April 2011 Geosphere highlights

(Press-News.org) Boulder, CO, USA - The April 2011 Geosphere includes three articles designated for the latest Geosphere theme: Exploring the Deep Sea and Beyond. Other topics include new entries to existing themes: New developments in Grenville geology; Making the Southern Margin of Laurentia; and Neogene Tectonics and Climate-Tectonic Interactions in the Southern Alaskan Orogen. Additionally, the April issue features several articles not associated with a specific theme.

Geosphere announces a new themed issue: The Exploring the Deep Sea and Beyond: Contributions to Marine Geology in Honor of William R. Normark issue, edited by Andrea Fildani, David J.W. Piper, and Dave Scholl, honors the work of
William R. Normark in the advancing the understanding of marine geology. The themed issue includes an article by Paull et al., published with the December 2010 Geosphere.

Introduction to the Theme: A volume in honor of William R. Normark, known to all of us as simply Bill, is an appropriate contribution to the memory of a great scientist, an extraordinary mentor, and an unrivaled friend. Editors and authors were energized by the opportunity to craft such a volume: To us Bill was a friend, a colleague, and an insightful peer always ready to share new ideas. For some of us, Bill was a mentor, an inspiration, and an unmatched role model. The variety and creativity reflected by these manuscripts revisit Bill's broad interests in earth sciences. His holistic approach to science and his natural talent for synthesizing large data sets made Bill the prototype of the modern scientist. Integration across disciplines, scientific rigor, and masterful syntheses together represent the "core" of Bill's legacy. This volume embodies Bill's ideas of exploring nature with every available tool while keeping his mind open to surprises around each corner. Throughout his career, targeted exploration remained the most effective scientific method to apply to revealing the ocean's secrets. This volume collects scientific contributions from recognized experts in different fields. Contributions are from marine geology, sedimentology, tectonics, seafloor geomorphology, and overarching earth sciences.

Modeling of submarine cyclic steps: Controls on their formation, migration, and architecture
Svetlana Kostic, San Diego State University, Civil, Construction and Environmental Engineering, 5500 Campanile Drive, San Diego, California 92182, USA; doi: 10.1130/GS601.1.

Migrating sediment waves constitute the most common bedform on the seafloor. There is a growing recognition that in many cases vast sediment-wave fields represent yet another manifestation of cyclic steps. This paper by Svetlana Kostic of San Diego State University outlines submarine cyclic steps in the context of the sediment waves of various origin and addresses frequent terminology confusion between net-depositional cyclic steps and sediment waves in general. The numerical experiments presented in the paper were designed to (1) classify submarine cyclic steps, and (2) identify the key parameters governing their formation, migration, and architecture.

Linking a late Miocene-Pliocene hiatus in the deep-sea Bounty Fan off South Island, New Zealand, to onshore tectonism and lacustrine sediment storage
Kathleen M. Marsaglia et al., Dept. of Geological Sciences, 18111 Nordhoff Street, California State University, Northridge, California 91330-8266, USA; doi: 10.1130/GS621.1.

This paper provides an excellent example of how the complex interplay between tectonic and paleoceanographic forces can affect the sedimentary record in deep-marine systems. Kathleen M. Marsaglia of California State University at Northridge and colleagues link a break in sediment accumulation (hiatus) on the submarine Bounty Fan off New Zealand to a combination of decreased sediment supply owing to tectonic disruption of onshore river drainage and a roughly simultaneous increase in ocean bottom-current strength. Evidence for this scenario includes the distribution of current-generated structures in sediment cores from the fan, the relative timing of an onshore transition from river to lake sedimentation, and a potential post-hiatus pulse of more weathered sediment into the Bounty Fan. This sediment pulse was possibly associated with the re-establishment of through-going drainage and the erosion and flushing of stored lake sediments through the system.

