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From the Himalaya to the Canadian Cordillera

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Boulder, Colo., USA – GSA's newest journal, Lithosphere, has put together several articles touching on the evolution and nature of Earth's crust and upper mantle. Topics include mountain-building and metamorphism in the Canadian Cordillera; the deep roots of an ancient continental arc exposed in Fiordland, New Zealand; the Sevier hinterland plateau, USA; the geologic history of the central and northern Tibetan Plateau; and Quaternary river diversion in the eastern Himalaya.

Record of orogenic cyclicity in the Alberta foreland basin, Canadian Cordillera

Garrett M. Quinn et al., Department of Geoscience, University of Calgary, Earth Science 118, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada. This article is online at http://lithosphere.gsapubs.org/content/early/2016/05/05/L531.1.abstract.

The record of orogenic cyclicity in the Alberta Foreland Basin, Canadian Cordillera, considers the evolution of the Canadian Rocky Mountains in the Jurassic and Cretaceous through analysis of sedimentary archives in the adjacent Alberta Foreland Basin. This study test a recent hypothesis that Cordilleran mountain systems like the Canadian Rockies evolve through cyclic periods of uplift and magmatic flare ups caused by the foundering of deep crustal roots into the mantle. By dating zircon grains from sandstones of the Alberta Foothills, apparently cyclic changes in sediment source areas to the basin are documented. These source areas include the southwestern U.S., the incipient Rocky Mountains, as well as igneous rocks formed as a result of mountain-building. Not only can changes in sediment sources be explained by the hypothesis of cyclic uplift, but the timing of enhanced sedimentation pulses in the basin is consistent with the timing of interpreted magmatic flare-ups.

Pre-Cenozoic geologic history of the central and northern Tibetan Plateau and the role of Wilson cycles in constructing the Tethyan orogenic system

Chen Wu et al., Structural Geology Group, China University of Geosciences (Beijing), Beijing 100083, China; corresponding author: An Yin. This article is online at http://lithosphere.gsapubs.org/content/early/2016/04/25/L494.1.abstract.

Swiss geologist Eduard Suess (1831-1914) first postulated the existence of a great ocean system, which he termed the Tethys, in the Paleozoic Era (between 500 and 250 million years ago) that separated the only two continental landmasses on Earth at the time: Supercontinent Gondwana in the south and Supercontinent Laurasia in the north. Later studies emphasize the antiquity of the northernmost branch of the Tethyan ocean system, with an inference that it had existed before the assembling and formation of the two supercontinents. In this study, scientists from China University of Geosciences (Beijing), the University of California at Los Angeles, and Chinese Academy of Sciences show that the oldest and northernmost ocean in the Tethyan system, commonly referred to as the Paleo-Tethys, was generated within the northern supercontinent Laurasia via a process known as the Wilson Cycle, first proposed by Tuzo Wilson (1908-1993) in 1966 for the evolution of the Atlantic ocean. The new finding reported by Wu et al. requires a fresh look at the existing theories on the origin of the Tethys, one of the greatest ocean systems in the history of the Earth.

Gneiss domes, vertical and horizontal mass transfer, and the initiation of extension in the hot lower crustal root of a continental arc, Fiordland, New Zealand

Keith A. Klepeis et al., Dept. of Geology, The University of Vermont, Burlington, Vermont 05405-0122, USA. This article is online at http://lithosphere.gsapubs.org/content/early/2016/01/25/L490.1.

This study uses structural analyses and zircon geochronology to determine how the deep roots of an ancient continental arc now exposed in Fiordland, New Zealand, formed and evolved over 35 million years. The Fiordland locality is important because it contains the largest (3,000 square kilometers) and deepest (approx. 65 km) known surface exposure of continental crust that formed at the base of an ancient continental arc. We show how the deep root of this arc was thermally and mechanically rejuvenated by the rapid influx of a large volume of magma during the Cretaceous. This event mobilized partially molten material within the crust, forming gneiss domes and leading the sinking of a dense root into the mantle. Our data reveal the three-dimensional patterns of this flow and the mechanisms that moved heat and mass through the Earth's lithosphere.

Middle Jurassic landscape evolution of southwest Laurentia using detrital zircon geochronology

Sally L. Potter-McIntyre et al., Southern Illinois University, Dept. of Geology, Parkinson Lab Mailcode 4324, Carbondale, Illinois 62902, USA. This article is online at http://lithosphere.gsapubs.org/content/early/2016/01/25/L467.1.abstract.

