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[1] This study presents the first results of the seismic character of the underthrusting Indian crust in the Sikkim Himalaya deduced through an analysis of -3600 receiver functions (RFs) abstracted from waveforms registered at 11 broadband stations spanning a 110 km long N-S profile from the foothills to the higher Himalaya. Common conversion point stacks of receiver functions prominently trace the northward dipping geometry of the Indian Moho beneath the Himalaya. Monte Carlo inversion of the azimuthal variations of the RFs at individual stations adopting the nearest neighborhood algorithm approach reveals that the crustal thickness varies from ~40 km to 61 km from south to north, with a dip varying between 4[degrees] and 10[degrees] among stations. A Moho doublet prominently seen at a depth of ~40 km in the higher Himalaya to the north of Main Boundary Thrust has been interpreted in terms of possible (partial) eclogitization of a granulitic Indian lower crust, akin to the finding just north of the study region beneath southern Tibet. A strong layer of anisotropy (~17%) localized within a low-velocity layer between 20 and 30 km has a NWSE oriented fast polarization direction counterintuitive to the convergence-parallel and range-perpendicular alignment expected in a convergent setting due to shear processes. Midcrustal transcurrent deformation in Sikkim and Bhutan, evidenced by a conjugate system of strike-slip faulting with NW to NE trending P axis orientations is the most feasible mechanism for causing a near strike parallel oriented fast axis of anisotropy in this segment of Himalaya. Citation: Singh, A., M. R. Kumar, and P. S. Raju (2010), Seismic structure of the underthrusting Indian crust in Sikkim Himalaya, Tectonics, 29, TC6021, doi: 10.1029/2010TC002722.

[1] Kinematic, kinematic vorticity ([W.sub.m]), and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the Kangmar gneiss dome, southern Tibet record ductile deformation patterns of a 'frozen' segment of a southward flowing midcrustal channel. Microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top-north and top-south shear in garnet zone rocks to dominantly top-north shear in staurolite/kyanite zone and deeper rocks. Kinematic vorticity values indicate an increase in pure shear component with depth from ~48% pure shear in chloritoid zone rocks through ~62% in garnet zone to staurolite/kyanite zone rocks to ~68% pure shear in an orthogneiss, the deepest exposed rocks. Deformation temperatures inferred from grain-scale microstructures and quartz lattice preferred orientations increase from ~300[degrees]C-400[degrees]C in chloritoid zone rocks to [greater than or equal to]600[degrees]C in the deepest exposed rocks. These temperatures are equivalent to temperatures derived from garnet-biotite thermobarometry, indicating that Wm was recorded during peak metamorphism. This ductile deformation zone was cut by the brittle southern Tibetan detachment system (STDS) that juxtaposed metasedimentary rocks upon the orthogneiss. On the basis of these relations, midcrustal rocks in the core of Kangmar Dome record: (1) general shear (vertical thinning and N-S horizontal extension) with a component of top-north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the STDS, (2) an increase in pure shear with structural depth, a consequence of an increase in lithostatic load, and (3) displacement of the high-temperature shear zone by the brittle STDS. Our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel. Citation: Wagner, T., J. Lee, B. R. Hacker, and G. Seward (2010), Kinematics and vorticity in Kangmar Dome, southern Tibet: Testing midcrustal channel flow models for the Himalaya, Tectonics, 29, TC6011, doi: 10.1029/2010TC002746.

New sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages, geochemical data, and a synthesis of existing stratigraphic, geochronologic, and geochemical results from the Tarim block and the Central Asian orogenic belt in northwestern China suggest the presence of a Permian (ca. 275 Ma) large igneous province (the Bachu large igneous province). The large igneous province consists predominantly of coevalmatic rocks (basalts and mafic-ultramafic intrusions) having an aerial coverage of more than 600,000 [km.sup.2], and its formation was accompanied by voluminous emplacement of A-type granites. This large igneous province, interpreted to be of mantle plume origin, is ~15 m.y. older than the ca. 260 Ma Emeishan large igneous province in southwestern China and ~25 m.y. older than the 251 Ma Siberian Trap in Russia. Such a sudden flair up of plume activity in the Permian may represent the early stage of the Pangean superplume event. The Permian plumes likely played a role in late Paleozoic rapid continental crustal growth in the Central Asian orogenic belt. In addition, there appear to be two types of mantle geochemical provinces (domains) in the region, a long-term enriched Tarim province and a subduction-metasomatized and depleted Central Asian orogenic belt province. doi: 10.1130/B30007.1

