Phillip A. Armstrong

Exhumation of the central Wasatch Mountains, Utah: 2. Thermokinematic model of exhumation, erosion, and thermochronometer interpretation

[1] The Wasatch fault is a � 370 km long normal fault in Utah that marks the boundary between the stable Colorado Plateau to the east and the extending Basin and Range to the west. Understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones (e.g., East Africa Rift, Transantarctic Mountains). We explore the thermokinematics of footwall exhumation and erosion in the Cottonwood Intrusive Belt of the central Wasatch Mountains. Emphasis is placed on using low-temperature thermochronometers to quantify (1) the spatial and temporal variability of exhumation and erosion rates, (2) the geometry of footwall tilt, (3) the fault dip angle, and (4) the magnitude and duration of exhumation. These processes are investigated using two-dimensional (2-D) thermal-kinematic models coupled with cooling-rate-dependent kinetic models which predict exhumed apatite fission track (AFT) and (U-Th)/He ages. The range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0.2 and 2.0 mm yr � 1 , footwall tilt hinge positions between 15 and 40 km distance from the fault, a single planar normal fault with dip angles of 45� and 60� , and exhumation magnitudes of up to 15 km at the fault. Simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat-producing layers. Erosion maintains a constant topographic profile. The kinematic and exhumation history of the Wasatch Mountains is investigated by comparing model predicted thermochronometer ages to observed AFT, ZFT, and (U-Th)/He ages. Predicted and observed ages are compared using a reduced chi-square analysis to determine a best fit kinematic model for the Wasatch Mountains. The preferred model includes exhumation occurring on either a 45� or 60� dipping fault, a footwall hinge located a minimum of 20–25 km from the fault, and a step decrease (deceleration) in the footwall exhumation rate at the fault from 1.2 to 0.8 mm yr � 1 at around 5 Ma. The model also suggests an exhumation duration of � 12 Myr ± 2 Myr). INDEX TERMS: 1035 Geochemistry: Geochronology; 8010 Structural Geology: Fractures and faults; 8015 Structural Geology: Local crustal structure; 8109 Tectonophysics: Continental tectonics— extensional (0905); 8130 Tectonophysics: Heat generation and transport; KEYWORDS: exhumation, erosion, normal faults, numerical modeling, heat flow, thermochronometers

Exhumation of the central Wasatch Mountains, Utah: 1. Patterns and timing of exhumation deduced from low-temperature thermochronology data

[1] The Wasatch Mountains are often cited as an example of normal fault growth and footwall flexure. They represent a tilted footwall at the edge of the Basin and Range extensional province, a major rift basin. Thus understanding the detailed spatial and elevation changes in coupled thermochronometer data, and how these changes can be interpreted, may aid in the analysis of thermochronometer data from other extensional regions around the world. We present a dense data set from the Cottonwood Intrusive Belt (CIB) of the Wasatch that includes apatite fission track (AFT), zircon fission track (ZFT), and apatite (U-Th)/He ages. ZFT, AFT, and apatite (U-Th)/He ages are 10, 5, and 3 Ma, respectively, adjacent to the Wasatch fault. AFT and (U-Th)/He ages increase slightly with distance east of the fault until about 15–20 km, where a more abrupt increase in these ages occurs at or near the Silver Fork-Superior fault zone. ZFT and AFT ages are concordant with 31–38 Ma pluton emplacement ages on the eastern side of range. Modeling of the data leads to the following interpretation: (1) Early cooling and � 3–4 km of exhumation for the middle and eastern parts of the range occurred in the late Oligocene-middle Miocene. (2) Beginning at 10–12 Ma, the locus of exhumation shifted westward toward the present range front, where the rocks cooled from >200� C in the last 10–12 Myr. Our data and interpretations are consistent with a model in which the locus of faulting and exhumation shifted opposite the direction of tilt, similar to that predicted by rolling-hinge extensional models. However, this westward shift and rapid Miocene to recent exhumation may be a local effect superimposed on lower fault displacement and exhumation rates elsewhere along the Wasatch. INDEX TERMS: 8109 Tectonophysics: Continental tectonics—extensional (0905); 1035 Geochemistry: Geochronology; 8010 Structural Geology: Fractures and faults; 8015 Structural Geology: Local crustal structure; KEYWORDS: Wasatch Mountains, exhumation, fission track, helium dating Citation: Armstrong, P. A., T. A. Ehlers, D. S. Chapman, K. A. Farley, and P. J. J. Kamp, Exhumation of the central Wasatch Mountains, Utah: 1. Patterns and timing of exhumation deduced from low-temperature thermochronology data, J. Geophys. Res., 108(B3), 2172, doi:10.1029/2001JB001708, 2003.

