Flysch deposition and preservation of coherent bedding in an accretionary complex: Detrital zircon ages from the Upper Cretaceous Valdez Group, Chugach terrane, Alaska

The Upper Cretaceous Valdez Group represents the flysch facies of the Mesozoic Chugach terrane accretionary complex in southern Alaska. The Valdez Group is dominated by litharenite sandstone and argillite deposited as coherent beds, unlike the older McHugh Complex mélange and massive sandstones. Detrital zircons from five sandstones sampled along an ~55 km transect through the Valdez Group were dated using U-Pb laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). The youngest populations from the two oldest samples, located along strike from each other, were 82-81 Ma. Three samples across strike and outboard of the others are separated by ~50 km, but each has a youngest population dated at ca. 68 Ma. All of these samples have major grain population ages that suggest erosion from the Coast Mountains Batholith, consistent with petrography and grain modes suggesting an arc source. No apparent age gap exists between the youngest McHugh Complex samples and the oldest Valdez Group samples, suggesting continuous deposition despite the different depositional and tectonic style. We propose a model in which the onset of coherently bedded flysch marks the transition from deposition in the trench or trench slope to deposition on the oceanic plate beyond the trench after it was filled at ca. 84 Ma, i.e., the time of the youngest mélange sedimentation. Preservation of coherent bedding resulted as large coherent blocks of Valdez Group rocks were imbricated into the subduction complex during continued subduction in Paleogene time. © 2011 Geological Society of America

Plate Margin Deformation and Active Tectonics Along the Northern Edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska and Yukon, Canada

The northwest directed motion of the Pacific plate is accompanied by migration and collision of the Yakutat terrane into the cusp of southern Alaska. The nature and magnitude of accretion and translation on upper crustal faults and folds is poorly constrained, however, due to pervasive glaciation. In this study we used high-resolution topography, geodetic imaging, seismic, and geologic data to advance understanding of the transition from strike-slip motion on the Fairweather fault to plate margin deformation on the Bagley fault, which cuts through the upper plate of the collisional suture above the subduction megathrust. The Fairweather fault terminates by oblique-extensional splay faulting within a structural syntaxis, allowing rapid tectonic upwelling of rocks driven by thrust faulting and crustal contraction. Plate motion is partly transferred from the Fairweather to the Bagley fault, which extends 125 km farther west as a dextral shear zone that is partly reactivated by reverse faulting. The Bagley fault dips steeply through the upper plate to intersect the subduction megathrust at depth, forming a narrow fault-bounded crustal sliver in the obliquely convergent plate margin. Since . 20 Ma the Bagley fault has accommodated more than 50 km of dextral shearing and several kilometers of reverse motion along its southern flank during terrane accretion. The fault is considered capable of generating earthquakes because it is linked to faults that generated large historic earthquakes, suitably oriented for reactivation in the contemporary stress field, and locally marked by seismicity. The fault may generate earthquakes of Mw <= 7.5

Strategies for effective unmanned aerial vehicle use in geological field studies based on cognitive science principles

Field geologists are increasingly using unmanned aerial vehicles (UAVs or drones), although their use involves significant cognitive challenges for which geologists are not well trained. On the basis of surveying the user community and documenting experts’ use in the field, we identified five major problems, most of which are aligned with well-documented limits on cognitive performance. First, the images being sent from the UAV portray the landscape from multiple different view directions. Second, even with a constant view direction, the ability to move the UAV or zoom the camera lens results in rapid changes in visual scale. Third, the images from the UAVs are displayed too quickly for users, even experts, to assimilate efficiently. Fourth, it is relatively easy to get lost when flying, particularly if the user is unfamiliar with the area or with UAV use. Fifth, physical limitations on flight time are a source of stress, which renders the operator less effective. Many of the strategies currently employed by field geologists, such as postprocessing and photogrammetry, can reduce these problems. We summarize the cognitive science basis for these issues and provide some new strategies that are designed to overcome these limitations and promote more effective UAV use in the field. The goal is to make UAV-based geological interpretations in the field possible by recognizing and reducing cognitive loa

Kinematic model for out-of-sequence thrusting: Motion of two ramp-flat faults and the production of upper plate duplex systems