The northeastern continental margin of Oman in the Saih Hatat region is characterized by high-pressure (HP) chloritoid- or carpholite-bearing metasediments and highly deformed mafic eclogites and blueschists in a series of tectonic units bounded by high-strain ductile shear zones. New data on the upper cover units of this HP nappe stack indicate that all of them underwent similar P conditions to the underlying Hulw structural unit (with a cooler exhumation pressure-temperature path). Early SSW directed crustal thickening during ophiolite emplacement created recumbent folds and strong schistose fabrics in these Permian-Mesozoic shelf carbonates and was followed by later NNE dipping normal sense shear zones and normal faults. The Mayh unit shows high strain in a 15-25 km long sheath fold that likely formed at carpholite grade pressures of 8-10 kbar. We show that there are no significant P differences across the Hulw shear zone (upper plate--lower plate discontinuity) or between the overlying Mayh, Yenkit-Yiti, and Ruwi units. Postpeak metamorphic exhumation of the HP rocks was therefore accomplished by bottom-to-SSW (rather than top-to-NNE) active footwall extrusion beneath a fixed, static, passive hanging wall. Footwall uplift beneath these passive roof faults resulted in progressive expulsion of the HP rocks from depths of ~80-90 km (eclogites) and mainly 30-35 km (blueschists and chloritoid-/ carpholite-bearing units) during the Campanian-Early Maastrichtian. Oman thus provides a detailed record of how continental material (thick platform shelf carbonates) progressively jammed a subduction zone and emphasizes the contrasting behavior between cover units and their underlying basement. doi: 10.1029/2010TC002669.

The Parry Sound domain is a granulite nappestack transported cratonward during reactivation of the ductile lower and middle crust in the late convergence of the Mesoproterozoic Grenville orogeny. Field observations suggest the following with respect to the ductile sheath: (1) Formation of a carapace of transposed amphibolite facies gneiss derived from and enveloping the western extremity of the Parry Sound domain and separating it from high-strain gneiss of adjacent allochthons. This ductile sheath formed dynamically around the moving granulite nappe through the development of systems of progressively linked shear zones. (2) Transposition initiated by hydration (amphibolization) of granulite facies gneiss by introduction of fluid along cracks accompanying pegmatite emplacement. Shear zones nucleated along pegmatite margins and subsequently linked and rotated. The source of the pegmatites was most likely subjacent migmatitic and pegmatite-rich units or units over which Parry Sound domain was transported. Comparison of gneisses of the ductile sheath with high-strain layered gneiss of adjacent allochthons show the mode of transposition of penetratively layered gneiss depended on whether or not the gneiss protoliths were amphibolite or granulite facies tectonites before initiation of transposition, resulting in, e.g., folding before shearing, no folding before shearing, respectively. Meter-scale truncation along high-strain gradients at the margins of both types of transposition-related shear zones observed within and marginal to Parry Sound domain mimic features at kilometer scales, implying that apparent truncation by transposition originating in a manner similar to the ductile sheath may be a common feature of deep crustal ductile reworking. doi: 10.1029/2009TC002537.

Active eclogitization has recently been inferred at depth beneath the Himalaya from geophysical observations, and the mechanical consequences of eclogitization can be observed in the field in the eroded and extended nappe stack of the Scandinavian Caledonides. There, Proterozoic metastable granulites and igneous protoliths underwent partial eclogitization during the collision of Baltica with Laurentia. The reaction began in pseudotachylites and veins and eventually formed a connected network of eclogite-facies shear zones that localized deformation and weakened the lower crust of Baltica during the collision with Laurentia. All these features can be compared with the seismic activity of the Indian Lower Crust, its strength loss beneath the Himalayan ranges, and its delayed density increase regarding its penetration in the eclogite facies. The Caledonian Bergen Arc eclogites and the Himalayan Ama Drime eclogites are both derived from continental crust. In both cases, these eclogites were formed contemporaneously with the activation of the main thrusts responsible for the construction of the orogenic wedges, the Main Central Thrust in the Himalayas, and the main thrust below the Jotun Nappe Complex in the Caledonides. The similarities in these two orogens, which compare both in size and structure, high-light the importance of eclogitization at depth as a mechanism for weakening of the lower crust and for decoupling of the crust and lithospheric mantle in collision zones. doi: 10.1029/2009TC002602.

The Middle Permian (c. 262 Ma) Emeishan Basalt Formation of SW China is a commonly cited example of a large igneous province (LIP) that formed as a result of a deep-mantle plume impinging on the base of the lithosphere and generating large regional-scale up-doming prior to volcanism. Recently, however, this assertion has been challenged on the grounds that some lava flows close to the centre of the LIP were erupted in a submarine setting. Here we analyse all the available biolithostratigraphical, petrological, geochemical and volcanological information and show that this supports the idea that the terrain was generated by a plume that originated in the mantle. Emeishan basalt magma melted from a hot mantle source with trace element contents that are similar to the source of Icelandic and intraplate ocean island basalts. However, the amount and lateral extent of uplift is significantly less than is predicted by conventional deep-mantle plume models. We conclude that large-scale doming is not a diagnostic feature of mantle plumes; surface topography can be greatly influenced by the type of lower mantle plume (thermal or thermochemical), how it passes through the transition zone, and how it interacts with the lithosphere. doi: 10.1144/0016-76492009-129.