Relations between hinterland and foreland shortening: Sevier orogeny, central North American Cordillera

The tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens. The classic central Cordilleran foreland fold-and-thrust belt in the United States (Late Jurassic to early Tertiary Sevier belt) and the more internal zones to the west (central Nevada thrust belt) provide data critical to understanding the developmere of internal and external parts of orogens. The Garden Valley thrust system, part of the central Nevada thnkst belt, crops out in south-central Nevada within a region generally considered to be the hinterland of the Jurassic to Eocene Sevier thrust belt. The thrust system consists of at least four principal thrust plates composed of strata as young as Pennsylvanian in age that are unconformably overlain by rocks as old as Oligocene, suggesting that contraction occurred between those times. New U/Pb dates on intrusions that postdate contraction, combined with new paleomagnetic data showing significant tilting of one area prior to intrusion, suggest that regionally these thrusts were active before -85-100 Ma. The thrust faults are characterized by long, relatively steeply dipping ramps and associated folds that are broad and open to close, upright and overturned. Although now fragmented by Cenozoic crustal extension, individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike. We correlate the structurally lowest thrust of the Garden Valley thrust system, the Golden Gate-Mount Irish thrust, southward with the Gass Peak thrust of southern Nevada. This correlation carries the following regional implications. At least some of the slip across Jurassic to mid-Cretaceous foreland thrusts in southern Nevada continues northward along the central Nevada thrust belt rather than noaheastward into Utah. This continuation is consistent with age relations, which indicate that thrusts in the type Sevier belt in central Utah are synchronous with or younger than the youngest thrusts in southern Nevada. This in turn implies that geometrically similar Sevier belt thrusts in Utah must die out southward before they reach Nevada, that slip along the southem Nevada thrusts is partitioned

Is the Wasatch fault footwall (Utah, United States) segmented over million-year time scales?

The Wasatch fault zone, Utah, is a 370-km-long segmented normal-fault system with topographic salients, depths of footwall exposure, geomorphic properties, and geophysical anomalies that suggest differential long-term footwall uplift and exhumation on segments that are partitioned by long-lived structural segment boundaries. Apatite (U-Th)/He ages from footwall samples along the range front from the five central footwall segments av- erage 5.3 6 1.0 Ma. Coupled two-dimensional thermokinematic and helium-diffusion mod- els suggest average long-term (;5 m.y.) exhumation rates of 0.2-0.4 mm/yr for most of the Wasatch front. The exception is the southern end of the Salt Lake City segment, where exhumation rates are two times as great as elsewhere along the Wasatch front. The rel- atively invariant He ages and exhumation rates imply that most of the Wasatch did not behave kinematically as independent footwall-segment blocks with differential exhumation amounts over the past 5 m.y. The structural boundaries, such as salients and intrabasin highs that partially delineate segments, may have persisted since the Pliocene and con- trolled the locations of the surface-rupture segments.

Alternating asymmetric topography of the Alaska range along the strike‐slip Denali fault: Strain partitioning and lithospheric control across a terrane suture zone

Contrasting lithospheric strength between terranes often results in the concentration of strain and deformation within the weaker material. Dramatic alternating asymmetric topography of the central and eastern Alaska Range along the active Denali fault is due to contrasting lithospheric strength between terranes and a suture zone, controlled by fault location with respect to the irregular boundary of a relatively stronger terrane backstop. Highest topography and greatest Neogene exhumation in the central Alaska Range occur on the concave side of the arcuate Denali fault, yet to the north and on the convex side of the fault in the eastern Alaska Range. The Denali fault largely lies along a Mesozoic suture zone between two large composite terranes (Yukon and Wrangellia composite terranes: YCT and WCT), but the McKinley strand of the fault cuts across an embayment of weaker suture-zone rocks (Alaska Range suture-zone, ARSZ) within the irregular southern boundary of the YCT (Hines Creek fault). Deformation (and uplift of the Alaska Range) is driven by slip and partitioning of strain along the Denali fault, occurring preferentially in weaker rocks of the ARSZ against the stronger YCT. Where the YCT lies well north of the McKinley strand, deformation is primarily to the north of the fault (eastern Alaska Range). Where the YCT is close to the fault, deformation is primarily to the south (central Alaska Range). While the trace of the McKinley strand approximates a small circle, two restraining bends (McKinley and Hayes) pinned equidistant from the ends of this strand localize uplift and exhumation.

