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Rupture along the Himalayan Front

Lithosphere articles published onine March-April 2015

Geological Society of America

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IMAGE: Figure 1. (A) Date and rupture patches for large historical Himalayan earthquakes (Rajendran and Rajendran, 2005; Kumar et al., 2006) with reference to the Uttarakhand region of the central seismic gap, and... view more

Credit: Morell et al. and Lithosphere

Boulder, Colo., USA - In their article for Lithosphere on 12 March, authors Kristin Morell and colleagues write, "The ?700-km-long 'central seismic gap' is the most prominent segment of the Himalayan front not to have ruptured in a major earthquake during the last 200-500 years. This prolonged seismic quiescence has led to the proposition that this region, with a population of more 10 million, is overdue for a great earthquake. Despite the region's recognized seismic risk, the geometry of faults likely to host large earthquakes remains poorly understood."

A little more than a month on, the area experience a magnitude 7.8 earthquake, centered in Nepal (25 Apr. 2015).

In their study, Morell and colleagues use a series of complementary geomorphic and erosion rate data to define the ramp-flat geometry of the active detachment fault that is likely to host a large earthquake within the hinterland of the northwest Himalaya. Their analysis indicates that this detachment is sufficiently large to host another great earthquake in the western half of the central Himalayan seismic gap.

Specifically, their data sets point to a distinctive physiographic transition at the base of the high Himalaya in the state of Uttarakhand, India, characterized by abrupt strike-normal increases in channel steepness and a tenfold increase in erosion rates.

When combined with previously published geophysical imaging and seismicity data sets, Morell and colleagues interpret the observed spatial distribution of erosion rates and channel steepness to reflect the landscape response to spatially variable rock uplift due to a structurally coherent ramp-flat system of the Main Himalayan Thrust. They write, "Although it remains unresolved whether the kinematics of the Main Himalayan Thrust ramp involve an emergent fault or duplex, the landscape and erosion rate patterns suggest that the décollement beneath the state of Uttarakhand provides a sufficiently large and coherent fault segment capable of hosting a great earthquake."

In conclusion, they note, "While this hypothesis remains speculative, it is supported by independent records of historical seismicity."

FEATURED ARTICLE

Geomorphology reveals active decollement geometry in the central Himalayan seismic gap

K.D. Morell et al., University of Melbourne, Melbourne, Victoria, Australia. Published online ahead of print on 12 Mar. 2015; http://dx.doi.org/10.1130/L407.1.

Other recently posted Lithosphere articles are listed below.

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.

Effects of Paleogene faults on the reconstruction of the metamorphic history of the northwestern Thor-Odin culmination of the Monashee Complex, southeastern British Columbia

Y.D. Kuiper et al., Colorado School of Mines, Golden, Colorado, USA. Published online ahead of print on 12 Mar. 2015; http://dx.doi.org/10.1130/L414.1.

The Monashee Complex is a dome-shaped exposure of highly metamorphosed ancestral North American rocks in the southern Canadian Cordillera. Its rocks were strongly deformed while they were part of a subhorizontal shear zone, during the Cordilleran deformation that led to today's mountains. Ages of Cordilleran deformation and metamorphism are progressively younger toward deeper structural levels. This has previously been explained by various models including downward heat transfer, juxtaposition of domains with different ages by faulting, or by upward movement of rocks out of a mid-crustal shear zone. Late sub-vertical strike-slip faults exist throughout the Cordillera, but can be difficult to recognize due to poor exposure and the existence of similar rocks on either side. One of these faults explains at least part of the observed age pattern in the Monashee Complex. The importance of these strike-slip faults may be greatly underestimated in many mountain belts.

Geology of the coastal Chiapas (Mexico) Miocene plutons and the Tonalá Shear Zone: Syntectonic emplacement and rapid exhumation during sinistral transpression

R.S. Molina-Garza et al., Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, México. Published online ahead of print on 12 Mar. 2015; http://dx.doi.org/10.1130/L409.1.

