A once-only cataclysmic event and other mysteries of earth's crust and upper mantle

(Press-News.org) Boulder, Colo., USA - The April 2014 Lithosphere is now available in print. Locations covered include the Acatlán Complex, Mexico; east Yilgarn craton, Australia; the eastern Paganzo basin, Argentina; the hotspot-related Yellowstone crescent, USA; and the western Alps. Locations investigated in four new papers published online on 2 April include the Banks Island assemblage in Alaska and British Columbia; The Diligencia basin of the Orocopia Mountains in California; a U.S. post-Grenville large igneous province; and South Island, New Zealand.

Abstracts are online at http://lithosphere.gsapubs.org/content/early/recent. Representatives of the media may obtain complimentary copies of LITHOSPHERE articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to LITHOSPHERE in articles published. Contact Kea Giles for additional information or assistance. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

IN PRINT A Late Triassic tectonothermal event in the eastern Acatlán Complex, southern Mexico, synchronous with a magmatic arc hiatus: The result of flat-slab subduction?
Mortiz Kirsch et al., Institut für Geologie, Universität Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany. April 2014 issue; http://lithosphere.gsapubs.org/cgi/content/abstract/6/2/63.

The Late Paleozoic to Early Mesozoic geologic history of the North American Cordillera is dominated by subduction-related processes along the western Pangean convergent margin. In southern Mexico, the spatial and temporal patterns of tectono-magmatic processes within this geodynamic framework are still poorly constrained. This paper by Moritz Kirsch and colleagues provides evidence for a Triassic tectono-thermal event in southern Mexico as indicated by 40Ar/39Ar step-heating laser-probe ages for amphibole, muscovite and biotite from Carboniferous-Permian rocks of the eastern Acatlán Complex. These data suggest rapid cooling from approx. 525 to 300 degrees Celsius between about 239 and 219 million years ago, accompanied by local shearing and thrusting. The cooling ages coincide with an apparent hiatus in magmatic arc activity in southern Mexico, and are interpreted to record a transient change from steep to flat subduction.

Archean andesites in the east Yilgarn craton, Australia: Products of plume-crust interaction?
Stephen J. Barnes and Martin J. Van Kranendonk, Earth Science and Resource Engineering, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Kensington, Perth, Western Australia 6151, Australia. April 2014 issue; http://lithosphere.gsapubs.org/cgi/content/abstract/6/2/80.

The late Archaean, 2700 million years ago, was a period of extraordinary volcanism and ore deposit formation. Vast volumes of magma, of a wide range in composition and temperature, including some of the hottest lavas ever erupted on the planet, were generated over a few tens of millions of years over huge areas. Was this a once-only cataclysmic event representing a major transition in the thermal structure of the Earth? Or was it a combination of plate tectonic processes similar to those in the modern Earth, but with some additional factor that preferentially preserved volcanic rocks of this age? Explanations for the late Archean bonanza have been polarized between business-as-usual plate tectonics, and models based on giant mantle plumes: bursts of superhot upwelling material from the core-mantle boundary. One of the key lies of evidence has been the presence of late Archean andesites, the signature rock type of modern "ring of fire" subduction zones. But how similar to modern arc andesites are Archean andesites? Based on comparison of the Archean rocks of Western Australia with modern arc volcanic rocks, we find crucial geochemical differences that are explicable by formation of the andesites in a plume setting. We present a model in which basalts derived from a mantle plume mix with siliceous magmas generated by melting of preexisting crustal rocks and erupt as andesites. We regard the east Yilgarn volcanic suite at 2700 million years old as the equivalent of a modern plume-related Large Igneous Province.

A thermochronometric view into an ancient landscape: Tectonic setting, development, and inversion of the Paleozoic eastern Paganzo basin, Argentina
Eva Enkelmann et al., Department of Geology, University of Cincinnati, 500 Geology Physics Building, Cincinnati, Ohio 45221-0013, USA. April 2014 issue; http://lithosphere.gsapubs.org/cgi/content/abstract/6/2/93.

