LARGE VERTICAL OFFSETS OF MOHO COINCIDE WITH MAJOR MESOZOIC TRANSFORM FAULTS IN NORTHERN ALASKA (Invited Presentation)

Northern and western Alaska contain several large Proterozoic and early Paleozoic continental blocks that experienced multiple orogenic events. These blocks are thought to have moved close to their present configuration during Mesozoic opening of the Canada basin, but the tectonic processes that juxtaposed them are still debated. Preliminary Earthscope transportable array (TA) data analysis provides estimates of depth to Moho that can be used to evaluate the location and character of tectonic boundaries. Laterally abrupt changes in Moho depth across some terrarne boundaries in northern Alaska support tectonic scenarios that include a significant component of transform motion.

Researchers generally agree that during the early Cretaceous the Pacific-facing margin of the Arctic Alaska microplate collided with an ocean island arc complex or complexes, producing blueschist and eclogite facies metamorphism in continental crust. The Ruby terrane, a continental block that currently lies south of the Arctic Alaska microplate, shares a common Devonian-late Jurassic tectonic history with it. We have previously proposed that a combination of transform and rotation displacement during and after the arc-continent collision slivered the collisional orogen into crustal blocks now juxtaposed by high-angle boundaries. One major dextral transform, we name the Ruby fault, bounds the Ruby terrane on the north. This fault translated the Ruby terrane south of the Arctic Alaska microplate and obliquely across the Yukon-Koyukuk basin. The southern margin of the Ruby terrane also was likely truncated by a high-angle fault (Poorman), juxtaposing it with Neoproterozoic to lower Paleozoic sediments of the Farewell terrane. Both the Ruby and Poorman faults are offset by the latest Cretaceous-early Cenozoic Kobuk and Kaltag faults and the Ruby fault truncates early Cretaceous structures.

The preliminary TA data analysis of this region includes three relatively closely spaced stations that show significantly thinner crust of the Ruby terrane bounded by thicker crust of the Farewell terrane and Yukon-Koyukuk basin. Significant vertical offset (up to 10 km) of the Moho occurs across the Ruby and Poorman faults. The timing constraints permit the Moho offset faults to have formed during opening of the Canada basin.