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 north-verging 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 *jtrop@bucknell.edu; ridge@purdue.edu Trop, J.M., and Ridgway, K.D., 2007, Mesozoic and Cenozoic tectonic growth of southern Alaska: A sedimentary basin perspective, in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O’Neill, J.M., eds., Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska: Geological Society of America Special Paper 431, p. 55–94, doi: 10.1130/2007.2431(04). For permission to copy, contact editing@geosociety.org. ©2007 The Geological Society of America. All rights reserved. GSA_SPE431_04_Trop.qxd 7/13/07 8:05 AM Page 55

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

Upper Jurassic-Lower Cretaceous sedimentary strata of the Nutzotin basin, the Nutzotin Mountains sequence, crop out in the Nutzotin and Mentasta Mountains of the eastern Alaska Range. These strata represent one of the best-exposed and leastmetamorphosed examples of a basin that is interpreted to have formed during collision of an allochthonous volcanic arc (i.e., the Wrangellia terrane) with a continental margin. New stratigraphic, geologic mapping, and provenance data indicate that the Nutzotin basin formed as a retroarc foreland basin along the northern margin (present coordinates) of the Wrangellia terrane. Coeval with basin development along the northern margin, sedimentary basins and plutons located along the southern margin of the Wrangellia terrane were being incorporated into a regional fold-and-thrust belt. This fold-and-thrust belt, located south of the Nutzotin basin, exposed multiple structural levels of the Wrangellia terrane that were eroded and provided sediment that was transported northward and deposited in the Nutzotin basin. New sedimentologic and stratigraphic data from the �3 km thick (minimum thickness) Nutzotin Mountains sequence define a three-part stratigraphy. The lower part consists of Upper Jurassic (Oxfordian to Tithonian) conglomerate with outsized limestone clasts (>10 m in diameter) and interbedded sandstone and shale that grade basinward into mainly black shale with minor micritic limestone and isolated lenses of conglomerate. The middle part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) normal-graded sandstone and shale interbedded with massive tabular sandstone and lenticular conglomerate. The upper part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous

Sedimentary record of the tectonic growth of a collisional continental margin: Upper Jurassic-Lower Cretaceous Nutzotin Mountains sequence, eastern Alaska Range, Alaska

Models of continental crustal magmagenesis commonly invoke the interaction of mafic mantle-derived magma and continental crust to explain geochemical and petrologic characteristics of crustal volcanic and plutonic rocks. This interaction and the specific mechanisms of crustal contamination associated with it are poorly understood. An excellent opportunity to study the progressive effects of crustal contamination is offered by the composite plutons of the Alaska Range, a series of nine early Tertiary, multiply intruded, compositionally zoned (Peridotite to granite) plutons. Large initial Sr and Nd isotopic contrasts between the crustal country rock and likely parental magmas allow evaluation of the mechanisms and extents of crustal contamination that accompanied the crystallization of these ultra-mafic through granitic rocks. Three contamination processes are distinguished in these plutons. The most obvious of these is assimilation of crustal country rock concurrent with magmatic fractional crystallization (AFC), as indicated by a general trend toward crustal-like isotopic signatures with increasing differentiation. Second, many ultramafic and mafic rocks have late-stage phenocryst reaction and orthocumulate textures that suggest interaction with felsic melt. These rocks also have variable and enriched isotopic compositions that suggest that this felsic melt was isotopically enriched and probably derived from crustal country rock. Partial melt from the flysch country rock may have reacted with and contaminated these partly crystalline magmas following the precipitation and accumulation of the cumulus phenocrysts but before complete solidification of the magma. This suggests that in magmatic mush (especially of ultramafic composition) crystallizing in continental crust, a second distinct process of crustal contamination may be super imposed on AFC or magma mixing involving the main magma body. Finally, nearly all rocks, including mafic and ultramafic rocks, have (
 87
Sr/
 86
Sr)
 i that are too high, and e(T) Nd that are too low, to represent the expected isotopic composition of typical depleted mantle. However, gabbro xenoliths with typical depicted-mantle isotopic compositions are found in the plutons. This situation requires either an additional enriched mantle component to provide the parental magma for these plutons, or some mechanism of crustal contamination of the parent magma that did not cause significant crystallization and differentiation of the magma to more felsic compositions. Thermodynamic modeling indicates that assimilation of alkali-and water-rich partial melt of the metapelite country rock by fractionating, near-liquidus basaltic magma could cause significant contamination while suppressing significant crystallization and differentiation.

