Cretaceous to Recent extension in the Bering Strait region, Alaska

Abstract

A key issue presented by the geology of northern Alaska concerns the demise of the Brooks Range going west toward the Bering Strait region. The main Brookian tectonic and stratigraphic elements continue into the Russian Far East, but the thick crustal root and high elevations that define the modern physiographic Brooks Range die out approaching the Bering and Chukchi shelves, which form an unusually broad area of submerged continental crust. Structural, geochronologic, and apatite fission‐track data indicate that at least three episodes of extension may have affected the crust beneath the Bering Strait region, in the middle to Late Cretaceous, Eocene‐early Oligocene, and Pliocene(?)‐Recent. This extension may explain the present thinner crust of the region, the formation of extensive continental shelves, and the dismemberment and southward translation of tectonic elements as they are traced from the Brooks Range toward Russia. Evidence for these events is recorded within a gently tilted 10‐ to 15‐km thick crustal section exposed on the western Seward Peninsula. The earliest episode is documented at high structural levels by the postcollision exhumation history of blueschists. Structural data indicate exhumation was accomplished in part by thinning of the crust during north‐south extension bracketed between 120 and 90 Ma by 40Ar/39Ar and U‐Pb ages. The Kigluaik Mountains gneiss dome rose through the crust during the later stages of this extension at 91 Ma. Similar gneiss domes occur within a broad, discontinuous belt of Cretaceous magmatism linking interior Alaska with northeast Russia; mantle‐derived melts within this belt likely heated the crust and facilitated extension. Apatite fission‐track ages indicate cooling below ≈120–85°C occurred sometime between 100 and 70 Ma, and the area subsequently resided at shallow crustal depths (<3–4 km) until the present. This suggests that denudation of deep levels of the crust by erosion and/or tectonism was mostly completed by the Late Cretaceous and thus that the present‐day thinner crust of the Bering Strait region developed primarily in the Cretaceous. Regional tilting and at least several more kilometers of local erosion followed in Eocene‐early Oligocene time as documented by fission‐track ages from deeper levels of the crustal section exposed in the Kigluaik Mountains. This event is generally coeval with development of the offshore transtensional Hope and Norton Basins which flank the Seward Peninsula to the north and south. Modern seismicity, active normal faults, and basin‐range topography document Pliocene(?) to Recent north–south extension across the region. Fission‐track data indicate that exhumation during this period was quite limited, less than 2–3 km. This inferred history of protracted extension in the Bering Strait region stands in sharp contrast to well‐documented Cretaceous and Tertiary north–south shortening in interior Alaska. This contrast in tectonic histories suggests a model in which contraction and westward extrusion of crustal fragments from interior Alaska by strike‐slip faulting were accommodated by north–south extension in the Bering Strait region. This resulted in the counterclockwise rotation of extruded crustal blocks, the extensional thinning of the western part of the Brooks Range crustal root, and the formation of transtensional basins and unusually broad continental shelves between Alaska and Russia.