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[1] The stratigraphic record of Mesozoic arc accretion in the North American Cordillera is preserved in a discontinuous belt of clastic strata that are exposed inboard (cratonward) of the allochthonous Wrangellia composite terrane in southern Alaska, western Canada, and Washington State. LA-ICPMS analyses of eight samples (n = 714 detrital zircon grains) collected at different stratigraphic intervals from the Jurassic-Cretaceous Kahiltna assemblage in southern Alaska reveals a bulk U-Pb age distribution of Precambrian-Mesozoic age grains (Mz 74%, Pz 11%, Pc 15%). A comparison of U-Pb ages from older to younger stratigraphic intervals within the Kahiltna assemblage reveals three stages of exhumation and basin development during arc accretion. Stages include (1) an initial Late Jurassic-Early Cretaceous pre/early collisional phase during which detritus was derived almost solely from Middle-Late Jurassic and Early Cretaceous magmatic sources of the outboard Wrangellia composite terrane (Mz 100%, Pz 0%, Pc, 0%), (2) a second Early Cretaceous syncollisional phase that reflects the introduction of Paleozoic and Precambrian detritus from the inboard Intermontane belt (Mz 84%, Pz 11%, Pc 5%) and an upsection increase in older detrital zircon grains compared to Mesozoic age grains (Mz 65%, Pz 11%, Pc 24%), and (3) a final Early-Late Cretaceous late/postcollisional phase that represents continued detrital contributions from Precambrian-Mesozoic source areas (Mz 19%, Pz 22%, Pc 59%) located inboard and outboard of the Kahiltna basin. Similar bulk trends in detrital zircon age populations have been reported from along-strike, age-equivalent strata of the Gravina belt (Mz 74%, Pz 20%, Pc 6%) in southeastern Alaska suggesting that similar provenance trends may exist in basins along this >2000 km -long collisional zone. Citation: Hampton, B. A., K. D. Ridgway, and G. E. Gehrels (2010), A detrital record of Mesozoic island arc accretion and exhumation in the North American Cordillera: U-Pb geochronology of the Kahiltna basin, southern Alaska, Tectonics', 29, TC4015, doi: 10.1029/2009TC002544.

Analysis of late Mesozoic and Cenozoic sedimentary basins, metamorphic rocks, and major faults 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 suture zone: (1) Deposition of 3-5 km of Upper Jurassic-Upper Cretaceous marine strata (the Kahiltna assemblage) recorded the initial collision of the island-arc assemblage 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 submarine-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 continentalmargin strata uplifted in the along-strike suture zone. A belt of retrograde metamorphism and a regional anticlinorium developed along the continental margin from 115 to 106 Ma, roughly coeval 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 the 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 Cantwell basin, in the northern part of the suture zone. Geologic mapping and compositional 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 to kyanite schists at ~25 km depth and ~650 [degrees] C, were exhumed and cooled through the biotite closure temperature (~300 [degres] C) by ca. 62 Ma. In the northern part of the suture zone, this time period was marked by shortening, uplift, and erosion of sedimentary strata of the Cantwell basin. (4) From 60 to 54 Ma, ~3 km of volcanic strata were deposited 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 resulted 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 ~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 deposits of the Tanana basin. These mapping relationships provide evidence that regional shortening continues to the present in the eastern and central Alaska Range. Keywords: Alaska Range, Cantwell Formation, Kahiltna assemblage, suture zone, Usibelli Group, Wrangellia.