Employing multiple geo- and thermochronometric methods to a suite of bedrock and sedimentary samples in west-central Argentina, we are able to reconstruct the thermal evolution of the basement rocks of the eastern Paganzo basin, identify two distinct sediment source areas that supplied sediment to the basin, and establish the timing of basin inversion. These data provide a thermochronometric view into an ancient landscape and record its thermal evolution from Paleozoic time to the present. The study area is located in the Sierra de Chepes (Argentina). This area was chosen because it includes an exceptionally well exposed Carboniferous paleo-glacier valley filled with Upper Carboniferous to Permian strata of the eastern Paganzo basin that were deposited on Early Paleozoic basement.

Miocene regional hotspot-related uplift, exhumation, and extension north of the Snake River Plain: Evidence from apatite (U-Th)/He thermochronology
James J. Vogl et al., Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA. April 2014 issue; http://lithosphere.gsapubs.org/cgi/content/abstract/6/2/108. From the abstract: Passage of North America over the Yellowstone hotspot has had a profound influence on the topography of the northern Rocky Mountains region. One of the most prominent hotspot-related topographic features is the Yellowstone crescent of high terrain, which consists of two elevated shoulders bounding the eastern Snake River Plain and converging at a topographic swell centered on the Yellowstone region. We have applied single-grain dating to apatites collected from the Pioneer-Boulder Mountains on the northern arm of the Yellowstone crescent of high terrain to constrain the timing, rates, and spatial distribution of exhumation. These data provide constraints on the timing and processes responsible for uplift related to passage of the hotspot.

The late Miocene to Holocene erosion pattern of the Alpine foreland basin reflects Eurasian slab unloading beneath the western Alps rather than global climate change
Ramona Baran et al., Dept. of Earth and Environmental Sciences, Ludwig-Maximilians-University Munich, Luisenstrasse 37, 80333 Munich, Germany. April 2014 issue; http://lithosphere.gsapubs.org/cgi/content/abstract/6/2/124.

From the abstract: We synthesized published data on the erosion of the Alpine foreland basin and apatite fission-track ages from the Alps to infer the erosional sediment budget history for the past five million years. The results yield a large ellipsoidal, orogen-crossing pattern of erosion, centered along the western Alps. We suggest that accelerated erosion of the western Alps and their foreland basin occurred in response to regional-scale surface uplift, related to lithospheric unloading of the Eurasian slab along the Eurasian-Adriatic plate boundary.

NEWLY POSTED U-Pb and Hf isotope analysis of detrital zircons from the Banks Island assemblage (coastal British Columbia) and southern Alexander terrane (southeast Alaska)
Clare J. Tochilin et al., Department of Geology, University of Arizona, Tucson, Arizona, USA. Published online ahead of print on 2 Apr. 2014; http://dx.doi.org/10.1130/L338.1.

The Banks Island assemblage (BIA) consists of metasedimentary rocks that are exposed on the outer islands of coastal British Columbia. These rocks are enigmatic in that quartz-rich metasedimentary components accumulated near a continent, whereas the surrounding Alexander and Wrangellia terranes have little continent-derived material. In an effort to learn more about the origin of the BIA, the authors have conducted U-Pb and Hf isotope analyses on detrital zircons from metasedimentary rocks of the BIA and from nearby strata of the southern Alexander terrane. These data confirm that the BIA consists largely of continent-derived material, whereas the southern Alexander terrane formed far from a continental margin. Comparison with previously published data from other regions of the Cordillera suggests that the BIA resembles rocks of the northern Alexander terrane. The authors accordingly suggest that the BIA formed adjacent to the northern Alexander terrane and was offset southward by approximately 1000 km to now reside adjacent to the southern portion of the terrane. This southward motion was accommodated along the Early Cretaceous Kitkatla shear zone. Comparison of these U-Pb ages and Hf isotope signatures with other regions supports the hypothesis that the BIA and northern Alexander terrane formed in the circum-Arctic region. U-Pb and Hf data for Cambrian-Ordovician rocks are similar to values from the Timanides, whereas Silurian-Lower Devonian rocks yield values shared with Caledonian rocks in Baltica and northern Greenland. BIA strata of suspected late Paleozoic age are more juvenile, perhaps recording motion of the Alexander terrane from the circum-Arctic into the paleo-Pacific realm.

