THE BLACK MOUNTAIN TECTONIC ZONE—A LONG-LIVED LITHOSPHERIC SHEAR ZONE IN THE YUKON-TANANA UPLAND OF EAST-CENTRAL ALASKA

The Yukon-Tanana Upland is a greater than 1000 km-long arcuate-shaped crystalline block bounded on the north and south by the Tintina and Denali fault systems, respectively. These two fault systems form a regional dextral wrench zone with several northeast-striking steep faults, such as the Shaw Creek fault, transecting the Upland. One vexing problem is the disparity in age and styles of Mesozoic ductile deformation in the eastern part of the Upland compared with those in the central part of the Upland. Rocks of the eastern area record Jurassic Ar-Ar metamorphic cooling ages and are intruded by plutons as old as Triassic, whereas rock in the central area records overwhelmingly Cretaceous Ar-Ar and U-Pb metamorphic cooling ages and are intruded by plutons no older than mid-Cretaceous.

The newly recognized Black Mountain tectonic zone is a belt of diverse northeast-trending geologic features in the Big Delta 1 degree by 3 degree quadrangle centered on the Black Mountain area. Geologic mapping in the B1 quadrangle of the Big Delta quadrangle has outlined a continuous zone of normal and strike-slip high-angle faults and shear zones, some of which appear to have late Tertiary displacement histories. As well, the zone contains areas of complexly intruded wall rocks and intermingled apopheses of the 107.6+/-1.2 Ma granodioritic Black Mountain intrusion of the Mount Harper batholith, mafic to intermediate composite dike swarms, base and precious metal mineralization, and Tertiary(?) volcanic activity. The surface expression defines a zone as much as 8.5 km wide and more than 33 km long. Recurrent activity along the tectonic zone, from at least mid-Cretaceous (~110 Ma) to late(?) Tertiary, suggests the presence of a long-lived deep crustal feature. The tectonic zone coincides with a profound northeast-trending aeromagnetic gradient that stretches between the Denali and Tintina fault systems. The gradient marks a 30 nanotesla regional shift, with higher values to the northwest. Preliminary depth analysis suggests that this gradient represents a deeply rooted contrast in crustal character across this zone. We suggest that the Black Mountain tectonic zone may have played a key role in controlling the emplacement of Cretaceous plutonism, gold mineralization, as well as Tertiary felsic volcanism in the Upland.