Tectonic Evolution of Northern Alaska
Cross Sections
The following cross sections are taken from Moore et al., 1994 in The Geology of Alaska. They begin in the mid-Jurassic and continue through early Cretaceous. The Brookian Orogeny occurred during this time period. The
map reconstructions show a greater amount of tectonic history, while the following cross sections show, in much greater detail, the development of a single orogeny. Note: the directions indicated on the sections (N and S) refer to present directions, not paleodirections. Also note: the Arctic Composite Terrane is herein referred to as the Arctic Alaska terrane. Same thing!
Timeline of the Brookian Orogeny:
- Mid Jurassic: Paleozoic/Mesozoic oceanic lithosphere begins to subduct "southward" below the Koyukuk arc (rocks which are south of the Brooks Range today), closing the Angayucham ocean. A subduction complex, composed of oceanic sediments and volcanic edifices scraped off the subducting slab, is formed in the subduction zone.
- Late Jurassic: Angayucham ocean is nearly closed. Passive margin sedimentation continues off the "southern" margin of the Arctic Alaska terrane (AAT). Initial rifting in the AAT.
- Early Cretaceous: Contact! The AAT reaches the subduction zone and is partially underthrust beneath oceanic crust. Thrust faults form, bringing slivers (allochthons) of AAT passive margin sediments up out of the subduction zone, over AAT rocks to the "north". The thrust faults bound a series of these imbricated allochthons.
- Mid Early Cretaceous: "North"-directed thrusting of passive margin sediments and oceanic crust continues. Rifting develops in the "north" of the AAT, forming the Canada Basin, later to become the Arctic Ocean.
- Late Early Cretaceous: Mid to lower crustal rocks, subducted to the P,T conditions of blueschist facies metamorphism, are uplifted via thrust tectonics. The lower crust thickens; imbrication, duplexing and out-of-sequence faulting occur and carry deep rocks nearer to the surface, where they are exposed by a combination of normal faulting and erosion.
- Latest Early Cretaceous: Thickening of the mid to lower crust continues, while the upper crust extends. Normal faults at the southern edge of the Brooks Range result in structural unroofing, allowing metamorphic rocks to come to the surface. Major clastic sedimentation occurs both southward into the Koyukuk Basin and northward into the Colville Basin.
Figures are from Moore et al., 1994 in The Geology of Alaska
