin, the Bruin Bay fault and associated structures represent a major, long-lived, tectonic boundary that has accommodated crustal-scale stresses generated by the subduction of oceanic lithosphere beneath continental North America since Middle to Late Jurassic time. Motion along the transpressional fault system has exerted a fundamental control on the temporal and areal distribution of forearc basin reservoir rock facies, yet little is known about the fault kinematics along the basin margin. Understanding the timing, rate, and continuity of sediment source-rock exhumation across major faults can provide important constraints on the kinematic history of faulted terranes, especially when considered with the depositional history of adjacent sedimentary basins. Thirty new apatite and eleven new zircon fission-track ages from samples collected at five locations across the Bruin Bay fault on the upper AKP, and the Lake Clark and Capps Glacier faults in the upper CI area span Middle Jurassic to Late Oligocene time. Preliminary results suggest that cooling episodes recorded in intrusive arc rocks are broadly coeval with periods of arc-derived proximal sedimentation in the forearc basin and are interpreted to represent cooling during exhumation of the arc roots. Commonly contrasting hangingwall and footwall cooling histories suggest that cooling was driven, in part, by differential exhumation across fault boundaries. On the upper AKP, Jurassic-age arc rocks record two periods of rapid exhumation (Middle to Late Jurassic time and Middle to Late Eocene time) that overlap with deposition of the Naknek and Copper Lake Formations, respectively. Preliminary results from Cretaceous-Tertiary granitoids along the northwestern CI basin margin record exhumation during Early to Late Eocene time that is coeval with West Foreland Formation and lower Hemlock Conglomerate deposition. Periods of Early Cretaceous and Late Eocene cooling of upper AKP basin deposits are interpreted as episodes of basin inversion and exhumation. Accelerated Eocene exhumation of the AKP and northwestern CI margin overlaps with the timing of Kula-Farallon ridge subduction and associated passing of a slab window, perhaps implying a link between Paleogene reorganization of the subducting plate and regional exhumation along the AKP-CI corridor.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009