This study presents a refined interpretation of what the landscape looked like on southwestern Laurentia during Mesozoic rifting of the supercontinent Pangea and the opening of the Gulf of Mexico. An abrupt change in sediment source occurred due to tectonic uplift of the southwestern flank of the Ancestral Rocky Mountains which caused a stream capture and drainage reorganization. This drainage reorganization input a large amount of water into the basin and caused the landscape to change from a desert with sand dunes that had persisted for tens of millions of years to large hypersaline lakes. The stream capture and drainage reorganization that created the lake system recorded in the Wanakah Formation and the Tidwell Member of the Morrison Formation likely evolved into the major river system that deposited the Salt Wash Member of the Morrison Formation.

Shallow-crustal metamorphism during Late Cretaceous anatexis in the Sevier hinterland plateau: Peak temperature conditions from the Grant Range, eastern Nevada, USA

Sean P. Long, School of the Environment, Washington State University, Pullman, Washington 99164, USA; and Emmanuel Soignard, Leroy Eyring Center For Solid State Science, Arizona State University, Tempe, Arizona 85287, USA. This article is online at http://lithosphere.gsapubs.org/content/early/2016/01/25/L501.1.abstract.

During the construction of mountain belts, the thermal conditions of the deforming crust progressively evolve, and can exert a significant influence on the corresponding deformation history. In the North American Cordilleran mountain belt, during the Late Cretaceous and early Tertiary periods, a high plateau called the Nevadaplano is hypothesized to have occupied what is now Nevada and western Utah, and was located west of the center of active crustal shortening in central Utah. This paper discusses evidence for significant heating of the upper ~10 km of the crust of the plateau in areas of eastern Nevada at about 80 million years ago, while the mountain belt was still being constructed. This anomalous heating indicates the possibility for significant thermal weakening of regions of the plateau crust, and may have contributed to a documented slowing of shortening rates in the adjacent zone of crustal shortening in central Utah.

Kinematics of Late Quaternary slip along the Yabrai Fault: Implications for Cenozoic tectonics across the Gobi Alashan block, China

Jingxing Yu and Wenjun Zheng, State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China. This article is online at http://lithosphere.gsapubs.org/content/early/2016/03/03/L509.1.abstract.

The Yabrai range-front fault accommodates deformation within the middle Gobi Alashan block between the Tibetan Plateau and the Ordos block. In this paper, the authors have determined that the Yabrai range-front fault is composed of three segments of varying fault strike and sense of motion based on geomorphic features and trench exposures. This work is remarkable in that it suggests that the deformation of the Tibetan Plateau has extended into the Gobi Alashan block and has built the deformation pattern for the southern Gobi Alashan block.

A synthesis of Jurassic and Early Cretaceous crustal evolution along the southern margin of the Arctic Alaska-Chukotka microplate and implications for defining tectonic boundaries active during opening of Arctic Ocean basins

Alison B. Till, U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508, USA. This article is online at http://lithosphere.gsapubs.org/content/early/2016/03/08/L471.1.abstract.

Tectonic models of Arctic Ocean basin opening remain controversial and are central to understanding the region's resource potential. One of the more significant impediments to construction of tectonic models is the uncertain role of a large piece of continental crust, the Arctic Alaska-Chukotka microplate. While many models favor opening the Canada Basin by rotation of the microplate away from Arctic Canada, it is too large to rotate as a single crustal block. This paper is a synthesis of the distribution, character, and timing of Jurassic and Early Cretaceous crustal deformation events along the margin of the microplate. The synthesis reveals that the microplate was probably two separate crustal pieces until the Aptian (Early Cretaceous; approximately 120 million years ago). The likely collision zone between Arctic Alaska and Chukotka is preserved as a zone of metamorphism and crustal thickening that crosses the eastern end of Chukotka and extends well into northern Alaska, along the southern flank of the Brooks Range.

Tectonic and climate controls on Quaternary river diversion in the eastern Himalaya

Jin-Yu Zhang et al., State Key Laboratory of Geological Processes and Mineral Resources and Structural Geology Group, China University of Geosciences, Beijing 100083, P.R. China; corresponding author: An Yin. This article is online at http://lithosphere.gsapubs.org/content/early/2016/05/05/L500.1.abstract.