Pennsylvanian--Permian synorogenic deposits (Minturn and Sangre de Cristo Formations) of the Central Colorado trough record an interplay of deformation and sedimentation in an Ancestral Rocky Mountains basin. The Central Colorado trough was a north-trending basin bordered by basement-involved highlands of the Uncompahgre uplift on the west and the Ancestral Front Range and Apishapa uplift on the east. Stratigraphic data show that the Central Colorado trough was an asymmetric basin in which coarse-grained sediments were deposited adjacent to the Sand Creek-Crestone thrust fault system of the Uncompahgre uplift. These deposits pinch out eastward against the Apishapa uplift along the eastern margin of the basin. Lithofacies analysis shows that the Central Colorado trough was filled by fan-delta, fluvial-delta, and turbidite deposits of the Middle Pennsylvanian Minturn Formation and by alluvial-fan, braided-stream, and meandering-stream deposits of the Upper Pennsylvanian--Permian Sangre de Cristo Formation. Geologic mapping has identified three syndepositional structures in the strata of the Central Colorado trough that indicate Pennsylvanian-Permian shortening: (1) the Gibson Peak growth syncline in the footwall of the Crestone thrust fault, which formed by syndepositional rotation of the Crestone Conglomerate Member of the Sangre de Cristo Formation during thrust displacement; (2) the Sand Creek thrust fault, which cuts the lower part of the Crestone Conglomerate Member but is covered by younger deposits of the Crestone Conglomerate Member; and (3) an intraformational angular unconformity in the Sangre de Cristo Formation that separates folded strata from overlying less deformed strata. All three structures indicate general east-west shortening during deposition. We interpret the Central Colorado trough as a flexural basin on the basis of syndepositional thrust-related structures, basin asymmetry, and lithofacies distribution. Displacement on the east-verging Sand Creek-Crestone thrust fault system appears to have controlled uplift of the central part of the Uncompahgre uplift and also subsidence in the adjacent basin. The Apishapa uplift, located along the eastern margin of the basin, is interpreted as a possible flexural forebulge related to crustal loading of thrust sheets along the western margin of the Central Colorado trough. Geologic mapping, subsurface data, and identification of syndepositional structures has also led to a better understanding of post-Paleozoic deformation of Pennsylvanian--Permian strata of the Central Colorado trough and the structural evolution of the present Sangre de Cristo Mountains. Keywords: fan deltas, Minturn Formation, Rocky Mountains, Sangre de Cristo Mountains, thrust faults.

The ~7000 m of Upper Triassic-Upper Cretaceous strata of the Wrangell Mountains basin depositionally overlie the allochthonous Wrangellia composite terrane in south-central Alaska. New sedimentologic, compositional, and geochronologic data from these strata provide a record of the migration of the terrane from an intraoceanic setting at low paleolatitudes (~12[degrees]N) to its present position along the continental margin of southern Alaska (~61[degrees]N). We recognize several distinct stages of basin development: (1) Upper Triassic-Lower Jurassic carbonate strata represent an intraoceanic carbonate platform built on a remnant volcanic are at low paleolatitudes (~12[degrees]N). (2) During the Middle to Late Jurassic, a subduction zone formed along the southern margin of the Wrangellia composite terrane, prompting development of an intraoceanic arc and backarc basin on top of the terrane. (3) A narrow thrust belt and retroarc foreland basin formed along the inboard margin of the arc during the latest Jurassic. The foreland basin and arc were subsequently folded, uplifted, and eroded during the latest Jurassic-Early Cretaceous as recorded by an angular unconformity and isotopic ages from clasts in conglomerate. Regional deformation of the foreland-basin strata, shortening and uplift of the Wrangellia composite terrane, and uplift and erosion of the Middle to Late Jurassic arc interpreted to represent the initial collision between the terrane and the continental margin of western North America. (4) Following regional deformation, a new volcanic arc was constructed inboard (northward) of the Wrangell Mountains basin. Upper Lower to Upper Cretaceous siliciclastic strata were deposited by shallow--to deep-marine deposystems in a continental-margin forearc basin. Distributions of lithofacies types and formation-thickness changes across intrabasinal normal faults document synextensional deposition. (5) The final stage of basin development was characterized by shortening and coarse-grained sedimentation along a fault system that separated the trenchward (southern) margin of the forearc basin from the subduction complex. (6) The Wrangell Mountains basin arrived at its current position by northward translation along orogen-parallel strike-slip fault systems. Comparison of the sedimentary record of the Wrangell Mountains basin, located on the outboard margin of the Wrangellia composite terrane, with the sedimentary record of the Nutzotin basin, located along the inboard margin of the terrane, demonstrates distinct changes in the locations of depocenters, the timing of deformation, and the composition of sediment. Similar stratigraphic and structural variations characterize outboard and inboard segments of the Wrangellia composite terrane in southeastern Alaska and coastal British Columbia. Keywords: Alaska, Mesozoic, sedimentology, tectonics, terranes, Wrangellia.

