Boulder, Colo., USA - Lithosphere articles posted 13 and 21 May cover several fascinating locations and geodynamic processes. One study investigates the kinematic evolution of the Himalayan orogen at a site in Nepal. Another paper addresses the "unroofing" of the Klamath Mountains in northern California/southern Oregon, USA. In the East African Rift area, researchers are examining how vegetation mediates slope erosion. Another group focusses on the largest salt lake of the Mediterranean region, Lake Tuz, Turkey.
All recently posted Lithosphere articles are listed below.
Abstracts are online at http://lithosphere.
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Lateral extrusion, underplating, and out-of-sequence thrusting within the Himalayan metamorphic core, Kanchenjunga, Nepal
T.K. Ambrose et al., University of British Columbia, Okanagan, Kelowna, British Columbia, Canada. Published online ahead of print on 21 May 2015; http://dx.
The ongoing India-Asia collision has often served as a template for understanding processes in continent-continent convergent margins. Recent studies of the Himalaya have begun to discover cryptic thrust-sense discontinuities within the exhumed, former mid-crustal core; however, their significance to the kinematic evolution of the orogen has remained enigmatic. This new study by T.K. Ambrose and colleagues helps elucidate the nature of these structures and presents an integrated kinematic model for the growth and evolution of the orogenic core. Their findings have significant bearing on the understanding of mid-crustal processes and how convergence is accommodated within large orogenic systems.
Unroofing the Klamaths -- Blame it on Siletzia?
R. Piotraschke et al., Pennsylvania State University, University Park, Pennsylvania, USA. Published online ahead of print on 13 May 2015; http://dx.
The Klamath Mountains of northern California and southern Oregon have long been enigmatic because of their anomalous height, uncertainties in the timing of their development, and their location at a major plate boundary transition. From a detailed study of the exhumational and structural evolution of the massif, new constraints on the timing of uplift of the Klamath Mountains have been determined. Rather than becoming exposed by regional uplift, the Klamaths became exposed as its crust was pulled from beneath adjacent, stable North America as the Farallon subduction zone was re-established after the docking of the Siletz terrane in central Oregon. This style of mountain building, although associated with a plate tectonic event, represents a new mechanism to produce a plate margin orogen.
Paradigm lost: Buoyancy thwarted by the strength of the Western Gneiss Region (U)HP terrane, Norway
J.P. Butler et al., Dept. of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada. Published online ahead of print on 13 May 2015; http://dx.
Ultrahigh-pressure (UHP) metamorphic rocks indicate that when continents collide their crust can be subducted to depths greater than 100 km. The current paradigm is that these UHP terranes are exhumed to the near surface when they detach from the underlying lithosphere and rise buoyantly along the subduction zone, a process typically envisaged as rapid (a few million years) and restricted to the early stages of continental collision. Yet in Norway, the Western Gneiss Region (WGR) UHP terrane experienced much longer-duration UHP metamorphism (approx. 15 million years) followed by exhumation after plate convergence ceased. Our two-dimensional geodynamic models show that the duration of UHP metamorphism and style of exhumation largely depend on crustal strength. The WGR crust must have been especially strong to resist detaching from the underlying lithosphere such that it remained at UHP conditions and only exhumed during later post-convergent extension, rather than by buoyancy-driven detachment and exhumation.
The effect of vegetation cover on millennial-scale landscape denudation rates in East Africa
Verónica Torres Acosta et al., Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Potsdam, Germany. Published online ahead of print on 13 May 2015; http://dx.
Although we have long understood that vegetation protects Earth's surface from erosion, understanding how climate shifts over longer time scales influence erosion is complicated by concurrent changes in vertical motions due to active tectonics (the summed motion of many earthquakes) and other processes. Our erosion rate measurements averaged over the past several thousand years from the East African Rift demonstrate that erosion rates are closely linked to slope (which in turn is linked to vertical motions), but vegetation cover controls how quickly erosion rates change with increasing slope. Minor shifts in vegetation could thus strongly influence erosion rates and the overall shape of the landscape.
Genetic framework of Neogene-Quaternary basin closure process in central Turkey
A. Gürbüz and N. Kazanci, Ankara Üniversitesi, Mühendislik Fakültesi, Jeoloji Mühendisligi Bölümü, 06100, Tandogan, Ankara, Turkey and Nigde Üniversitesi, Mühendislik Fakültesi, Jeoloji Mühendisligi Bölümü, 51240, Nigde, Turkey. Posted online ahead of print on 13 May 2015; http://dx.
The largest salt lake of the Mediterranean region, Lake Tuz, represents the last breaths of a dying sedimentary basin under the effects of internal and external forces of Earth. Lake Tuz ("tuz" means "salt" in Turkish) is a very large but very shallow lake that located in central Turkey with a surface area of over 1500 square kilometers. Researchers from Nigde and Ankara Universities of Turkey have modeled the Lake Tuz basin using both surface and subsurface data for the last approx. 10 million years. The current saline lake is a relic of a previously very large and deep freshwater lake that covered this largest intracontinental basin of the country. The old (paleo) lake basin was connected to the sea until the triggering of basin closure process by the tearing of a subducted slab beneath Anatolia. As a result, a series of internal forces (isostasy, volcanism, and faulting) separated the lake basin from the Mediterranean Sea prominently, divided the old lake basin into two subbasins, forced the Lake Tuz basin to migrate northward, and trapped the lake in a very shallow depression in the middle of country as it is today. During the basin closure process, climatic change played a relatively minor role and was closely associated with the results of internal forces.
Tectonic and sedimentary linkages between the Belt-Purcell basin and southwestern Laurentia during the Mesoproterozoic ca. 1.60-1.40 Ga
J.V. Jones III et al., U.S. Geological Survey, Alaska Science Center, Anchorage, Alaska, USA. Published online ahead of print on 21 May 2015; http://dx.
Bedrock exposures in western North America preserve three known remnants of ancient sedimentary basins that formed between 1.5 and 1.4 billion years ago and contain distinctive and globally rare 1.6 to 1.5 billion-year-old detrital zircon grains. These basin remnants extend from Arizona to western Yukon, Canada, and provide one of the few direct links tying North America to formerly adjacent continents such as Australia and Antarctica in the supercontinent Columbia that formed 1.6 billion years ago. James V. Jones and colleagues review detrital zircon data and geologic relationships that link the geological histories of these basins within the evolution of the North American continent and also provide key insights into the timing, geometry, and possible mechanisms of supercontinent formation and breakup.
Low-temperature thermochronology of the Black and Panamint mountains, Death Valley, California: Implications for geodynamic controls on Cenozoic intraplate strain
T.S. Bidgoli et al., University of Kansas, Lawrence, Kansas, USA. Published online ahead of print on 21 May 2015; http://dx.
Death Valley is one of the most iconic geologic locales in the world and host to some of the best-studied faults in scientific literature. However, despite decades of research, significant debate surrounds the timing of development of major normal and strike-slip faults in the region and potential drivers of that deformation. Here, Tandis S. Bidgoli and colleagues present new apatite and zircon (U-Th)/He ages and thermal modeling results that shed light on these issues. The new data are evaluated within the framework of regional fault timing data to show that Basin and Range extension and dextral transtension associated with the eastern California shear zone occur in discrete phases, a conclusion that complements several recent studies in the region. More importantly, the study is the first to firmly establish a link between the initiation of the dextral faults west of Death Valley and lithospheric delamination approx. 3.5 million years ago.