Boulder, Colo., USA - New papers published in the June issue of Lithosphere cover the geology of Western Europe; the Osa Peninsula of Costa Rica; the Norwegian Caledonides; the Central Asian Orogenic Belt; the Karakoram shear zone and Greater Himalaya Sequence, NW India; the Garlock fault and the southern Sierra Nevada-eastern Tehachapi Mountains, USA; and the Chinese Altai. The issue features multi-national research teams, including authors from Belgium, Scotland, China, and Japan, as well as the USA.
Abstracts are online at http://lithosphere.
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Local stress sources in Western Europe lithosphere from the geoid anomalies
T. Camelbeeck et al., Royal Observatory of Belgium, Uccle, BE-1180, Belgium; http://lithosphere.
This paper by T. Camelbeeck and colleagues describes a method to evaluate the internal forces generated locally in the lithosphere by lateral variations of topography and density. The method applied to Western Europe suggests that these local forces can play a significant role in the present-day tectonic activity of some regions, like in the Pyrenees.
Upper plate deformation in response to flat slab subduction inboard of the aseismic Cocos Ridge, Osa Peninsula, Costa Rica
T.W. Gardner et al., Trinity University, Geosciences, One Trinity Place, San Antonio, Texas 78209, USA; http://lithosphere.
This paper by T.W. Gardner and colleagues describes the deformation of the upper part of Earth's crust on the Osa Peninsula on the southern Pacific coast of Costa Rica. Along this part of Central America, where the thick and buoyant, aseismic Cocos Ridge intersects the Middle America Trench, the Cocos plate is subducting under the Panama microplate at a very low angle (3 degrees to 10 degrees). As a result, the crust of the Panama microplate under the Osa Peninsula, 20 km inboard of the plate boundary, is quite thin, ~3 to ~ 10 km. Radiometric dating and elevations of relative young sediments, less than 125, 000 years old, indicates rapid vertical deformation of small crustal blocks, generally less that 5 km on a side. Rates of uplift range from 1.7 m per thousand years to 8.5 m per thousand years. The spatial distribution of deformation rates, topography, and structural fabric on the Osa Peninsula is, to the first order, controlled by short wavelength, high-relief bathymetric features on the subducting Cocos Ridge. Permanent deformation is largely accomplished by simple shear on an array of subvertical faults that allow the upper plate to adjust to variations in the slope and size of incoming ridges and seamounts. The deformation on the Osa Peninsula is ephemeral in the sense that topographic collapse and subsidence follows uplift as three-dimensional bathymetric features continue to subduct underneath the Osa Peninsula.
Extension, disruption, and translation of an orogenic wedge by exhumation of large ultrahigh-pressure terranes: Examples from the Norwegian Caledonides
Hannes K. Brueckner, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA; and Simon J. Cuthbert, School of Science, University of the West of Scotland, Paisley Campus, Paisley PA1 2BE, UK; http://lithosphere.
Some of the most impressive mountain systems, like the Alps or Himalayas, develop when two continents drift toward each other and ultimately collide. During collision, the edge of one of the continents is pulled deep (>100 km) into Earth's mantle through a process called "subduction." There the very high pressures and temperatures transform ("recrystallize") the subducted continental crust into metamorphic rocks characterized by very dense minerals, including in some cases tiny diamonds. Despite these transformations, the continental crust remains buoyant relative to the surrounding rocks of the mantle, and so will return toward the surface much as wooden plank submerged into a lake will float back to the top of the lake. Earth's mantle is not a liquid, of course, but surprisingly behaves as a fluid over millions of years. The return of continental crust is called "eduction," the opposite of subduction, which brings these high pressure/high temperature rocks out of the mantle and up into the cores of the overlying mountains. In this paper, Hannes K. Brueckner and Simon J. Cuthbert propose that this process can stretch and fragment the overlying mountains and even carry detached portions of the mountains away from the mountain core and toward its fringe. They suggest that this process operated 400 million years ago when "Baltica" (now known as Norway and Sweden) collided with "Laurentia" (now known as Greenland) to create the Caledonian mountain system. Baltica subducted westward beneath Laurentia and was partially transformed into a terrane composed of high pressure/high temperature metamorphic rocks. The subsequent eastward and upward exhumation (eduction) of these rocks stretched and disrupted the overlying Caledonides and even carried pieces of the Caledonides "piggyback" tens to maybe even a hundred or so kilometers to the east.