Structural, sedimentological, and provenance data from Paleogene synorogenic deposits of the east-central flank of the Bighorn Mountains provide new information about the development of footwall growth synclines, the evolution of fault-related folds, and the erosional unroofing history of intraforeland uplifts. Three conglomerate units, the upper conglomerate member of the Fort Union Formation and the Kingsbury and Moncrief Members of the Wasatch Formation, are incorporated within an asymmetric, east-verging growth syncline in the footwall of the main range-bounding thrust system. Three stages of footwall deformation are recorded within these conglomerates. Analysis of mapped progressive unconformities, retrodeformed balanced cross sections, and conglomerate clast composition data define these stages as part of a continuum of deformation associated with the development of footwall growth synclines. Development of an anticline-syncline pair marked the earliest stage of growth syncline formation (stage I). Rotation of the shared fold limb resulted in amplification of the growth syncline. Fine-grained, synorogenic sediment derived from easily eroded Mesozoic mudstone bypassed the growth syncline during this stage. By the end of Lebo Shale deposition, an average of 12.1% of shortening and 6.46 km of uplift had occurred along the range margin. Continued growth syncline development was marked by the deposition of the Kingsbury Conglomerate. The Kingsbury Conglomerate was derived from resistant, middle and lower Paleozoic carbonate strata in the uplifted source terrane. Intraformational unconformities, recording as much as 55 [degrees] of bed rotation, were developed within the Kingsbury Conglomerate as fold limb rotation occurred coeval with deposition. Cross sections indicate that during this early stage of fault-related folding, an average of 16.9 % shortening and 8.12 km of uplift occurred along the eastern flank of the Bighorn Mountains (end of stage I). The intermediate stage (stage II) of footwall growth syncline development involved partial truncation of the growth syncline by the advancing thrust faults and deposition of the Moncrief Conglomerate. The lower portion of the Moncrief Conglomerate was rotated basinward in the developing growth syncline. The final stage of deformation (stage III) was dominated by the thrust faulting of middle and lower Paleozoic strata eastward over steeply dipping Mesozoic strata and rotated Eocene synorogenic conglomerate. During this stage of deformation, the Moncrief Conglomerate was deformed, as the initially blind thrusts propagated into the near-surface conglomerate deposits, truncated the entire footwall syncline, and overrode the synorogenic conglomerate package. Cross sections in areas where this final stage of deformation is well developed indicate that an average of 24.1% shortening and 9.7 km of uplift had occurred along the eastern margin of the Bighorn Mountains. The caliber of synorogenic deposition in the Powder River basin was linked directly to the lithologic composition of the Bighorn Mountains. Approximately half of the 3.6-kin-thick source-stratigraphic section of the eastern Bighorn Mountains was eroded prior to accumulation of conglomerate. The majority of this eroded material was derived from Mesozoic mudstone and poorly indurated sandstone that were incapable of generating coarse detritus. The first Paleogene conglomerates deposited along the east-central Bighorn Mountains, therefore, do not represent the initiation of Laramide uplift, but instead represent the exposure of coarse-clast-forming rock types from the lower half of the hanging-wall stratigraphic section (i.e., the Mississippian Madison Limestone and Ordovician Bighorn Dolomite).