The natural range of submarine canyon-and-channel longitudinal profiles
Jacob A. Covault et al., Chevron Energy Technology Company, Clastic Stratigraphy Research and Development, 6001 Bollinger Canyon Road, San Ramon, California 94583, USA; doi: 10.1130/GS610.1.

Submarine canyons are conduits through which sediment and dissolved loads are transported across continental margins to the deep sea, thereby playing a vital role in seascape evolution and global geochemical cycling. Processes that sculpted canyons during their lifetimes are manifested in the shapes of longitudinal profiles measured down their axes. Jacob A. Covault of Chevron Energy Technology Company and colleagues differentiated 20 submarine canyon longitudinal profiles across various types of continental margins on the basis of relative convexity or concavity, and according to their similarities to best-fitting mathematical functions. Profiles generally reflect the competing influences of uplift and construction of depositional relief of the seafloor and its degradation by erosion related to mass wasting. Results of this study provide a new catalog of the breadth and general controls of the shapes of submarine sediment-delivery systems, which can be applied to predictive models of depositional architecture and continental margin evolution.

Fresh Science for Existing Themed Issues THEME: NEW DEVELOPMENTS IN GRENVILLE GEOLOGY New insights on the evolution of the Lyon Mountain Granite and associated Kiruna-type magnetite-apatite deposits, Adirondack Mountains, New York State
Peter M. Valley et al., Dept. of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada; doi: 10.1130/GS624.1.

Peter M. Valley of Memorial University of Newfoundland and colleagues use a multidisciplinary approach (including field relations, petrography, geochemistry, and U-Th-Pb geochronology) to understand the type, timing, and sequence of complex fluid alteration and magnetite-apatite mineralization associated with the Lyon Mountain granite. The results presented in this paper suggest that the initial deposits formed during granite emplacement and coincide with the middle to late Ottawan orogeny. Secondary mineralization and remobilization of these early formed ores occurred 20 to 60 million years later during extension of the Adirondack Highlands as external Na-rich fluids penetrated the Lyon Mountain granite during orogenic collapse.

THEME: MAKING THE SOUTHERN MARGIN OF LAURENTIA THEMED ISSUE Evaluation of Nd isotope data for the Grenville Province of the Laurentian shield using a geographic information system
E.S. Moore and A.P. Dickin, School of Geography & Earth Sciences, McMaster University, Hamilton, Ontario L8S 4K1, Canada; doi: 10.1130/GS612.1.

The Laurentian Shield forms the core of the North American continent and has a geologic history spanning billions of years. The samarium-neodymium radiometric dating method provides an ideal tool for charting the growth of the continent over geologic history, due to its ability to yield an estimate of the initial age of "crustal formation" for a given rock unit, and because of its resistance to later metamorphic disturbance. Hence, Nd isotope analysis can be used to map crustal terranes that were formed at different times and later added to the margins of the continent by plate tectonic collisions. Authors E.S. Moore and A.P. Dickin of McMaster University note that this paper represents the first use of a Geographic Information System (GIS) to analyze crustal formation ages in a large area of the Laurentian shield from Ontario to western Quebec. GIS contouring of Sm-Nd crustal formation ages is used to test geologic models for the location of major crustal boundaries, including the line of crustal suturing between the Archean continental nucleus and younger accreted terranes, and the most important thrust boundary associated with the 1-billion-year-old Grenville mountain building event. The result of this work is a more accurate geologic map of crustal terrane boundaries within the Laurentian Shield, and hence an improved understanding of its evolution.

Stratigraphy and age of Upper Jurassic strata in north-central Sonora, Mexico: Southwestern Laurentian record of crustal extension and tectonic transition
David J. Mauel, Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA; doi: 10.1130/GES00600.1.

