Audrey D. Huerta

The thermal structure of collisional orogens as a response to accretion, erosion, and radiogenic heating

Thermal models of collisional orogens generally predict temperature structures that are much cooler than those recovered by thermobarometric studies. Here we demonstrate that high-temperature, low-pressure metamorphism and the development of inverted geotherms within collisional belts may be the result of accretion and erosion acting on crust enriched with heat-producing elements. A new two-dimensional finite difference model, described here, incorporates the subduction of lithosphere with heat-producing material in the upper crust, accretion of crustal material from the subducting plate to the upper plate, and surface erosion of the upper plate. These processes result in the development of a wedge of heat-producing material within the upper plate. The rate of heat production within the wedge and maximum depth of the wedge are the most important parameters controlling the magnitude of upper plate temperatures. Our model yields inverted upper plate geotherms when heat production rates exceed 0.75 μW/m3 and the heat-producing wedge extends to a depth greater than 35 km. Temperatures in excess of 500°C at depths of 20–30 km are computed when heat production rates are greater than ∼1.75 μW/m3 and the wedge extends to a depth >50 km. Other processes, such as shear heating, fluid flow, or mantle delamination, need not be invoked to explain geologic evidence of high temperatures or inverted thermal gradients in collisional systems.

The Interdependence of Deformational and Thermal Processes in Mountain Belts

Crustal temperatures within collisional orogens are anomalously high compared with temperatures at comparable depths in stable continents, which is evidence of thermal processes that are fundamental to orogenesis. These temperatures can be explained by the redistribution of crust enriched in heat-producing elements through the accretion of crust from the down-going plate to the upper plate and surface erosion. With the use of geologically reasonable rates, the model results predict high temperatures (over 600°C) and inverted upper-plate geotherms (about 100°C over 20 kilometers) at shallow depths (20 to 40 kilometers) by 25 to 35 million years after collision. This study emphasizes the interdependence of deformational, surficial, and thermal processes.

Upper mantle structure of central and West Antarctica from array analysis of Rayleigh wave phase velocities

The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two-plane-wave tomography method, and are inverted for shear velocity using a Monte-Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70-100 km thick lithosphere, underlain by a low velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome, and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity.

The mantle transition zone beneath West Antarctica: Seismic evidence for hydration and thermal upwellings

Although prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here we use P wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high-quality PRFs using an iterative, time domain deconvolution method filtered with a Gaussian width of 0.5 and 1.0, corresponding to frequencies less than ∼0.24 and ∼0.48 Hz, respectively. Single-station and common conversion point stacks, migrated to depth using the AK135 velocity model, indicate that mantle transition zone thickness throughout most of West Antarctica does not differ significantly from the global average, except in two locations; one small region exhibits a vertically thinned (210 ± 15 km) transition zone beneath the Ruppert Coast of Marie Byrd Land and another laterally broader region shows slight, vertical thinning (225 ± 25 km) beneath the Bentley Subglacial Trench. We also observe the 520 discontinuity and a prominent negative peak above the mantle transition zone throughout much of West Antarctica. These results suggest that the mantle transition zone may be hotter than average in two places, possibly due to upwelling from the lower mantle, but not broadly across West Antarctica. Furthermore, we propose that the transition zone may be hydrated due to >100 million years of subduction beneath the region during the early Mesozoic.

Wilson cycles, tectonic inheritance, and rifting of the North American Gulf of Mexico continental margin

The tectonic evolution of the North American Gulf of Mexico continental margin is characterized by two Wilson cycles, i.e., repeated episodes of opening and closing of ocean basins along the same structural trend. This evolution includes (1) the Precambrian Grenville orogeny; (2) formation of a rift-transform margin during late Precambrian opening of the Iapetus Ocean; (3) the late Paleozoic Ouachita orogeny during assembly of Pangea; and (4) Mesozoic rifting during opening of the Gulf of Mexico. Unlike the Atlantic margins, where Wilson cycles were first recognized, breakup in the Gulf of Mexico did not initially focus within the orogen, but was instead accommodated within a diffuse region adjacent to the orogen. This variation in location of rifting is a consequence of variations in the prerift architecture of the orogens. The Appalachian-Caledonian orogeny involved substantial crustal shortening and formation of a thick crustal root. In contrast, the Ouachita orogeny resulted in minimal crustal shortening and thickening. In addition, rather than a crustal root, the Ouachita orogen was underlain by the lower plate of a relatively pristine Paleozoic subduction system that is characterized by a shallow mantle. A finite element model simulating extension on the margin demonstrates that this preexisting structure exerted fundamental controls on the style of Mesozoic rifting. The shallow mantle created a strong lithosphere beneath the orogen, causing extension to initiate adjacent to, rather than within, the orogen. On the Atlantic margins, the thick crustal root resulted in a weak lithosphere and initiation of extension within the interior of the orogen. Major features of the modern Gulf of Mexico margin, including the Interior Salt Basin, outboard unextended Wiggins arch, and an unusually broad region of extension beneath the coastal plain and continental shelf, are direct consequences of the prerift structure of the margin.

