Jeff A. Benowitz

Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

40 Ar/ 39 Ar, apatite fission-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from ∼33 Ma to ∼18 Ma for 40 Ar/ 39 Ar biotite, ∼18 Ma to ∼6 Ma for K-feldspar minimum closure ages, and ∼15 Ma to ∼1 Ma for apatite fission-track ages, and apatite (U-Th)/He cooling ages range from ∼4 Ma to ∼1 Ma. There has been at least ∼11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite 40 Ar/ 39 Ar thermochronology. Variations in exhumation history along and across the strike of the fault are influenced by both far-field effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least ∼22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (∼past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.

Persistent long-term (c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermochronology

Abstract To address Miocene–present episodic v. persistent exhumation, we utilize a simple graphical procedure that vertically stacks spatially diverse K-feldspar 40Ar/39Ar multi-domain diffusion (MDD) models from the length of the approximately 100 km-long high-peak region of the Eastern Alaska Range. We supply additional constraints with 40Ar/39Ar mica dating because the higher closure-temperature-window places limits on the initiation of rapid Eastern Alaska Range exhumation. We also provide a broad 40Ar/39Ar K-feldspar minimum closure age data set to add more detail on spatial patterns in the regional exhumation history for the Eastern Alaska Range. We find that rapid and persistent exhumation has occurred in the Eastern Alaska Range since about 24 Ma at a long-term rate of approximately 0.9 km/Ma, but that this rapid exhumation is spatially variable through time. Onset of rapid Eastern Alaska Range exhumation is coincident with the initiation of rapid exhumation in SW Alaska, the Western Alaska Range and the Chugach–Saint Elias Range at around 25 Ma, implying a region-wide deformational response to a change in tectonic forcing. The initiation of highly coupled flat-slab subduction of the Yakutat microplate is probably responsible for this prolonged period of rapid exhumation in southern Alaska. Supplementary material: Sample locations from the Eastern Alaska Range, and 40Ar/39Ar data tables and age spectrum figures are available at www.geolsoc.org.uk/SUP18603.

The role of thrust faulting in the formation of the eastern Alaska Range: Thermochronological constraints from the Susitna Glacier Thrust Fault region of the intracontinental strike-slip Denali Fault system

Horizontal-slip along restraining bends of strike-slip faults is often partitioned into a vertical component via splay faults. The active Susitna Glacier Thrust Fault (SGTF), as shown by its initiation of the 2002 M7.9 Denali Fault earthquake, lies south of, and intersects the dextral strike-slip Denali Fault. Geochronology and thermochronology data from samples across the SGTF constrain the regions tectonic history and the role of thrusting in the formation of the eastern Alaska Range south of the Denali fault. U-Pb zircon ages indicate intrusion of plutons in the footwall (~57 Ma) and hanging wall (~98 Ma). These U-Pb zircon ages correlate to those from the Ruby Batholith/Kluane Terrane ~400 km east along the Denali Fault, supporting geologic correlations and hence constraints on long-term slip rates. 40Ar/39Ar mica and K-feldspar data from footwall and hanging wall samples (~54 to ~46 Ma) reflect cooling following magmatism and/or regional Eocene metamorphism related to ridge subduction. Combined with apatite fission track data (ages 43–28 Ma) and thermal models, both sides of the SGTF acted as a coherent block during the Eocene and early Oligocene. Contrasting apatite (U-Th)/He ages across the Susitna Glacier (~25 Ma footwall, ~15 Ma hanging wall) suggest initiation of faulting during the middle Miocene. Episodic cooling and exhumation is related to thrusting on known or hypothesized faults that progressively activate due to varying partition of strain along the Denali Fault associated with changing kinematics and plate interaction (Yakutat microplate collision, flat-slab subduction and relative plate motion change) at the southern Alaskan plate margin.