This paper describes rocks in coastal Chiapas, in southern Mexico, which contain the exhumed record of the roots of the extinct Chiapanecan Miocene Arc. After and during emplacement, the Miocene arc rocks, including numerous deformed granitic plutons, were affected by a major sinistral-transpressive fault system. The record of fault activity is well-exposed in a 3 to 4 km-wide, ~150 km-long, greenschist facies mylonite belt we name the Tonalá Shear Zone. The fault is interpreted as a relict transform boundary between the Caribbean and North American plates, and the transpressive component is interpreted to be the driver of shortening in the Sierra Chiapas fold and thrust belt to the north. Arc magmatism is now located ~150 km north of the coast, and this rapid migration is attributed to shallowing of the subducting slab in response to evolution of the North America-Caribbean-Cocos triple plate junction.

Late Miocene upper crustal deformation within the interior of the southern Puna Plateau, central Andes

R. Zhou, University of Toronto, Toronto, Ontario, Canada; and L.M. Schoenbohm, University of Toronto Mississauga, Mississauga, Ontario, Canada. Published online ahead of print on 12 Mar. 2015; http://dx.doi.org/10.1130/L396.1.

This study by Zhou and Schoenbohm highlights the critical role that lower lithospheric processes may play in deformation at Earth's surface. Although proposed by numerical studies, the idea that lithospheric dripping or delamination may cause upper crustal deformation has not yet been well explored with real-world data. The authors conduct a multidisciplinary analysis on the southern Altiplano-Puna Plateau, the world's second largest continental plateau, and show that a critical but data-poor region underwent coeval basin formation and deformation from about 11.7 to 10.5 million years ago until at least about 7.8 million years ago. Taking into consideration regional deformation, volcanic, geochemical and geophysical data, the authors conclude that the observed late Miocene rejuvenation of the southern Puna interior was caused by a small-scale lithospheric dripping event. Zhou and Schoenbohm demonstrate that structural analysis within individual sedimentary basins is a critical indicator, when combined with data from other disciplines, for understanding the underlying geodynamic processes.

Kikiktat volcanics of Arctic Alaska -- Melting of harzburgitic mantle associated with the Franklin large igneous province

Grant M. Cox et al., McGill University, Montréal, Québec, Canada. Published online ahead of print on 12 Mar. 2015; http://dx.doi.org/10.1130/L435.1.

From the abstract: The Kikiktat volcanics (new name) of the northeastern Brooks Range of Arctic Alaska are exceptionally well-preserved Neoproterozoic continental tholeiites. This volcanic suite includes high-temperature picritic compositions, making them an excellent probe of mantle composition and temperature underlying the northern margin of Laurentia during the breakup of Rodinia. Detrital zircons from a volcaniclastic sample directly overlying basaltic flows of the Kikiktat volcanics were dated at 719.47 plus or minus 0.29 million years old by U-Pb chemical abrasion-thermal ionization mass spectrometry. This age suggests that the Kikiktat volcanics are an extension of the Franklin large igneous province. Petrogenetic modeling indicates a simple crystallization sequence of olivine to plagioclase to clinopyroxene, recording anhydrous low-pressure fractionation of a picritic parental melt. The composition of this parental liquid requires melting of harzburgite in the spinel stability field, while temperature estimates of the primary melt indicate elevated mantle potential temperatures. In contrast to the ca. 720 Ma Natkusiak basalts of Victoria Island, the Kikiktat volcanics have very low Ti concentrations, consistent with melting of harzburgitic mantle possibly by thermal conduction of an underlying plume. These data are consistent with Neoproterozoic to early Paleozoic tectonic reconstructions that restore the North Slope of Arctic Alaska to the northeastern margin of Laurentia and not directly adjacent to Victoria Island.

Tracking paleo-deformational fields in the Mesozoic central Sierra Nevada arc: implications for intra-arc cyclic deformation and arc tempos

W. Cao et al., University of Southern California, Los Angeles, California, USA. Published online ahead of print on 12 Mar. 2015; http://dx.doi.org/10.1130/L389.1.

