History of the Chukchi borderland and the Amerasia basin, Arctic Ocean

Abstract

Structural and stratigraphic interpretation of 2D multi-channel seismic (MCS) reflection profiles through recognition of the sub-surface reflection patterns and integration of the seismic interpretation with the other geophysical and geological data reveal the history of the Chukchi Borderland. This investigation provides new constraints for the tectonic development of the Amerasia Basin. North-striking normal faults of the Chukchi Borderland dissect the continental basement into the Chukchi Plateau, Northwind Basin and Northwind Ridge from west to east. A well-developed angular unconformity (Au) separates the stratigraphic section into sub and super-Au seismic units. Sub-Au units include: (1) seaward dipping reflections (SDRs) observed in the juncture between the North Chukchi-Toll Basins and Chukchi Plateau; (2) growth and folded strata in the Northwind Basin; (3) thrust faults in the Northwind Basin and over the Northwind Ridge; and (4) a clinoform sequence that downlaps onto the extended continental crust of the Canada Basin, supported by presence of SDRs and diapiric reflections within the crust. Au is inferred to correlate to the Hauterivian (LCu) and the Middle Jurassic (Ju) unconformities of the Alaska North Slope. The SDRs indicate that the southwestern margin of the Chukchi Borderland may be a rifted continental margin. Loosely constrained age control of a super-Au unit (inferred condensed section, perhaps correlative to Hauterivian pebble shale or the Jurassic upper Kingak shale units of Alaska North Slope) implies that the rifted margin subsided no later than the earliest Cretaceous, providing a plausible time constraint for Middle Jurassic-earliest Cretaceous rifting in the North Chukchi Basin. The growth strata and north-striking normal faults of the Northwind Basin are continuous with the extensional structures of the Mississippian Hanna Trough, providing a geologic linkage between the two. The folding and thrust faults reveal a phase of contraction confined to sub-Au units of the south and eastern Northwind Basin and Northwind Ridge. The clinoform sequence of the Northwind Ridge-Canada Basin is inferred to correlate with the Upper Jurassic-Lower Cretaceous Kingak shale unit of Alaska North Slope, implying that the extension of the crust beneath the western Canada Basin occurred no later than the Middle Jurassic. Super-Au strata (~16 km) onlap the condensed section, SDRs, growth and passive margin strata from west to east, tapering down to a few kilometers north and eastward across the seismic grid. These are part of the Aptian through Cenozoic Brookian megasequence, a series of clinothems, deposited across the foreland of the Chukotka and Brooks Range orogens. These strata were deposited by northward-migrating depositional systems that progressively filled the North Chukchi Basin and buried the southern flank of the Chukchi Borderland, and deposited along the Northwind margin of the Canada Basin. Another unit of growth strata is observed in the Northwind Basin, indicating another phase of extension of the Boderland. The Upper Cretaceous section of the Brookian megasequence is displaced by normal faults over the Chukchi Plateau and inferred age-equivalent strata over the Northwind Ridge. These constrain the second phase of extension of the interior Borderland to the Late Cretaceous to Paleocene. The recognition of the sub-Au units and continuity of the super-Au units across the area, north-striking normal faults, and the absence of east-directed thrust faults between the Northwind Ridge and Canada Basin invalidate one model proposed for tectonic development of the Amerasia Basin. Models that require significant relative motion between the Chukchi Shelf and Borderland since the Middle Jurassic are precluded by these observations.