Early Paleozoic transform-margin structure beneath the Mississippi coastal plain, southeast United States

A geophysical transect across the central Gulf of Mexico coastal plain shows that the early Paleozoic continental margin of southern Laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late Paleozoic Ouachita orogeny and formation of the modern Gulf of Mexico coastal plain. The thickness of the crystalline crust decreases abruptly across the margin over a distance of ∼50 km, from 35 km beneath the Black Warrior foreland basin to 10 km beneath the Ouachita fold-and-thrust belt. This abrupt decrease in crustal thickness is similar to modern transform margins, but very different from most rifted margins, which display much more gradual transitions in crustal thickness. The geophysical data indicate an absence of synrift intrusive and volcanic rocks, underplated mafic rocks at the base of the crust, and abnormally thick oceanic crust adjacent to the margin. The lack of these features is also characteristic of modern transform margins. Combined with transects across the margin farther west, the data confirm previous suggestions that the central Gulf of Mexico coastal plain overlies an ∼800-km-long transform segment of the late Proterozoic–early Paleozoic southern Laurentian continental margin that extends continuously from western Arkansas to southeast Alabama.

A Seismic Transect Across West Antarctica: Evidence for Mantle Thermal Anomalies Beneath the Bentley Subglacial Trench and the Marie Byrd Land Dome

West Antarctica consists of several tectonically diverse terranes, including the West Antarctic Rift System, a topographic low region of extended continental crust. In contrast, the adjacent Marie Byrd Land and Ellsworth-Whitmore mountains crustal blocks are on average over 1 km higher, with the former dominated by polygenetic shield and stratovolcanoes protruding through the West Antarctic ice sheet and the latter having a Precambrian basement. The upper mantle structure of these regions is important for inferring the geologic history and tectonic processes, as well as the influence of the solid earth on ice sheet dynamics. Yet this structure is poorly constrained due to a lack of seismological data. As part of the Polar Earth Observing Network, 13 temporary broadband seismic stations were deployed from January 2010 to January 2012 that extended from the Whitmore Mountains, across the West Antarctic Rift System, and into Marie Byrd Land with a mean station spacing of ~90 km. Relative P and S wave travel time residuals were obtained from these stations as well as five other nearby stations by cross correlation. The relative residuals, corrected for both ice and crustal structure using previously published receiver function models of crustal velocity, were inverted to image the relative P and S wave velocity structure of the West Antarctic upper mantle. Some of the fastest relative P and S wave velocities are observed beneath the Ellsworth-Whitmore mountains crustal block and extend to the southern flank of the Bentley Subglacial Trench. However, the velocities in this region are not fast enough to be compatible with a Precambrian lithospheric root, suggesting some combination of thermal, chemical, and structural modification of the lithosphere. The West Antarctic Rift System consists largely of relative fast uppermost mantle seismic velocities consistent with Late Cretaceous/early Cenozoic extension that at present likely has negligible rift related heat flow. In contrast, the Bentley Subglacial Trench, a narrow deep basin within the West Antarctic Rift System, has relative P and S wave velocities in the uppermost mantle that are ~1% and ~2% slower, respectively, and suggest a thermal anomaly of ~75 K. Models for the thermal evolution of a rift basin suggest that such a thermal anomaly is consistent with Neogene extension within the Bentley Subglacial Trench and may, at least in part, account for elevated heat flow reported at the nearby West Antarctic Ice Sheet Divide Ice Core and at Subglacial Lake Whillans. The slowest relative P and S wave velocity anomaly is observed extending to at least 200 km depth beneath the Executive Committee Range in Marie Byrd Land, which is consistent with warm possibly plume-related, upper mantle. The imaged low-velocity anomaly and inferred thermal perturbation (~150 K) are sufficient to support isostatically the anomalous long-wavelength topography of Marie Byrd Land, relative to the adjacent West Antarctic Rift System.