This study proposed a cyclic pattern of intra-arc deformation that mimic the magmatic flare-up and lull cycle of the Sierra Nevada arc. W. Cao and colleagues suggest potential feedbacks exist between arc deformation, crustal thickening and magmatism, which may play an important role in regulating the arc tempo.

Coupling sequential restoration of balanced cross sections and low-temperature thermochronometry: The case study of the Western Carpathians

Ada Castelluccio et al., University of Padua, Padova, Italy. Published online ahead of print on 23 Apr. 2015, http://dx.doi.org/10.1130/L436.1.

From the abstract: In this paper, a new approach is applied to test a proposed scenario for the tectonic evolution of the Western Carpathian fold-and-thrust belt?foreland system. A north-south balanced section was constructed across the fold-and-thrust belt, from the Polish foreland to the Slovakia hinterland domain. Its sequential restoration allows us to delineate the tectonic evolution and to predict the cooling history along the section. In addition, the response of low-temperature thermochronometers (apatite fission-track and apatite [U-Th]/He) to the changes in the fold-and-thrust belt geometry produced by fault activity and topography evolution are tested. The effective integration of structural and thermochronometric methods provides, for the first time, a high-resolution thermo-kinematic model of the Western Carpathians from the Early Cretaceous onset of shortening to the present day. The interplay between thick- and thin-skinned thrusting exerts a discernible effect on the distribution of cooling ages along the profile. Our analysis unravels cooling of the Outer Carpathians since ca. 22 Ma. The combination of thrust-related hanging-wall uplift and erosion is interpreted as the dominant exhumation mechanism for the outer portion of the orogen. Younger cooling ages (13-4 Ma) obtained for the Inner Carpathian domain are mainly associated with a later, localized uplift, partly controlled by extensional faulting. These results, which help unravel the response of low-temperature thermochronometers to the sequence of tectonic events and topographic changes, allow us to constrain the tectonic scenario that best honors all available data.

Eocene extension and meteoric fluid flow in the Wildhorse detachment, Pioneer metamorphic core complex, Idaho

R.R. McFadden et al., Salem State University, Salem, Massachusetts 01970, USA. Published online ahead of print on 23 Apr. 2015; http://dx.doi.org/10.1130/10.1130/L429.1.

From the abstract: The relationship between microstructure and fluid flow traced by hydrogen isotope ratios (delta-D) is examined within the Wildhorse detachment system of the Pioneer metamorphic core complex in south-central Idaho. Within the detachment footwall, 100-m-thick mylonitic quartzite containing minor white mica and K-feldspar displays a NW-trending stretching lineation and consistent top-to-the-NW sense of shear criteria. Microstructures within the detachment footwall comprise two groups: quartz ribbons and relict quartz grains flattened within the foliation, with porphyroclastic white mica fish; and intensely deformed and recrystallized quartz with high-aspect-ratio white mica arranged within C' shear bands. White mica delta-D values are highly negative and cluster around -145 parts per million in high-aspect-ratio white mica and around -120 parts per million in porphyroclastic white mica fish. The most negative values are interpreted to reflect interaction with meteoric fluids from a high-elevation catchment (3000?4000 m), and the less negative values are interpreted to represent incomplete hydrogen isotope exchange between the meteoric fluid and the pre-extensional metamorphic fluid delta-D values in the white mica porphyroclasts. A suite of tightly clustered 40Ar/39Ar ages from synkinematic white mica in the detachment footwall dates deformation, recrystallization, fluid-rock interaction, and therefore the presence of high topography at 38-37 Ma; these ages are consistent with the cooling/exhumation history of the high-grade core of the Pioneer metamorphic core complex in the late Eocene. The 38-37 Ma 40Ar/39Ar ages are substantially younger than previously published ages of high topography in British Columbia to the north (49-47 Ma), in line with the hypothesis that high topography propagated from north to south in the northern segment of the North American Cordillera through Eocene time.

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