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Mesozoic and Cenozoic tectonic growth of southern Alaska: A sedimentary basin perspective

By
Jeffrey M. Trop
Jeffrey M. Trop
1
Department of Geology, Bucknell University, Moore Avenue, Lewisburg, Pennsylvania, 17837, USA
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Kenneth D. Ridgway
Kenneth D. Ridgway
2
Department of Earth and Atmospheric Sciences, 550 Stadium Mall Drive, Purdue University, West Lafayette, Indiana 47907-2051, USA
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Published:
January 01, 2007

Mesozoic and Cenozoic sedimentary strata exposed throughout southern Alaska contain a rich archive of information on the growth of collisional continental margins through the processes of terrane accretion, magmatism, accretionary prism development, and subduction of oceanic spreading ridges. Two major collisional events define the tectonic growth of southern Alaska: Mesozoic collision of the Wrangellia composite terrane and Cenozoic collision of the Yakutat terrane. The sedimentary record of these two collisional events can be summarized as follows. (1) Middle Jurassic volcaniclastic and sedimentary strata represent shallow-marine deposition in narrow subbasins adjacent to the volcanic edifice of the south-facing, intraoceanic Talkeetna arc. (2) Late Jurassic syndepositional regional shortening resulted in thick sections of conglomerate in proximal parts of both retroarc and forearc basins. In distal retroarc depocenters, fine-grained turbidite sedimentation commenced in a series of basins that presently extend for >2000 km along strike. This time interval also marked cessation of magmatism and rapid exhumation of the Talkeetna oceanic arc. We interpret these observations to reflect initial oblique collision, younging to the northwest, of the Wrangellia composite terrane with the continental margin of western North America. (3) During Early Cretaceous time, Jurassic retroarc basin strata were incorporated into an expanding northverging thrust belt, and sediment was bypassed into more distal collisional retroarc basins located within the suture zone. Compositional data from these collisional basins show that the Wrangellia composite terrane was exhumed to deep stratigraphic levels. Detrital zircon ages from strata in these basins record some sediment derivation from source areas with North American continental margin affinity. Our data clearly show that the Wrangellia composite terrane and the former continental margin were in close proximity by this time. Accretion of this oceanic terrane and associated basinal deposits marked one of the largest additions of juvenile crust to western North America. The collision of the Wrangellia composite terrane also resulted in a change in subduction parameters that eventually prompted development of a new south-facing arc system, the Chisana arc. Construction of this arc was contemporaneous with renewed forearc basin subsidence and sedimentation. (4) Late Early Cretaceous to early Late Cretaceous time was characterized by regional deformation of retroarc collisional basin strata by south-verging thrust faults that are part of a regional thrust belt that extends throughout the northwestern Cordillera. (5) Latest Cretaceous time was characterized by synorogenic sedimentation in forearc and retroarc basins related to regional shortening and exhumation of a coeval continentalmargin arc and older collisional basinal deposits. Forearc depocenters subsided into deep-water settings between the arc and expanding accretionary prism. Nonmarine to marginal-marine strata accumulated in retroarc depocenters influenced by syndepositional thrust-fault deformation. (6) Growth of the southern Alaska continental margin during Paleocene to Early Eocene time is defined by regional nonmarine deposition, magmatism within the suture zone, and expansion of the accretionary prism. Oblique subduction of an oceanic spreading ridge prompted diachronous deformation, synorogenic sedimentation, and magmatism. Subduction of progressively more buoyant, topographically higher lithosphere (the spreading ridge) followed by less buoyant, topographically lower lithosphere prompted coarsegrained alluvial-fluvial sedimentation and slab-window magmatism in remnant forearc basins. (7) Regional transpressive deformation characterized southern Alaska during Middle Eocene to Oligocene time. This deformation generated coarse-grained alluvial-fluvial sedimentation in narrow fault-bound basins along major strike-slip faults, including the Denali and Castle Mountain faults. (8) A second major phase of terrane collision and basin development shaped the southern margin of Alaska during latest Oligocene to Holocene time. Northward translation and collision of the Yakutat terrane, an excised continental fragment of western North America, prompted growth of the largest coastal mountain range on Earth, construction of a new magmatic arc, exhumation of older fore-arc basinal strata, and renewed uplift of the Alaska Range. The sedimentary record of this collisional event is contained in a collisional foreland basin on the north side of the Alaska Range, intraarc basins in the Wrangell Mountains, and a collisional foreland basin within the Yakutat terrane. This phase of collision continues to the present as evidenced by active mountain building, large-magnitude earthquakes, and some of the highest sediment accumulation rates on Earth.

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GSA Special Papers

Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska

Kenneth D. Ridgway
Kenneth D. Ridgway
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Jeffrey M. Trop
Jeffrey M. Trop
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Jonathan M.G. Glen
Jonathan M.G. Glen
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J. Michael O'Neill
J. Michael O'Neill
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Geological Society of America
Volume
431
ISBN print:
9780813724317
Publication date:
January 01, 2007

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