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

The western Chugach Mountains and Prince William Sound are located in a syntaxial bend, which lies above flat-slab subduction of the Yakutat microplate and inboard of the Yakutat collision zone of southern Alaska. The syntaxis is characterized by arcuate fault systems and steep, high topography, which suggest focused uplift and exhumation of the accretionary prism. We examined the exhumation history with low-temperature thermochronometry of 42 samples collected across the region. These new apatite (U-Th)/He, apatite fission-track, zircon (U-Th)/He, and zircon fission-track ages, combined with ages from surrounding regions, show a bull’s-eye pattern, with the youngest ages focused on the western Chugach syntaxis. The ages have ranges of ca. 10–4 Ma, ca. 35–11 Ma, ca. 33–25 Ma, and ca. 44–27 Ma, respectively. The youngest ages are located on the south (windward) side of the Chugach Mountains and just north of the Contact fault. Sequentially higher closure temperature systems are nested across Prince William Sound in the south, the Chugach Mountains, and the Talkeetna Mountains to the north. Computed exhumation rates typically are 0.2 mm/yr across Prince William Sound, increase abruptly to ∼0.7 mm/yr across and adjacent to the Contact fault system, and decrease to ∼0.4 mm/yr north of the core of the Chugach Mountains. The abrupt age and exhumation rate changes centered on the Contact fault system suggest that it may be a critical structural system for facilitating rock uplift. Our data are most consistent with Yakutat flat-slab subduction starting in the Oligocene, and since then ∼11 km of rock uplift north of the Contact fault and ∼4–5 km of rock uplift in Prince William Sound to the south. These data are consistent with a deformation model where the western Chugach core has approached long-term exhumational steady state, though exhumation rates have probably increased in the last ∼5 m.y. We interpret that rock uplift in the overriding wedge has been driven dominantly by underplating, with long-term vertical displacement concentrated at the southern edge of the Chugach Mountains and centered on the Contact fault system. Though our data do not unequivocally differentiate between Pliocene tectonic- or climate-related causes for increased exhumation in the last ∼5 m.y., we interpret the increased rates to be due to increased influx of underplated sediments that are derived from erosion in the Saint Elias orogen collision zone.

Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska

Megathrust splay fault systems in accretionary prisms have been identified as conduits for long-term plate motion and significant coseismic slip during subduction earthquakes. These fault systems are important because of their role in generating tsunamis, but rarely are emergent above sea level where their long-term (million year) history can be studied. We present 32 apatite (U-Th)/He (AHe) and 27 apatite fission-track (AFT) ages from rocks along an emergent megathrust splay fault system in the Prince William Sound region of Alaska above the shallowly subducting Yakutat microplate. The data show focused exhumation along the Patton Bay megathrust splay fault system since 3–2 Ma. Most AHe ages are younger than 5 Ma; some are as young as 1.1 Ma. AHe ages are youngest at the southwest end of Montague Island, where maximum fault displacement occurred on the Hanning Bay and Patton Bay faults and the highest shoreline uplift occurred during the 1964 earthquake. AFT ages range from ca. 20 to 5 Ma. Age changes across the Montague Strait fault, north of Montague Island, suggest that this fault may be a major structural boundary that acts as backstop to deformation and may be the westward mechanical continuation of the Bagley fault system backstop in the Saint Elias orogen. The regional pattern of ages and corresponding cooling and exhumation rates indicate that the Montague and Hinchinbrook Island splay faults, though separated by only a few kilometers, accommodate kilometer-scale exhumation above a shallowly subducting plate at million year time scales. This long-term pattern of exhumation also reflects short-term seismogenic uplift patterns formed during the 1964 earthquake. The increase in rock uplift and exhumation rate ca. 3–2 Ma is coincident with increased glacial erosion that, in combination with the fault-bounded, narrow width of the islands, has limited topographic development. Increased exhumation starting ca. 3–2 Ma is interpreted to be due to rock uplift caused by increased underplating of sediments derived from the Saint Elias orogen, which was being rapidly eroded at that time.