About 100 years ago, two great Himalayan geologists — Sir Sidney Burrard and Sir. Henry Hayden — postulated that the Yarlung River north of the Himalaya had been episodically blocked due to Himalayan tectonic activities. As a result, this river may have spilled over the Himalayan crest departing from its current course. This geologically plausible inference has never been proven until the study presented by Zhang et al. reported in Lithosphere. In their studies, the authors present geomorphologic and sedimentological evidence to support this long-speculated geological process in the Himalaya. An important implication of this work is that the river course may have shifted rapidly at a time scales of few thousand years during episodes of rapid climate change in the region.

Diachronous tectono-metamorphism in the northern Canadian Cordillera

Reid D. Staples et al., Dept. of Earth Sciences, Simon Fraser University, 8888 University Dr., Burnaby, British Columbia, Canada V5A 1S6. This article is online at http://lithosphere.gsapubs.org/content/8/2/165.abstract.

From the abstract: Development of amphibolite-facies transposition fabrics in the northern Canadian Cordilleran hinterland occurred diachronously in the Permian-Triassic, Early Jurassic, Middle Jurassic to Early Cretaceous, and Early to mid-Cretaceous. Rocks tectonized in the Permian-Triassic and Early Jurassic were exhumed in the Early Jurassic, while rocks immediately to the northeast (toward the foreland) were not buried and heated until the Middle Jurassic to mid-Cretaceous. Early Jurassic to mid-Cretaceous emplacement of the Yukon-Tanana terrane on the North American continental margin, together with the imbrication of parautochthonous rocks, formed a foreland-propagating orogenic wedge. Cooler rocks in front of the wedge were progressively buried and metamorphosed to amphibolite facies from the Jurassic to mid-Cretaceous as they were underthrust into a spatially and temporally transient distributed ductile shear zone near the base of the overriding wedge. Rocks previously incorporated into this zone were displaced upward and exhumed through the combined effects of renewed underplating at depth and compensating extensional and erosional denudation above to maintain a critically tapered wedge. Extensional exhumation of the metamorphic hinterland in the mid-Cretaceous marked the collapse and end of orogen-perpendicular wedge dynamics in operation since the Early Jurassic. Rocks incorporated into the midcrustal shear zone in the Middle Jurassic to mid-Cretaceous were exhumed in the mid-Cretaceous along southeast-directed (orogen-parallel) extensional faults from beneath a supracrustal "lid" tectonized in the Permian-Triassic and Early Jurassic. Like the Himalayan orogen and eastern Alps, orogen-parallel extension developed in an orthogonal plate-convergent setting, simultaneous with, and bounded by, orogen-parallel strike-slip faulting that facilitated northwestward lateral extrusion of rocks normal to the direction of convergence.

Observations on normal-fault scarp morphology and fault system evolution of the Bishop Tuff in the Volcanic Tableland, Owens Valley, California, U.S.A.

David A. Ferrill, Director, Department of Earth, Material, and Planetary Sciences, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166, USA. This article is online at http://lithosphere.gsapubs.org/content/early/2016/03/28/L476.1.abstract.

From the abstract: Mapping of normal faults cutting the Bishop Tuff in the Volcanic Tableland, northern Owens Valley, California, using side-looking airborne radar data, low-altitude aerial photographs, airborne light detection and ranging (LiDAR) data, and standard field mapping yields insights into fault scarp development, fault system evolution, and timing. Fault zones are characterized by multiple linked fault segments, tilting of the welded ignimbrite surface, dilation of polygonal cooling joints, and toppling of joint-bounded blocks. Maximum fault zone width is governed by (i) lateral spacing of cooperating fault segments and (ii) widths of fault tip monoclines. Large-displacement faults interact over larger rock volumes than small-displacement faults and generate larger relay ramps, which, when breached, form the widest portions of fault zones. Locally intense faulting within a breached relay ramp results from a combination of distributed east-west extension, and within-ramp bending and stretching to accommodate displacement gradients on bounding faults. One prominent fluvial channel is offset by both east- and west-dipping normal faults such that the channel is no longer in an active flowing configuration, indicating that channel incision began before development of significant fault-related geomorphic features. The channel thalweg is "hanging" with respect to modern (Q1) and previous (Q2) Owens River terraces, is incised through the pre-Tahoe age terrace level (Q4, 131-463 ka), and is at grade with the Tahoe age (Q3, 53-119 ka) terrace. Differential incision across fault scarps implies that the channel remained active during some of the faulting history, but it was abandoned between Q2 and Q3 time, while faulting continues to the present day.

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