Lithosphere-Asthenosphere Architecture in the Transition from the Amerasian Basin to the Laptev Shelf, from Gravity Modeling and Crustal-Scale Seismic Imaging

James Granath

Processing of 16-second, long-offset seismic records in the ION BasinSPAN program is typically supported by gravity modeling along the trace of the lines to help derive deep seismic velocities. In the case of the Eastern Russia and Siberia (ERAS-1) free air gravity grids from the open-file Arctic Gravity Project were sampled along the SPAN lines. This compilation of marine and airborne data (in 5 km grid cells) is good quality and easily sufficient to capture both long and short wavelength anomalies produced by the sedimentary section, crystalline crust, as well as deeper density contrasts in the upper mantle and asthenosphere. The resulting models show an extraordinarily crisp picture of the structure of the crust and upper mantle. Interpretation consistency is achieved by using constant densities for all modeled layers: water (1.03 g/cc), four sedimentary rock layers (2.0 to 2.5 g/cc), three crystalline crustal layers (2.67 to 3.05 g/cc), and mantle (3.2 g/cc). This ERAS-1 survey is unique among the SPAN surveys in that it was acquired in the immediate area of an active mid-ocean ridge where shallow asthenosphere is expected at shallow depths. Even though the lithosphere-asthenosphere boundary (LAB) is not visible in the seismic imaging, the best gravity models show the presence of the asthenosphere within the 40 km depth imaging window. Where the Gakkel Ridge projects into the Laptev Shelf, matches between calculated and observed gravity profiles were dependent on introducing an LAB-related density contrast of -0.1 to -0.22 g/cc. Near the intersection of the Gakkel Ridge and the shelf edge this LAB is as shallow as 6.5 km, and plunges to depths > 50 km just 30 km inboard of the shelf edge. The Moho is well imaged across all of these lines and is interpreted to lie between 20 and 32 km beneath the Laptev Shelf. The Laptev Shelf is characterized by a set of well-known rift basins. The gravity modeling shows that the sedimentary section filling those rifts reaches 9-10 km in thickness above a density contrast between 2.5 and 2.65 g/cc at the top of crystalline basement. This basement interface is markedly rugose in a basin-and-range style fault block array along the rim of the shelf, suggesting an extensional gradient between the transform at the shelf edge and the discrete rift basins to the south. The associated faults planes appear to root into a brittle-ductile transition near 13 to 15 km depth.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013