Mesozoic and Cenozoic tectonics of the eastern and central Alaska Range: Progressive basin development and deformation in a suture zone

摘要

Analysis of late Mesozoic and Cenozoic sedimentary basins, metamorphic rocks, and major fauls in the eastern and central Alaska Range documents the progressive development of a suture zone that formed as a result of collision of an island-arc assemblage (the Wrangellia composite terrane) with the former North American continental margin. New basin-analysis, structural, and geochronologic data indicate the following stages in the development of the surface zone: (1) Deposition of 3-5 km of Upper Jurassic-Upper Cretaceous marine strate (the Kahiltna assemblage) recorded the initial collision of the island-arc assemblege with the continental margin. The Kahiltna assemblage exposed in the northern Talkeetna Mountains represents a Kimmeridgian-Valanginian backarc basin that was filled by northwestward-flowing seubmarine-fan systems that were transporting sediment derived from Mesozoic strata of the island-arc assemblage. The Kahiltna assemblage exposed in the southern Alaska Range represents a Valanginian-Cenomanian remnant ocean basin filled by west-southwestward-flowing submarine-fan systems that were transporting sediment derived from Paleozoic continental-margin strata uplifted in the along-strike suturre zone. A belt of retrograde metamorphism and a regional anticlinorium developed along the continental margin from 115 to 106 Ma, roughly coeveral with the end of widespread deposition in the Kahiltna sedimentary basins. (2) Metamorphism of submarine-fan deposits of the Kahiltna basin, located near the leading edge of teh island-arc assemblage, occurred at ca. 74 Ma, as determined from a new U-Pb zircon age for a synkinematic sill. Coeval with metamorphism of deposits of the Kahiltna basin in the southern part of the suture zone was development of a thrust-top basin, the Canwell basin, in the northern part of the suture zone. Geologic mapping and comositional data suggest that the 4 km of Upper Cretaceous nonmarine and marginal marine sedimentary strata in this basin record regional subaerial uplift of the suture zone. (3) Shortening and exhumation of the suture zone peaked from 65 to 60 Ma on the basis of metamorphic and geochronologic data. In the southern part of the suture zone, submarine-fan deposits of the Kahiltna basin, which had been metamorphosed tokyanite schists at approx 25 km depth and approx 650 deg C, were exhumed and cooled through the biotite closure temperature (approx 300 deg C) by ca. 62 Ma. In the northern part of the suture zone, this time perod was marked by shortening, uplift, and erosion of sedimentary strata of the Cantwell basin. (4) From 60 to 54 Ma, approx 3 km of volcanic strata were depoisted over deformed sedimentary strata of the Cantwell basin, and several granitic plutons (the McKinley sequence) were emplaced along the suture zone. (5) Following igneous activity, strike-slip displacement occurred from ca. 54 to 24 Ma along the Denali fault system, which had developed in the existing suture zone. Late Eocene-Oligocene strike-slip displacement resultd in the formation of several small sedimentary basins along the Denali fault system. (6) Regional transpressive shortening characterized the suture zone from ca. 24 Ma to the present. Flexural subsidence, related to regional shortening, is represented by late Eocene to Holocene nonmarine deposits of the Tanana foreland basin. Regional subsidence resulted in Miocene coal seams up to 20 m thick and well-developed lacustrine deposits. Overlying the Miocene deposits are approx 1.2 km of Pliocene and Holocene conglomeratic deposits. Compositional and paleocurrent data from these younger deposits record regional Neogene uplift of the suture zone and recycling of detritus from older basins to the south that had become incorporated into the up-lifted suture zone. Geologic mapping of major thrust faults along the northern and southern margins of the suture zone documents Paleozoic strata thrust over both Pliocene fluvial deposits and Quaternary glacial depos