Detrital geochronology of pre-Mississippian strata in the northeastern Brooks Range, Alaska: Insights into the tectonic evolution of northern Laurentia

The Arctic Alaska terrane of northern Alaska and Yukon is one of several exotic terranes in the North American Cordillera with putative early Paleozoic connections to the northern Caledonian-Appalachian orogen. The U-Pb and 40 Ar/ 39 Ar isotopic data from detrital minerals in pre-Mississippian sedimentary units of the northeastern Brooks Range are presented here to investigate the consequences of Caledonian orogenesis on sediment dispersal trends and the paleogeography of northern Laurentia. Neoproterozoic–Cambrian siliciclastic rocks of the informal Firth River group and the Neruokpuk Formation were most likely deposited along a passive margin that sourced Archean and Paleoproterozoic basement rocks of the Canadian shield and reworked Mesoproterozoic and younger sedimentary units. These strata are overlain by a Lower Ordovician–Lower Devonian succession of fine-grained siliciclastic turbidites, herein referred to as the Clarence River group, which records a prominent shift in provenance most likely associated with the onset of the Caledonian-Appalachian orogeny in northeast Laurentia. U-Pb detrital zircon age populations of ca. 470–420 and 990–820 Ma, along with 40 Ar/ 39 Ar detrital muscovite ages of ca. 470–430 Ma, support provenance connections with the East Greenland Caledonides, Pearya, and Svalbard. Partially reset 40 Ar/ 39 Ar ages in these sedimentary successions are linked to low-grade metamorphism associated with the Early–Middle Devonian Romanzof orogeny, a poorly understood tectonic event in the Brooks Range that is possibly associated with the emplacement of an allochthonous oceanic assemblage, herein named the Whale Mountain allochthon.

The leading wisps of Yellowstone: Post–ca. 5 Ma extension-related magmatism in the upper Wind River Basin, Wyoming (USA), associated with the Yellowstone hotspot tectonic parabola

The upper Wind River Basin in northwest Wyoming (USA) is located ~80– 100 km southeast of the Yellowstone Plateau volcanic field. While the upper Wind River Basin is a manifestation of primarily Cretaceous to Eocene Laramide tectonics, younger events have played a role in its formation, including Eocene Absaroka volcanism, Cenozoic lithospheric extension, and the migration of the North American plate over the Yellowstone hotspot tail. New 40Ar/39Ar ages coupled with existing K-Ar results from intrusives and lavas in the upper Wind River Basin show that igneous activity younger than ca. 5 Ma occurred locally. Field and geochemical data show that these <ca. 5 Ma upper Wind River Basin magmas were either erupted or emplaced along normal fault zones at different locations and range in composition from tholeiitic basalt (Spring Mountain) to calc-alkaline basaltic andesite through dacite (Lava Mountain, Crescent Mountain, and Wildcat Hill), and include a lamprophyre intrusion (Pilot Knob). Together, these igneous rocks define the Upper Wind River Basin volcanic field (UWRB). All UWRB rocks have large ion lithophile element enrichments, high field strength element depletions, and other geochemical characteristics associated with subduction and that are identical to those of the Miocene Jackson Hole volcanics, even though the former erupted in an intraplate setting. Our results suggest that UWRB magmatism, as well as the Jackson Hole volcanics and other small-volume, similarly aged intermediate to felsic magmatism in eastern Idaho, are the result of the interaction between the North American plate and the progression of the tectonic parabola associated with the Yellowstone hotspot tail.

Cenozoic sinistral transpression and polyphase slip within the Bruin Bay fault system, Iniskin-Tuxedni region, Cook Inlet, Alaska