The Cantwell Formation consists of a lower sedimentary sequence as much as 4000 m thick and an upper volcanic sequence with a maximum thickness of 3750 m that was deposited in the Cantwell basin, south-central Alaska. Previous to this study, the Cantwell basin was interpreted as a Paleogene, nonmarine (mainly fluvial), pull-apart basin that formed in response to dextral, strike-slip displacement on the Denali fault system. This study proposes that the Cantwell basin formed as part of the Mesozoic accretionary phase of deformation, prior to the development of the Cenozoic postaccretionary Denali fault system. Our reinterpretation is based on several new lines of data. (1) Age. New data based on palynologic analyses of 135 fine-grained samples indicate that the lower Cantwell Formation was deposited during the late Campanian and early Maastrichtian. On the basis of previous regional tectonic studies and this new age constraint. the formation of the Cantwell basin was coeval with regional Late Cretaceous shortening associated with accretionary tectonics in southern Alaska. (2) Depositional systems. Our analysis of the Cantwell Formation demonstrates that sedimentation occurred mainly in stream-dominated alluvial fan, axial braided stream, and lacustrine settings. These depositional systems were strongly influenced by a southward dipping, asymmetric basin floor. The presence of abundant terrestrially derived organic material, together with palynological assemblages that include marine dinoflagellates and the associated presence of oncolites, may be suggestive of a time of marginal marine influence during the deposition of the upper part of the lower Cantwell Formation. The late Campanian to early Maastrichtian timing of this possible marine influence is within the range of the Bearpaw transgressive event of the Cordilleran foreland basin and allows for regional stratigraphic correlation of the Cantwell basin with other sedimentary basins in northwestern North America. (3) Structural controls on basin formation. Mapping of intraformational angular unconformities and progressively tilted strata along the southern margin of the Cantwell basin provides direct evidence that thrust fault deformation and lower Cantwell Formation sedimentation were synchronous. Distinctive Cantwell Formation conglomerate clasts derived from the uplifted hanging walls of nearby thrust sheets adjacent to the southern basin margin also support a syndepositional thrusting interpretation. Provenance data and the concentration of proximal alluvial fan deposits along the northwestern basin margin adjacent to the Hines Creek fault indicate that it, too, was active during deposition of the Cantwell Formation. On the basis of the new data, the Cantwell basin is interpreted to have formed as a thrust-top basin (i.e., piggyback basin) along the Late Cretaceous suture zone between the accreting Wrangellia composite terrane and the North American continental margin. In contrast to previous studies, this reinterpretation of the formation of the Cantwell basin implies that the lower Cantwell Formation is not a synorogenic deposit directly associated with strike-slip displacement along the Denali fault system. Therefore, the Cantwell basin cannot be used to constrain the timing for the early development of the Denali fault system.

New and previously published detrital zircon U-Pb ages from sediment in major rivers of south-central Alaska archive several major episodes of magmatism associated with the tectonic growth of this convergent margin. Analysis of detrital zircons from major trunk rivers of the Tanana, Matanuska-Susitna, and Copper River watersheds (N = 40, n = 4870) documents major <250 Ma age populations that are characteristic of the main phases of Mesozoic and Paleogene magmatism in the region as documented from limited U-Pb ages of igneous rocks. Key points from our detrital record include: (1) Major magmatic episodes occurred at 170, 150, 118, 95, 72, 58, and 36 Ma. The overall pattern of these ages suggests that felsic magmatism was episodic with periodicity ranging between ~14 and 32 m.y. with an average of ~22 m.y. (2) Magmatism in south-central Alaska shows similar age trends with both the Coast Mountains batholith and the along-strike Alaska Peninsula forearc basin strata, demonstrating a spatial and temporal relationship of felsic magmatism along the entire northern Cordilleran margin. (3) Topography and zircon fertility appear to influence the presence and/or absence of detrital zircon populations in individual watersheds. Results from this study indicate that regionally integrated detrital zircon populations from modern trunk rivers are faithful recorders of Mesozoic and Paleogene magmatic events along a convergent margin, but there appears to be a lag time for major rivers to record Neogene and ongoing magmatic events. [ABSTRACT FROM AUTHOR]

