Dextral-slip reactivation of an arc-forearc boundary during Late Cretaceous-early Eocene oblique convergence in the northern Cordillera

Abstract

The Border Ranges fault system forms the most fundamental crustal boundary in southern Alaska, separating crystalline basement with Paleozoic and Mesozoic arc sequences, the Wrangellia terrane, from a long-lived accretionary complex, the Chugach terrane. Although the Border Ranges fault system originated as a subduction zone megathrust, few strands of it preserve that original history. This study shows that a dextral fault zone, herein named the Hanagita fault system, has overprinted the subduction zone megathrust. This dextral fault zone is 5-10 km wide and has been traced continuously for ∼250 km. The Hanagita fault system is probably continuous with dextral slip systems described in Glacier Bay and Baranof Island in southeast Alaska, which would extend its length to ∼700 km. Within this fault zone, elements of Wrangellia and the Chugach terrane are complexly intermixed. Geologic mapping and 4 0 Ar/ 3 9 Ar geochronology of white mica crystallized during dextral slip indicate the strike-slip faulting may have begun by ∼85 Ma and was continuous from ∼70 Ma until ∼51 Ma. Accretion of a thick flysch sequence along this margin began in the Maastrichtian (latest Cretaceous) and continued uninterrupted through middle Eocene. Migration of a ridge-trench-trench triple junction along this margin during the late Paleocene-early Eocene introduced hot fluids to the fault zone and plutons on the southern edge of the fault system. Dextral slip occurred before, during, and after the triple junction migration. Thus, the Hanagita fault system records long-lived oblique convergence between two separate oceanic plates and North America and is a relatively well-preserved example of displacement partitioning along the backstop of an accretionary prism. The Hanagita fault system includes several through-going dextral high-angle faults that are continuous on the scale of 10s to 100+ km. These strike WNW, subparallel to the structural grain in the accretionary prism to the south. Between these strands are numerous shear zones and faults that are continuous on the scale of several 100 m to several km. The majority of these also show subhorizontal to oblique slip, but the dip of the fault planes is more variable than the through-going faults. The western part of the fault system has north-dipping thrust and oblique slip faults that formed approximately synchronously with south-dipping oblique slip faults 100 km to the east. The faults formed at progressively lower temperatures through time based on cross-cutting relations and 4 0 Ar/ 3 9 Ar white mica dates of fault fabrics. The oldest shear zones formed at low greenschist facies, and the youngest structures are discrete brittle faults and gouge zones. Rocks within the fault system include crystalline rocks from the inboard side of the fault system and melange of the McHugh Complex, the oldest part of the accretionary prism in this area. The crystalline rocks include abundant Late Jurassic diorite (U/Pb zircon date) with 4 0 Ar/ 3 9 Ar hornblende cooling ages of 142-146 Ma, which correlates with the Chitina batholith of the Wrangell Mountains and the St. Elias Plutonic Suite of the St. Elias range. Other crystalline rocks include an ∼170 Ma diorite (hornblende 4 0 Ar/ 3 9 Ar) associated with abundant metabasite and leucocratic plutonic rock. No known correlative to these units occurs for a minimum of 600 km to the southeast along the Border Ranges fault system. Thus, the total amount of dextral displacement along the Hanagita fault system from the Late Cretaceous to Middle Eocene is conservatively estimated at 600 km but could be >1000 km, based on correlation of the 170 Ma pluton and related rocks to a similar suite on the west side of Vancouver Island. This displacement is also consistent with regional geology elsewhere in the accretionary prism. The timing of displacement on the fault system coincides with dextral slip on several other major faults, parallel but more inboard from the margin, incl