The Late Jurassic was a period of changing tectonic regimes along the southern margin of present day North America. It was during this period that the Pangean supercontinent began to break apart and the Gulf of Mexico began its formation. Although much is known, many details concerning the changing plate tectonics remain poorly understood and highly debated among geologists who study this time period and region. David J. Mauel of New Mexico State University and colleagues present new data on the age and depositional setting of Upper Jurassic strata in north-central Sonora, Mexico. These strata represent the fill of a Late Jurassic marine depocenter that they term the Altar-Cucurpe Basin. This basin was part of a larger northwest trending marine embayment named the Arivechi-Cucurpe seaway, which was likely linked to the Gulf of Mexico during its existence. Mauel and colleagues interpret the sediments that ultimately filled the Altar-Cucurpe Basin to have been derived primarily from the eruptive products of coeval volcanism related to continental rifting. The Late Jurassic volcanic center from where the ash rich sediments were likely sourced has been shown to have been located in south-central Arizona and north-central Sonora by previous workers. The new data presented here help to improve understanding of the age range of magmatism in this extensional setting during a time when magmatism was not particularly abundant in the rest of Mexico and the southwestern United States. In addition, the authors present a revised Jurassic-Lower Cretaceous stratigraphy for the region that suggests several conglomeratic units, formerly interpreted as Late Jurassic pull-apart basin deposits and related to the Mojave-Sonora megashear, are not of Late Jurassic age. The Mojave Sonora megashear is a hypothetical continental-scale strike slip fault proposed to have been active during Late Jurassic time.

THEME: NEOGENE TECTONICS AND CLIMATE-TECTONIC INTERACTIONS IN THE SOUTHERN ALASKAN OROGEN Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali Fault of the eastern Alaska Range
Jeff A. Benowitz et al., Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775, USA; doi: 10.1130/GS589.1.

The Denali Fault, an active continental-scale strike-slip fault, has an east-west and north-south asymmetric topographic signature (the Alaska Range) making the region an ideal location for a case study to examine the contribution of local and regional structures on strain partitioning and the timing of exhumation adjacent to the fault. Jeff A. Benowitz of the University of Alaska at Fairbanks and colleagues applied a full range of thermochronometers on granitoid samples collected from the eastern segment of the Alaska Range, to increase their understanding of the role of both far-field effects and local structural irregularities on exhumation patterns in the region. The authors infer deformation and rapid exhumation has been occurring in the eastern Alaska Range since at least ca. 22 Ma, most likely related to the continued collision of the Yakutat microplate with the North American plate. Late Pleistocene to recent (past ~1 million years) tectonically driven exhumation in the eastern Alaska Range is not range-wide, but is instead focused in the Nenana Mountain region, possibly related to variations in fault plane geometry. Exhumation patterns suggest that the master strand of the Denali Fault plays a first-order role on topographic development in the region. During the Plio-Quaternary, episodes of climatic instability and overall global cooling are temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of Denali Fault system.

HIGHLIGHTS OF NON-THEMED ARTICLES IN THE APRIL 2011 ISSUE

A reassessment of Mojavia and a new Cheyenne Belt alignment in the eastern Great Basin
Stephen T. Nelson et al., Dept. of Geological Sciences, S-389 ESC, Brigham Young University, Provo, Utah 84602, USA; doi: 10.1130/GS595.1.

Continents are made up of individual pieces of different ages and somewhat different compositions. The boundaries between individual pieces are easily recognized if they are exposed at the surface. However, many boundaries are covered by younger sedimentary rocks, and this is true in the western United State. This paper by Stephen T. Nelson of Brigham Young University and colleagues indicates that the pieces that make up the western United States may be fewer in number than previously recognized. In addition, a crustal boundary thought to extend east-west along the axis of the Uinta Mountains, separating 1.7 from >2.5 b.y. crust, is mislocated. The boundary appears to lie well to the north.

Structure of the San Fernando Valley region, California: Implications for seismic hazard and tectonic history
V.E. Langenheim et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA; doi: 10.1130/GS597.1.