Constraining rates of thrusting and erosion : Insights from kinematic thermal modeling

We present a thermal model of a simple thrust system that can be used to determine thrust rates and erosion rates from low-temperature thermochronology. Unlike previous models, this model incorporates the effects of erosion both during and after thrusting. In particular, we examine the modeled evolving thermal structure and pressure-temperature-time evolution of hanging-wall rocks that undergo fault-bend folding due to transport over a blind footwall ramp. In all cases, rocks cool as they move over the footwall ramp, potentially providing a common pinpoint for determining thrust rates. In the simplest case, low-temperature thermochronology of minerals that pass through their closure temperature over the ramp will yield details on thrust kinematics (thrust rate, timing of initiation, and duration of thrusting). Additional cooling ages of a more comprehensive sample suite can capture cooling due to erosion. In these latter cases, model results can place limitations on erosion rates.

Seismic evidence for lithospheric foundering beneath the southern Transantarctic Mountains, Antarctica

The 3000-km-long Transantarctic Mountains (TAMs), which separate cratonic East Antarctica from tectonically active West Antarctica, remain one of the least understood of Earth’s major mountain ranges. The tectonic mechanism that generates the high elevation, as well as the processes that produce major differences between various sectors of the TAMs, are still uncertain. Here we present newly constructed seismic images of the crust and uppermost mantle beneath central Antarctica derived from recently acquired seismic data, indicating ongoing lithospheric foundering beneath the southern TAMs. These images reveal an absence of thick, cold cratonic lithosphere beneath the southern TAMs. Instead, an uppermost-mantle slow seismic anomaly extends across the mountain front and 350 km into East Antarctica, beneath a high plateau near the South Pole. Under the slow anomaly, a relatively high-wavespeed root is found at ~200 km depth, connected with the East Antarctic lithosphere, suggesting that sinking lithosphere has been replaced at shallow depths by warm, slow-velocity asthenosphere. A mantle lithosphere foundering model is proposed to interpret these images, which best explains the present large area of high elevation and the uplift of the TAMs, as well as Miocene-age volcanism in the Mount Early region.

Correcting GIS-based slope aspect calculations for the Polar Regions

Intr oductionSlope aspect is an important geomorphic parameter . Theaspect of a slope controls its solar irradiation, thus affectinga wide diversity of processes (for a partial review see Mooreet al. 1991). Slope aspect can be calculated from a digitalelevation model using one of several GIS-based algorithms(e.g. Moore et al. 1991, Burrough & McDonnell 1998).These algorithms are designed for mid-latitude regions,calculating slope aspect as an angle relative to grid north. Inpolar regions, however , this approach suf fers from twoproblems:1. In the commonly-used polar stereographic projections,grid north is parallel to either the prime meridian (forthe south pole) or the meridian at 180i (for the northpole) while geographic north varies with longitude.2. In the polar regions, the direction of geographic northcan vary significantly over a relatively small area.As a result, GIS-based slope aspect calculations do not givecorrect values when using polar projections.The method we present here provides a simple techniquefor correcting GIS-calculated slope aspect to determine thetrue, geographic slope aspect. This technique is critical forwork using slope aspect in the polar regions.Aspect corr ectionT o obtain a corrected slope aspect map, we must subtracteach grid cellOs longitude from the GIS-calculated aspect(Fig. 1). In the South Pole S tereographic Projection (seeSnyder 1987, p. 154) the longitude, !, of a data point in thisprojection is independent of the ellipsoid used and is givenby where the four -quadrant arctangent is used. In this equationx and y are the coordinates of the data point after theremoval of false easting and false northing (i.e., when thecoordinates of the pole are 0,0). In the North PoleS tereographic Projection, the longitude is given by ESRI states that incorrect slope aspects in the polar regionsrepresent Oa known limit for our softwareO (personalcommunications, ESRI support, February 2006). Thus, wehave written a Matlab function (Appendix A) to convert theESRI-determined slope aspect into slope aspects that arereferenced to geographic north. This function uses the GIS-calculated aspect data, the grid spacing, and the (x,y)coordinates of the lower left-hand corner of the grid asinputs. The data can be transferred between ArcGIS andMatlab in the form of ASCII files. The Matlab function (Appendix A) allows the user tochoose output ranges of 0i to 360i or -180i to 180i. Thecode also addresses the two special cases of 1) grid cells thatdo not have an elevation in the DEM (ONoDataO cells inArcGIS), and 2) cells that are flat, and thus have no aspect.By convention, the ONoDataO cells are set to a value of -9999 and the flat cells are set to a value of -1. If the userchooses an output range of -180i to 180i the flat cells areset to a value of 9999.AcknowledgementsThis research was partially funded by NSF grant OPP-0534036. W e thank the referee for their helpful comments.