Tertiary tectonics of the Border Ranges Fault System, Chugach Mountains, Alaska: Deformation and uplift in a forearc setting

The Border Ranges fault system (BRFS) locally separates supracrustal Upper Cretaceous-Tertiary rocks of the Cook Inlet/Matanuska Valley forearc basin on the Peninsular terrane from metamorphosed Cretaceous subduction complex rocks of the Chugach terrane to the south. This 5- to 10-km-wide zone of concentrated deformation along the inboard edge of southern Alaska's accretionary prism has suffered a protracted history, beginning with its inception as a “megathrust” by Early Cretaceous time and continuing into early Tertiary time, when it was transected by a system of steeply arcward dipping normal- and dextral-slip faults. Based on detailed geologic mapping, structural analysis, metamorphic mineral assemblage data, and K-Ar and fission track geochronology, the Tertiary history of displacements along a central part of this forearc fault system has been divided into several phases of deformation. These include a Paleocene phase of regional uplift of the forearc basin together with block faulting along the BRFS to rapidly uplift and subaerially expose low-grade metamorphic rocks of the subduction complex (Upper Cretaceous Valdez Group) within 15 Ma of their inferred underplating at deep structural levels of the accretionary prism. Continued normal-separation faulting along the BRFS in late Paleocene/early Eocene time to further uplift the subduction complex was recorded by deposition of a syntectonic alluvial fan system (Chickaloon Formation) across the BRFS and along the southern margin of the forearc basin. A third, mid Eocene, increment of normal-separation faulting along the BRFS juxtaposed greenschist facies fabrics of early Eocene age in the subduction complex against the Chickaloon Formation. This latter uplift event predated intrusion of early late Eocene (∼43–48 Ma, zircon fission track minimum ages) felsic dikes into the present terrane-bounding fault and was accompanied by no more than a few tens of kilometers of dextral-slip faulting along the BRFS, a conclusion that does not accord with recent suggestions for thousands of kilometers of displacement along that fault system in early-middle Tertiary time, based on paleomagnetic data. Field observations in the study area indicate that the great oroclinal bend of southern Alaska is postearly Eocene and possibly prelate Oligocene in age. A final period of deformation in middle to late Tertiary time was marked by periods of minor north-south contraction of the forearc basin and by local reverse-slip reactivation of earlier faults. Fission track age data on apatite indicate rapid cooling and probably uplift of both subduction complex and adjacent parts of the forearc basin in Miocene time (∼17–24 Ma). Episodic uplift of the subduction complex, either locally by normal-separation faulting along its “backstop,” or by a more regional uplift also affecting the adjacent forearc basin, overlapped in time or closely followed discrete pulses of large-scale sediment underplating along the Gulf of Alaska subduction margin. This coincidence suggests that major pulses of sediment underplating along southern Alaska's convergent margin may have led to isostatic uplift of its “critical-taper” accretionary prism, an uplift that was in part accommodated by normal-separation faulting along its mechanical backstop.