Kinematic models developed here suggest a bewildering array of structural styles can be generated during out-of-sequence thrusting. Many of these structures would be difficult to distinguish from a normally stacked thrust sequence and the process can produce younger-on-older faults that could easily be misinterpreted as normal faults. This paper considers a small subset of this problem within a large model space by considering structures that develop along a pair of ramp-flat faults that are moving simultaneously, or sequentially. Motion on the lower ramp warps the structurally higher fault due to fault-bend folding and when the fault ruptures through the warp it transfers a horse to the upper hanging wall. Continuity of the process generates what is referred to here as an “upper plate duplex” to distinguish the structure from a conventional duplex. Kinematic parameters are developed for two models within this general problem: 1) a system with a fixed ramp in the lower thrust, overridden by an upper thrust; and 2) a double-duplex system where a conventional duplex develops along the lower fault at the same time as an upper plate duplex is formed along the upper fault. The theory is tested with forward models using 2D Move software and these tests indicate different families of structural styles form in association with relative scaling of ramp systems, slip-ratio between faults, and aspect ratios of horse blocks formed in the upper-plate duplex. A first-order result of the analysis is that an upper plate duplex can be virtually indistinguishable from a conventional duplex unless the trailing branch lines of the horses are exposed or imaged; a condition seldom met in natural exposures. Restoration of an upper-plate duplex produces counterintuitive fault geometry in the restored state, and thus, restorations of upper plate duplexes that erroneously assume a conventional duplex model would produce restored states that are seriously in error. In addition, in most of the models some fault segments place younger rocks on older rocks which could be easily misinterpreted as normal fault systems. In some models younger-on-older juxtapositions are significant and if scaled to crustal scale would produce core-complex style structures that would be difficult to recognize as contractional features. Collectively, these observations imply that many areas where simultaneous contraction and extension are inferred may be entirely contractional with younger-on-older relationships generated by out-of-sequence thrust systems. Examples where this process may have occurred are in southwestern North America and the Moine thrust system and future studies should evaluate these systems in light of these models. Distinguishing upper plate duplex from conventional duplex is potentially important in economic evaluations of thrust systems because fluid migration paths would be very different in the two alternatives. The process may also be important in seismogenic mechanisms, particularly in subduction megathrusts, because faults warping faults could produce fault irregularities that would form transient asperities along the fault

Challenges and Strategies for increasing the number of Hispanic students in the geoscience program at the University of Texas at El Paso

The geology program at the University of Texas at El Paso (UTEP) includes a PhD program with a strong research base located in a community with a nearly 80% Hispanic population. Because of our location on the US-Mexican border and the policy of the University to serve our local population, we are a major source of Hispanic geoscientists at all levels. However, the number and percentage of Hispanic students decreases steadily as the academic level increases. This decline may reflect our success in placing our graduates in advanced degree programs at other universities and in jobs. In contrast, our undergraduates are primarily Hispanic, due in part to a strong recruiting pipeline that makes use of multiple programs to interest local pre-college students in geology. It begins with campus tours and talks at local schools followed by a summer program (Pathways) for high school freshmen and sophomores that introduces about 50 outstanding local students to geoscience through a variety of activities with our faculty. The Pathways program is followed by the METALS program which involves 10 high school juniors and seniors in a 2 week field trip with students from San Francisco State University, Purdue University, and the University of New Orleans. Undergraduate students at UTEP are often supported as research assistants who work in labs and on projects that lead to presentations at major meetings and summer internships at other Universities through both the Pathways and the METALS programs. The students who come through this pipeline typically are heavily recruited by other universities and employers. We now need to counteract this loss of potential Hispanic graduate students with a pipeline program that recruits heavily from HSIs that do not offer the research opportunities and advanced degrees that UTEP can provide. Thus, we are working to identify promising geoscience students who do not yet considered themselves to be likely PhD students. Because many of our Hispanic students are first generation college students, the PhD is often not something they envision for themselves but by working with Hispanic undergraduates and MS students in a variety of settings we can begin to change that mindset and make them realize that UTEP offers a unique opportunity to pursue an advanced degree

Timing and origin of migmatitic gneisses in south Karakoram: Insights from U-Pb, Hf and O isotopic record of zircons