The early Paleogene was a dynamic period marked by long-term climatic trends and rapid climate events superimposed upon a hot greenhouse state. The response of the terrestrial hydrological cycle to these climate variations can be investigated at the continental scale by analysis of Paleogene strata in Laramide basins and the Gulf of Mexico. New U-Pb geochronologic data from detrital zircons in Paleogene fluvial strata of the Tornillo Basin in west Texas indicate relatively continuous deposition from 63 to 47 Ma. The combination of detrital zircon ages with existing carbon isotope chemostratigraphic data allows us to revise the age model of these strata and to correlate stratigraphic changes with Paleogene climate variations. Sedimentologic data indicate that, during the Paleocene (63-55.5 Ma), high-sinuosity, fluvial systems meandered on a forested floodplain with well-developed soils and swamps. The fluvial regime changed abruptly to low-sinuosity, braided fluvial systems with floodplains characterized by intermittent wetting at the Paleocene-Eocene thermal maximum (55.5 Ma). Braided fluvial regimes persisted thereafter and culminated during the early Eocene climate optimum (52-50 Ma) with deposition of a coarse sheet of sand and gravel. We interpret the transition from meandering to braided fluvial systems as a shift in discharge and sediment supply associated with the more seasonal precipitation of the hot, subtropical monsoonal climate of the early Eocene. After 54 Ma, this shift in fluvial regime is superimposed by an upwardcoarsening grain-size trend probably driven by enhanced regional Laramide tectonism. These data provide the first reconstruction of Paleogene subtropical fluvial dynamics in western North America and correlate with regional and global changes in Paleogene climate and landscapes. [ABSTRACT FROM AUTHOR]

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Lithofacies analysis, magnetostratigraphy, and seismic profiles of Pliocene-Pleistocene foreland basin deposits of Taiwan provide a framework to evaluate the stratigraphic development of a collisional marine foreland basin. We have recognized several scales of stratigraphic packages and unconformities in deposits of the Taiwan foreland basin. Small-scale (20 to 150 m thick) stratigraphic sequences contain upward-shallowing, marine lithofacies successions that are bracketed by thin coquina sandstones. We interpret the small-scale stratigraphic packages as 'parasequences' in the traditional sequence stratigraphy model, the thin coquina sandstones representing marine-flooding intervals. The average duration of individual small-scale packages was in the range of 37.5 k.y., on the basis of our magnetostratigraphy. These sequences are interpreted as the product of eustatic sea-level change possibly related to the orbital time series of obliquity. Intermediate-scale stratigraphic sequences are 150 to 1000 m thick and are bounded by unconformities that are well exposed in outcrop and can be clearly identified in seismic sections. The unconformity surfaces have several hundred meters of relief and represent periods of major fluvial valley incision in the foreland basin. One of the unconformities is locally an angular one that we interpretas representing a growth structure that formed during structural uplift of the proximal margin of the foreland basin at ca. 1.25 Ma. Across this angular unconformity, there were marked increases in rates of sediment accumulation and tectonic subsidence in the foreland basin. Other major unconformities that bound intermediate-scale stratigraphic sequences are high-relief disconformities. These unconformities may be the product of eustatic changes, because there has been little change in rates of sediment accumulation and tectonic subsidence across these unconformities. The duration of individual, intermediate-scale packages ranges from ~100 000 to 700 000 yr, on the basis of magnetostratigraphy and biostratigraphy. We interpret the intermediate-scale sequences as 'sequences' in the traditional sequence stratigraphy model. Our analysis of the Pliocene-Pleistocene deposits of the Taiwan foreland basin has several implications for understanding the stratigraphic evolution of this collisional marine foreland basin. (1) Deposition in the Taiwan foreland basin appears to have been punctuated by at least five episodes of erosion and major fluvial valley incision. Large volumes of sediment were eroded from the proximal margin of the foreland basin and transported to more distal parts of the foreland basin or to depocenters outside the foreland basin system during all stages of basin development. (2) The presence of high-relief unconformities and growth structures in the Pliocene-Pleistocene foreland basin deposits suggests a well-developed wedge-top depozone in the foreland basin system. (3) The Pliocene-Pleistocene strata of the foreland basin of Taiwan record ~2.3 m.y. of deposition, on the basis of our magnetostratigraphy. Sediment accumulation rate was on the order of ~950 m/m.y, during the earlier stages of basin development. During the later stages of basin development, sediment accumulation rate increased to ~1900 m/m.y. Sediment accumulation rates in the collisional marine foreland basin of Taiwan are much higher than previously published rates from more extensively studied retroarc foreland basins and collisional nonmarine foreland basins. Keywords: foreland basin, magnetostratigraphy, Pliocene-Pleistocene, Taiwan, unconformities.