Tectonic processes, from rifting to collision via subduction, in SE Asia and the western Pacific: A key to understanding the architecture of the Central Asian Orogenic Belt
Koji Wakita et al., Geological Survey of Japan, Tsukuba, 3058567, Japan; http://lithosphere.
The Central Asian Orogenic Belt (CAOB) is a very attractive research area for the world's Earth scientists. The history of mountain building of the CAOB is dynamic and fantastic, but it is complicated to unravel. Therefore, says Koji Wakita of the Geological Survey of Japan, "We need to compare it with comparable, well-established, modern mountain belts." The region of continental Southeast Asia and the Western Pacific ocean provides such a modern analog of past geological processes. Wakita and colleagues review the processes of accretion of continental blocks during the Tertiary in Southeast Asia and the western Pacific with the aim of better understanding the evolution of the Central Asian Orogenic Belt, which is a Neoproterozoic to mid-Phanerozoic orogenic collage surrounded by the East European, Siberian, Tarim, and North China Cratons. In the Western Pacific, there is abundant evidence of sequential plate tectonic processes from accretion to continent-arc/continent collision, via exhumation and supra-subduction. Early processes involved accretion at a mid-oceanic ridge, sea-floor spreading with diagnostic ocean plate stratigraphy, subduction, accretion, the formation of island arcs, and back-arc extension. Two important types of tectonic setting and evolution are recognized along the present Pacific convergent margin; sediment/crust accretion and tectonic erosion. Subduction is also associated with back-arc extension, particularly in Indonesia and the southwest Pacific region, and arc-arc collisional belts are well developed in Taiwan, the Philippines, and Japan. Wakita and colleagues show that the CAOB during its history between 250 and 1000 million years ago was also created by subduction of oceanic crust with diagnostic stratigraphy, generation of island arcs, arc-arc collisions, back-arcs, and accretion of continental blocks.
Age and origin of granites in the Karakoram shear zone and Greater Himalaya Sequence, NW India
Forrest Horton and Mary L. Leech, Geosciences Department, San Francisco State University, 1600 Holloway Ave, San Francisco, California 94132, USA; http://lithosphere.
New ages for rocks at the core of the Himalaya and in the Karakoram fault zone in northwestern India indicate that there was less granite magmatism in the westernmost Himalaya and that magmatism ended there earlier than in the central and eastern Himalaya. The differences in the evolution of the western Himalaya could be due to strike-slip motion on the Karakoram fault because it behaved like a tectonic plate boundary in the last ~20 million years.
Exhumation of the southern Sierra Nevada-eastern Tehachapi Mountains constrained by low-temperature thermochronology: Implications for the initiation of the Garlock fault
A.E. Blythe and N. Longinotti, Geology Department, Occidental College, Los Angeles, California 90041, USA; http://lithosphere.
New apatite and zircon fission-track and apatite (U-Th)/He data from nine samples collected on a north-south transect across the southern Sierra Nevada-eastern Tehachapi Mountains constrain the cooling and exhumation history over the past ~70 million years. The data presented here by A.E. Blythe and N. Longinotti are consistent with northward-directed tilting and exhumation beginning 20 million years ago, probably as the result of north-south extension in the Mojave Desert on an early strand of the Garlock fault with down-to-the-south offset. A third minor phase of rapid exhumation beginning about 10 million years ago is suggested by the data; this may indicate the beginning of left-lateral slip on the Garlock fault.
Permian hornblende gabbros in the Chinese Altai from a subduction-related hydrous parent magma, not from the Tarim mantle plume
Bo Wan et al., Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China; http://lithosphere.
In the Chinese Altai, on the northern side of the Erqis fault, the ~10-m-wide Qiemuerqieke gabbro is composed almost entirely of hornblende and plagioclase. The results presented here by Bo Wan and colleagues, integrated with published data, support a model of juvenile crustal growth by a subduction-related process.