Observations on outcrop of a regionally developed sand injectite are used to infer and estimate the pore-pressure conditions in the shallow crust that caused the fluidization and injection of tens of cubic kilometres of sand. The estimated pore-fluid pressures at the base of the injection complex (at 1500 m burial depth, below a regionally developed shale-dominated seal) are from 22.26 to 25.08 MPa, which respectively correspond to 0.81 and 0.95 lithostatic pressure. A theoretical basis for prediction of sand injection is defined and applied to the prediction of pore pressure at the time of sand injection, the depth at which seal failure occurred, and the density and granular content of the fluidized flow. Lateral variations in the style and abundance of sandstone intrusions are described and these all fit into a remarkably uniform tripartite division of parent units, an intrusive complex and an extrusive complex. A sill zone (intrusions are dominated by sills) occurs in a restricted stratigraphic interval 200-270 m thick. Location of the base of the sill zone is directly related to the thickness of the overburden, and an isobaric surface at the time of sand injection, the lithostatic equilibrium surface, is defined at the base of the sill zone. When the sills formed an extended period of supralithostatic pressure occurred within the sill zone. doi: 10.1144/0016-76492010-004.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.04.007 Byline: David E. Smith (a), Maria T. Zuber (a), Roger J. Phillips (b), Sean C. Solomon (c), Gregory A. Neumann (d), Frank G. Lemoine (d), Stanton J. Peale (e), Jean-Luc Margot (f), Mark H. Torrence (g), Matthieu J. Talpe (a), James W. Head (h), Steven A. Hauck (i), Catherine L. Johnson (j), Mark E. Perry (k), Olivier S. Barnouin (k), Ralph L. McNutt (k), Jurgen Oberst (l) Keywords: Mercury; Mercury, Surface; Mercury, Interior; Geophysics Abstract: On 14 January and 6 October 2008 the MESSENGER spacecraft passed within 200km of the surface of Mercury. These flybys by MESSENGER provided the first observations of Mercury from a spacecraft since the Mariner 10 flybys in 1974 and 1975. Data from the Mercury Laser Altimeter (MLA) provided new information on the equatorial shape of Mercury, and Doppler tracking of the spacecraft through the flybys provided new data on the planet's gravity field. The MLA passes were on opposite hemispheres of the planet and span collectively [approximately equal to]40% of the equatorial circumference. The mean elevation of topography observed during flyby 1, in the longitude range 0-90[degrees]E, is greater than that seen during flyby 2 in the longitude range 180-270[degrees]E, indicating an offset between centers of mass and figure having a magnitude and phase in general agreement with topography determined by Earth-based radar. Both MLA profiles are characterized by slopes of [approximately equal to]0.015[degrees] downward to the east, which is consistent with a long-wavelength equatorial shape defined by a best-fitting ellipse. The Doppler tracking data show sensitivity to the gravitational structure of Mercury. The equatorial ellipticity of the gravitational field, C.sub.2,2, is well determined and correlates with the equatorial shape. The S.sub.2,2 coefficient is [approximately equal to]0, as would be expected if Mercury's coordinate system, defined by its rotational state, is aligned along its principal axes of inertia. The recovered value of the polar flattening of the gravitational potential, J.sub.2, is considerably lower in magnitude than the value obtained from Mariner 10 tracking, a result that is problematic for internal structure models. This parameter is not as well constrained as the equatorial ellipticity because the flyby trajectories were nearly in the planet's equatorial plane. The residuals from the Doppler tracking data suggest the possibility of mascons on Mercury, but flyby observations are of insufficient resolution for confident recovery. For a range of assumptions on degree of compensation and crustal and mantle densities, the allowable crustal thickness is consistent with the upper limit of about 100km estimated from the inferred depth of faulting beneath a prominent lobate scarp, an assumed ductile flow law for crustal material, and the condition that temperature at the base of the crust does not exceed the solidus temperature. The MESSENGER value of C.sub.2,2 has allowed an improved estimate of the ratio of the polar moment of inertia of the mantle and crust to the full polar moment (C.sub.m/C), a refinement that strengthens the conclusion that Mercury has at present a fluid outer core. Author Affiliation: (a) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA (b) Planetary Science Directorate, Southwest Research Institute, Boulder, CO 80302, USA (c) Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA (d) Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (e) Department of Physics, University of California, Santa Barbara, CA 93106, USA (f) Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA (g) Stinger Ghaffarian Technologies, Inc., 7701 Greenbelt Road, Suite 400, Greenbelt, Maryland 20770, USA (h) Department of Geological Sciences, Brown University, Providence, RI 02912, USA (i) Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USA (j) Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 (k) Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA (l) German Aerospace Center, Institute of Planetary Research, D-12489 Berlin, Germany Article History: Received 16 September 2009; Revised 6 April 2010; Accepted 8 April 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.02.018 Byline: Rosemary M. Killen (a), Andrew E. Potter (b), Ronald J. Vervack (c), E. Todd Bradley (d), William E. McClintock (e), Carrie M. Anderson (a), Matthew H. Burger (a) Keywords: Mercury, Atmosphere; Mercury, Surface Abstract: From observations of the metallic species sodium (Na), potassium (K), and magnesium (Mg) in Mercury's exosphere, we derive implications for source and loss processes. All metallic species observed exhibit a distribution and/or line width characteristic of high to extreme temperature - tens of thousands of degrees K. The temperatures of refractory species, including magnesium and calcium, indicate that the source process for the atoms observed in the tail and near-planet exosphere are consistent with ion sputtering and/or impact vaporization of a molecule with subsequent dissociation into the atomic form. The extended Mg tail is consistent with a surface abundance of 5-8% Mg by number, if 30% of impact-vaporized Mg remains as MgO and half of the impact vapor condenses. Globally, ion sputtering is not a major source of Mg, but locally the sputtered source can be larger than the impact vapor source. We conclude that the Na and K in Mercury's exosphere can be derived from a regolith composition similar to that of Luna 16 soil (or Apollo 17 orange glass), in which the abundance by number is 0.0027 (0.0028) for Na and 0.0006 (0.0045) for K. Author Affiliation: (a) NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States (b) National Optical Astronomy Observatory, Tucson, AZ 85719, United States (c) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, United States (d) Department of Physics, University of Central Florida, Orlando, FL 32816, United States (e) Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, United States Article History: Received 27 October 2009; Revised 17 February 2010; Accepted 18 February 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.05.007 Byline: Matthew H. Burger (a), Rosemary M. Killen (a), Ronald J. Vervack (b), E. Todd Bradley (c), William E. McClintock (d), Menelaos Sarantos (e), Mehdi Benna (a), Nelly Mouawad (f) Keywords: Mercury, Atmosphere; Mercury, Surface; Atmospheres, Structure; Magnetospheres Abstract: We present a Monte Carlo model of the distribution of neutral sodium in Mercury's exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to [approximately equal to]10.sup.6-10.sup.7 cm.sup.-2 s.sup.-1, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER flyby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion. Author Affiliation: (a) Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (b) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA (c) Department of Physics, University of Central Florida, Orlando, FL 32816, USA (d) Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA (e) Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (f) Astronomy Department, University of Maryland, College Park, MD 20742, USA