Paleotectonics of a complex Miocene half graben formed above a detachment fault: The Diligencia basin, Orocopia Mountains, southern California
Raymond V. Ingersoll et al., Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California 90095-1567, USA. Published online ahead of print on 2 Apr. 2014; http://dx.doi.org/10.1130/L334.1.

From the abstract: The Diligencia basin in the Orocopia Mountains of southeastern California has been one of the primary areas used to test the hypothesis of more than 300 km of dextral slip along the combined San Andreas/San Gabriel fault system. The Orocopia Mountains have also been the focus of research on deposition, deformation, metamorphism, uplift and exposure of the Orocopia Schist, which resulted from flat-slab subduction during the latest Cretaceous/Paleogene Laramide orogeny. The uppermost Oligocene/Lower Miocene Diligencia Formation consists of more than 1500 m of nonmarine strata, including 21- to 24-million-year-old basalt flows and intrusions…

U-Pb geochronology of 1.1 Ga diabase in the southwestern United States: Testing models for the origin of a post-Grenville large igneous province
Ryan M. Bright et al., Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Published online ahead of print on 2 Apr. 2014; http://dx.doi.org/10.1130/L335.1.

The Grenville orogeny involved the collision of a continent to the southern and eastern margins of North America during the period 1.3 to 1.1 billion years ago. Shortly after this collision, in the southwest United States, was a widespread magmatic event that resulted intrusions and lava flows. This study produced new ages on rocks in this magmatic province, named the Southwest Laurentia large igneous province, using the U-Pb decay scheme on a mineral called baddeleyite (ZrO2). The new ages suggest that this event is much shorter in duration than previously thought, lasting less than 20 million years from 1100 to 1080 million years ago. The cause of magmatism may have been the break-off of a subducting slab or a large mantle plume that split into two parts, one forming the Southwestern Laurentia large igneous province and the other responsible for the coeval magmatism in the Mid-Continent Rift exposed in the northern Midwest of the United States.

Two-stage development of the Paparoa Metamorphic Core Complex, West Coast, South Island, New Zealand: Hot continental extension precedes sea-floor spreading by -25 m.y.
Daniel O. Schulte et al., Technische Universität Darmstadt, Darmstadt, Germany. Published online ahead of print on 2 Apr. 2014; http://dx.doi.org/10.1130/L348.1.

Metamorphic core complexes can be precursors of continental breakup and result from extreme extension where the lower crust is dragged to the surface below large-scale extensional faults. The Paparoa Metamorphic Core Complex (PCC) on the West Coast of the South Island in New Zealand developed in the mid-Cretaceous and conditioned the crust for the eventual breakup of the Gondwana Pacific margin and formation of the Tasman Sea. The PCC has two detachment systems with opposite senses of shear: a top-to-the-NE detachment in the north and a top-to-the-SW detachment in the south. The southern detachment was synchronous with a granite intrusion and played the dominant role, while the northern detachment is a late minor feature of the core complex structure accommodating the final exhumation of the metamorphic core. Further thermochronological data reveal a complicated history of burial, reheating, and exhumation after the PCC's initial exhumation, which can be related to events associated with the Late Cretaceous period of continental breakup. Our work shows that initial extension in the mid-Cretaceous proceeded under high-temperature conditions and preceded sea-floor spreading about 25 million years ago.

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