Synthesis by V.E. Langenheim of the U.S. Geological Survey and colleagues of geologic and geophysical data, including 13 industry seismic-reflection profiles, reveals an asymmetric basin beneath San Fernando Valley, the site of two damaging earthquakes in 1971 and 1994. The deepest part is in the northern part of the valley, with basin depths reaching 5-8 km (Sylmar subbasin). Gravity and seismic-reflection data show a concealed basement high beneath the southeastern part of the valley that influenced Miocene depositional patterns and may have influenced damage from the 1994 Northridge earthquake. These data map the North Leadwell fault zone, a zone of previously unrecognized Miocene normal faults that extend across the valley and may connect to the Oak Ridge fault system. Another Miocene fault system along the northeast margin of the valley coincides in part with the neotectonic Verdugo fault and separates uplift of the Santa Susana Mountains from subsidence in the Sylmar subbasin (containing nearly 3.7 km of Quaternary gravels), highlighting the influence of Miocene structures on young faulting and landscape.

High-resolution three-dimensional imaging and analysis of rock falls in Yosemite Valley, California
Greg M. Stock et al., National Park Service, Yosemite National Park, 5083 Foresta Road, Box 700, El Portal, California 95318, USA; doi: 10.1130/GS617.1.

Rock falls commonly occur from the granitic walls of Yosemite Valley, modifying this iconic landscape but also posing significant potential hazards to the roughly 4 million visitors to the park each year. Detailed documentation of rock falls in Yosemite has previously been difficult because of the sheer size and inaccessibility of the walls, but recent breakthroughs in digital imaging provide valuable new tools for studying these events. Greg M. Stock of the U.S. National Park Service and colleagues combine repeat gigapixel photography and ground-based laser scanning (LiDAR) to investigate large rock falls that occurred from Glacier Point in Yosemite Valley in October 2008. These high-resolution data and images characterize the rock-fall detachment surface and adjacent cliff area, quantify the rock fall volume at 5,663 cubic meters, evaluate the geologic structure that contributed to failure, and suggest a likely shear failure mode, a classic example of granitic exfoliation. Their results demonstrate the utility of remote sensing techniques for understanding rock falls from the largely inaccessible, vertical rock faces of Yosemite Valley, and for providing highly accurate and precise data needed for rock-fall hazard assessment.

Oblique convergence, arc-parallel extension, and the role of strike-slip faulting in the High Himalaya
Richard H. Styron et al., Department of Geology, University of Kansas, 1475 Jayhawk Boulevard, Lawrence, Kansas 66045, USA; doi: 10.1130/GS606.1.

A major, but often ignored, element of active Himalayan and south Tibetan tectonics is extension of the Himalayan arc parallel to the strike of the range. In this study, Richard H. Styron of the University of Kansas and colleagues test the more prominent models explaining this deformation. By quantifying the rates of tectonic deformation through analysis of published GPS data, as well as geologic field observations, the authors are able to show that the observed "stretching" of the Himalaya is consistent with forces applied to the range from the Indian plate as it collides with the Himalaya; because the Himalaya is curved in map view, the eastern and northwestern Himalaya are forced away from the central Himalaya, as they are dragged along the southern margin of the Tibetan Plateau by the underlying Indian plate. This scenario, dubbed the "oblique convergence" model, is very common in subduction zones worldwide, but has received less attention in the Himalayan and Tibetan literature than models seeking to explain arc-parallel extension and translation from body forces or an active "bending" of the Himalaya. This study was supported in part by a grant from the U.S. National Science Foundation.

### Keywords: Marine geology, sediment wave-fields, New Zealand, seafloor geomorphology, submarine canyon, Grenville, Lyon Mountain granite, Adirondack Mountains, Laurentian Shield, Geographic Information System, Denali Fault, Yakutat microplate, Cheyenne Belt, Mojavia, San Fernando Valley, 1994 Northridge earthquake, North Leadwell fault zone, Yosemite Valley, LiDAR, Himalayan arc, Mexico, Altar-Cucurpe Basin

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