Evidence for multiple mechanisms of crustal contamination of magma from compositionally zoned plutons and associated ultramafic intrusions of the Alaska Range

Terry L. Pavlis, Dept. of Geological Sciences, University of Texas, El Paso, Texas 79968, USA; Jeffrey M. Amato, Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA; Jeffrey M. Trop, Dept. of Geology and Environmental Geosciences, Bucknell University, Lewisburg, Pennsylvania 17837, USA; Kenneth D. Ridgway, Dept. of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, Indiana 47907, USA; Sarah M. Roeske, Earth and Planetary Sciences Dept., University of California, Davis, California 95616, USA; and George E. Gehrels, Dept. of Geosciences, University of Arizona, Tucson, Arizona, 85721, USA

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Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America

The eastern Alaska Range, between 141°W (International Boundary) and 145°W long. in south-central Alaska, provides clues to the tectonic development of northwestern North America. The Denali fault system, a major structural feature extending in an arcuate path from the Bering Sea to the Gulf of Alaska, transects the eastern Alaska Range and separates extremely diverse geologic terranes. North of the Denali fault lies a widespread terrane of highly deformed, metamorphosed sedimentary and minor igneous rocks of Precambrian to Devonian age. South of the Denali fault system these rocks are absent, and the oldest rocks exposed are a heterogeneous series of Pennsylvanian(?) or Permian volcanic and volcaniclastic rocks derived from a late Paleozoic volcanic island arc probably built directly on oceanic crust. These rocks are overlain by a succession of Permian marine clastic beds and limestones; Triassic carbonaceous shales, subaerial tholeiitic basalt flows, and mari e limestones; and Jurassic-Cretaceous argillite, graywacke, and conglomerate. The cumulative thickness of the succession locally exceeds 10,000 ft (3,050 m). Sedimentation culminated in middle(?) Cretaceous time with a short-lived and restricted episode of andesitic volcanism. Relatively undeformed continental sedimentary rocks of Cretaceous age, or younger, and late Cenozoic terrestrial volcanic flows overlie the older rocks with marked angular unconformity. Linear bodies of serpentinized ultramafic rocks are present with the Permian rocks to the west in the central Alaska Range and to the east in Canada. In the eastern Alaska Range, ultramafic rocks have not been observed south of the Denali fault, but they do occur locally along the fault zone and in the older terrane just north of the fault. All pre-Late Cretaceous rocks south of the Denali fault system have been cut by high-angle normal faults and by numerous reverse and thrust faults that dip north toward the Denali fault. The Jurassic-Cretaceous marine sedimentary rocks also exhibit complex folding, locally isoclinal, and fold axes plunge at low angles generally toward the northwest. The geologic data suggest that the oceanic terrane south of the Denali fault collapsed against, and was added to, the continental American plate, probably in Early Triassic time. Since then, this terrane has undergone multiple deformation as later oceanic plates impinged against the continental margin. The Denali fault, which represents an ancient subduction zone, was reactivated as a ridge-arc dextral transform fault--probably during the early Pliocene--in response to a change in the direction of spreading in the North Pacific oceanic plate. The Totschunda fault system, which diverges from the Denali structure near 144°W long. and trends southeasterly toward the Fairweather fault in the Gulf of Alaska, is another major right-lateral strike-slip fault that may have developed as re ently as the middle Pleistocene. At present, the Denali fault system apparently is inactive southeast of the Denali-Totschunda junction.

D. H. Richter

Papers

Structure and Stratigraphy of Eastern Alaska Range, Alaska: Regional Arctic Geology of Alaska