The Bruin Bay fault system defines the northwestern tectonic boundary of the Cook Inlet forearc basin for ~450 km along the southern Alaskan forearc. The age, origin, and tectonic significance of the fault system are not well understood. We present field observations and a population of minor fault slip data (n = 296) collected within the Bruin Bay fault system from the Iniskin-Tuxedni region of the Cook Inlet. The minor faults cut Triassic–Paleogene strata and are subdivided into two kinematically distinct populations. Population A (n = 233, 79%) includes strike-slip and reverse faults that altogether record subhorizontal, southeast-trending tectonic shortening and bulk sinistral transpression. Population B (n = 63, 21%) includes strike-slip faults that are compatible with subhorizontal, northeast-trending shortening and southeast-trending extension and are younger. Gently deformed mafic and felsic dikes that intrude cataclasite within the Bruin Bay fault zone at two localities yield late Paleogene biotite (ca. 37 Ma), whole-rock (ca. 33 Ma), and plagioclase (ca. 31 Ma) 40Ar/39Ar ages. The ages of deformed strata and crosscutting dikes indicate that sinistral transpression (population A) occurred during the Paleogene prior to ca. 37 Ma, but some deformation persisted through at least the early Oligocene. Results place the Bruin Bay fault system in the Paleogene tectonic context of southern Alaska. We discuss several competing hypotheses to interpret the tectonic evolution of the fault system. We suggest that the majority of the Paleogene deformation likely occurred during either a spreading ridge subduction event or accretion of the Chugach–Prince William terrane to the southern Alaskan margin. INTRODUCTION Convergence along ocean-continent subduction margins shapes the geology of the overriding plate for hundreds of kilometers inboard of the trench (e.g., the American Cordillera). Accretionary prisms, forearc basins, and volcanic arcs are geologic features characteristic of ocean-continent subduction margins that are commonly separated from each other by large faults systems (e.g., Border Ranges fault system, Alaska; Pavlis and Roeske, 2007; Wilson et al., 2012; Sagaing fault system, Myanmar; Vigny et al., 2003; Wang et al., 2014; the megasplay fault, Nankai margin, Japan; Gulick et al., 2010; Sumatran fault, Indonesia; Sieh and Natawidjaja, 2000). Their tectonic development varies widely and is strongly dependent on the plate kinematics of the convergent margin, sediment flux at the trench, configuration of the subducting plate (e.g., buoyancy or roughness), and/or arrival of allochthonous terranes (Dickinson and Seely, 1979; Kopp, 2013; Noda, 2016). Long-lived subduction margins can lead to the reactivation of preexisting structures (Holdsworth et al., 1997) and the development of fault systems with slip and deformation histories that record both the initial tectonic setting and their reactivation under different stress regimes (Cembrano et al., 1996; Murphy et al., 1999). During plate convergence, these structures can also facilitate the transfer of slip deep into continental interiors (Storti et al., 2003). The transpressive nature of these longlived structural zones is recorded in their magmatic and deformation history, which can be constrained through structural, petrological, geochronological and thermochronology analysis (e.g., Stewart et al., 1999; Benowitz et al., 2011; Niemi et al., 2013). Therefore, the composite geologic history of long-lived structures can help to constrain plate boundary conditions over significant amounts of time. The Cook Inlet Basin (CIB) in south-central Alaska is a >25,000 km2 forearc basin that records Jurassic–modern tectonic evolution of the southern Alaskan subduction margin (Fig. 1; see reviews by Trop and Ridgway, 2007; LePain et al., 2013). The CIB is part of the Wrangellia composite terrane (Fig. 1), which is located between the Mesozoic to Cenozoic accretionary prism (Chugach– Prince William terrane) and interior Alaska. The CIB is bound on either side by regional long-lived tectonic boundaries known as the Bruin Bay (Magoon et al., 1976; Wilson et al., 2012) and Border Ranges fault systems (e.g., Pavlis and Roeske, 2007). The late Mesozoic–early Cenozoic tectonic setting of south-central Alaskan subduction margin is widely debated. For example, some studies conclude that many of the geologic features characteristic of the Alaskan forearc (i.e., Peninsular and Chugach–Prince William terranes; Fig. 1) were strongly affected by the Paleogene subduction of a trench-ridge-trench triple junction in a paleogeographic setting that resembled the modern margin (e.g., Bradley et al., 2000; Haeussler et al., 2003a, 2003b). In contrast, other work indicates that much of this record (i.e., the Chugach–Prince William terrane) was likely translated a long distance (as much as 3000 km) along the North American margin to its present location between the Late Cretaceous and Eocene (e.g., Plumley GEOSPHERE GEOSPHERE; v. 13, no. 6 doi:10.1130/GES01464.1 15 figures; 1 supplemental file CORRESPONDENCE: pmbetka@utexas .edu CITATION: Betka, P.M., Gillis, R.J., and Benowitz, J.A., Cenozoic sinistral transpression and polyphase slip within the Bruin Bay fault system, Iniskin-Tuxedni region, Cook Inlet, Alaska: Geosphere, v. 13, no. 6, p. 1806–1833, doi:10.1130/GES01464.1. Received 22 November 2016 Revision received 4 August 2017 Accepted 22 September 2017 Published online 26 October 2017 For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.