CRUSTAL EVOLUTION AS SEEN IN A TRANSECT ACROSS ALASKA, FROM PACIFIC TO ARCTIC MARGINS

The evolution of the Alaskan crust can be inferred from a composite crustal cross section put together from data obtained from the Trans-Alaska Crustal Transect (TACT). TACT, a collaborative project between the USGS and universities, extended approximately 1500 km from the trench in the Gulf of Alaska to the Arctic coast, largely following the trans-Alaska oil pipeline. The data collected included refraction/wide-angle reflection, vertical-incidence reflection, geologic, gravity, magnetic, magnetotelluric, earthquake, and petrophysical data.

The most spectacular structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, the Chugach Mountains are being uplifted by the subduction/collision of Pacific plate (PAC) and the Yakutat terrane (YAK) with North America. Just inboard of this collision is a stack of subcreted oceanic layers similar to the PAC/YAK, that are interpreted as remnants of the extinct Kula (or Resurrection) plate that was converging rapidly on North America in the latest Cretaceous and early Cenozoic. Continental Moho just north of this subcreted stack is more than 55 km deep. Near the Arctic margin, the Brooks Range and southern part of the North Slope are underlain by striking duplex structures that extend ever deeper into the crust from north to south. In one interpretation, these duplexes overlie a crustal-scale wedge consisting of continental crust of the North Slope passive margin and an attached fragment of mantle that has moved southward with respect to the North American plate. Moho has been depressed to nearly 50-km depth. The tectonic history leading to the current structure of northern Alaska is complex, with several compressional episodes and at least one extensional episode occurring between Late Jurassic/Early Cretaceous and the present.

Central Alaska consists of continental-marginal terranes and at least one oceanic arc that collided with these terranes in the Late Jurassic/Early Cretaceous and was partially obducted. Some of these terranes were oroclinally folded in the Early Cretaceous, and many were extended and intruded in the mid-Cretaceous. Offset along major strike-slip faults (Tintina and Denali faults) followed. Moho in central Alaska is everywhere between 30 and 35 km deep.