The timing and origin of partial melting in collision belts is crucial to understand the thermotectonic evolution and the relationship between HT metamorphism and magmatism in over-thickened crust. In the present study, we used the in-situ isotopic (Hf, O and U-Pb) record of zircons to investigate the timing and origin of migmatitic gneisses exposed in the core of the Dassu dome in south Karakoram. The new U-Pb zircon dating identified the Proterozoic inherited cores (1.8-1.9 Ga and 2.3-2.5 Ga) surrounded by a Neogene overgrowth with ages ranging from similar to 6 to similar to 20 Ma. These ages imply that the partial melting in the Karakoram Metamorphic Complex lasted from \textgreater20 Ma to similar to 6 Ma and can be correlated with the Miocene magmatism in the adjacent Baltoro region. Oxygen isotopic data from Proterozoic inherited cores (1.8-1.9 Ga) and Neogene overgrowths are indistinguishable and generally vary from 8 parts per thousand to 9.5 parts per thousand. These values are slightly higher than the most igneous zircons (6.5-8 parts per thousand Valley et al., 2005) indicating an igneous precursor with heavy initial 0 composition that later might have equilibrated with low temperature environment or some involvement of supracrustal material is likely. However, a few low U/Th, relatively old inherited cores (2.3-2.5 Ga) showed mantle-like (delta O-18 = 5.3 +/- 0.6 parts per thousand, Valley et al., 2005) values of delta O-18 = 5.5 +/- 2.7 parts per thousand. The present-day weighted mean epsilon Hf (0) of the Proterozoic inherited cores ranges from -50 +/- 1.0 to -44.3 +/- 1.2. In contrast, the Neogene rims are 15-20 epsilon-units higher than the inherited core with present-day epsilon Hf (0) = -30.6 +/- 0.9. This implies that the Hf composition of the Neogene overgrowth is not controlled exclusively by the dissolution of the inherited cores and that contamination by external melts is likely. We suggest a contribution from the Neogene, less-evolved magmatism in the Baltoro region (epsilon Hf (0) = similar to-4 to -10). The elevated oxygen composition is not consistent with the contribution from pristine mantle-derived magmas. The observed homogeneous and uniform Hf-O isotopic composition of the Proterozoic inherited cores suggest their derivation from mildly evolved infracrustal sources with minor input from supracrustal material. The older inherited zircons (2.3-2.5 Ga) were precipitated from juvenile mantle derived magmas. (C) 2016 Elsevier Ltd. All rights reserved

The tectonic significance of the Early Cretaceous forearc-metamorphic assemblage in south-central Alaska based on detrital zircon U-Pb dating of sedimentary protoliths

A complex array of faulted arc rocks and variably metamorphosed forearc accretionary complex rocks form a mappable arc-forearc boundary in southern Alaska known as the Border Ranges fault (BRF). We use detrital U-Pb zircon dating of metasedimentary rocks within the Knik River terrane in the western Chugach Mountains to show that a belt of Early Cretaceous amphibolite-facies metamorphic rocks along the BRF was formed when older mélange rocks of the Chugach accretionary complex were reworked in a sinistral-oblique thrust reactivation of the BRF during a period of forearc plutonism. The metamorphic subterrane of the Knik River terrane has a maximum depositional age of 156.5 ± 1.5 Ma and a detrital zircon age spectrum that is indistinguishable from the Potter Creek assemblage of the Chugach accretionary complex, supporting correlation of these units. These ages contrast strongly with new and existing data that show Triassic to Earliest Jurassic detrital zircon ages from metamorphic screens in the plutonic subterrane of the Knik River terrane, a fragmented Early Jurassic plutonic assemblage generally interpreted as the basement of the Peninsular terrane. Based on these findings, we propose new terminology for the Knik River terrane. We propose the terms: (1) “Carpenter Creek metamorphic complex” for the Early Cretaceous “metamorphic subterrane”; (2) “western Chugach trondhjemite suite” for the Early Cretaceous forearc plutons within the belt; (3) “Friday Creek assemblage” for a transitional mélange unit that contains blocks of the Carpenter Creek complex in a chert-argillite matrix; and (4) “Knik River metamorphic complex” in reference to metamorphic rocks engulfed by Early Jurassic plutons of the Peninsular terrane that represent the roots of the Talkeetna arc). The correlation of the Carpenter Creek metamorphic complex with the Chugach mélange indicates that the trace of the Border Ranges fault lies ~1–5 km north of the map trace shown on geologic maps, although like other segments of the Border Ranges fault, this boundary is blurred by local complexities within the Border Ranges fault system. Ductile deformation of the mélange is sufficiently intense that few vestiges of its original mélange fabric exist, suggesting the scarcity of rocks described as mélange in the cores of many orogens may result from misidentification of rocks that have been intensely overprinted by younger, ductile deformation.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author