[1] Apatite and zircon (U-Th)/He cooling ages are used to quantify the timing of exhumation associated with thrust faulting in the High Zagros fold-thrust belt. Single-grain cooling age data were collected from (1) Cambrian sandstone in various thrust sheets, (2) sandstone and basement clasts derived from structurally controlled salt plugs or fault-bounded slices, and (3) syntectonic Neogene siliciclastics strata. In the northwestern (Kuhrang) and central (Kuh-e Lajin) High Zagros, apatite (U-Th)/He (AHe) ages range from ~26.7 to ~0.38 Ma. Most cooling and exhumation occurred in the early to middle Miocene, constrained by AHe ages ~19-15 Ma from the High Zagros thrust sheet, localized faults, and reset cooling ages from Bakhtiyari deposits. In the southeastern High Zagros (Kuh-e Dinar), cooling occurred later with AHe ages ranging from ~16.5 to ~0.79 Ma. Here most cooling and exhumation occurred in the late Miocene, constrained by AHe ages ~12-8 Ma from the High Zagros fault, and exhumed Paleozoic clasts in synorogenic strata. Zircon (U-Th)/He (ZHe) ages from bedrock samples across the High Zagros are reflective of the precollisional thermal history. The preservation of precollisional ZHe ages limits the pre-Miocene maximum burial temperature of the exhumed strata to < 180 [degrees]C, and indicate < 7-9 km of maximum exhumation in the central Zagros. This study shows that thrust activity in the High Zagros and continental suturing along the Zagros suture was underway by at least 19 Ma, and initiated no later than latest Oligocene to early Miocene time (~23 Ma). Citation: Gavillot, Y., G. J. Axen, D. F. Stockli, B. K. Horton, and M. D. Fakhari (2010), Timing of thrust activity in the High Zagros fold-thrust belt, Iran, from (U-Th)/He thermochronometry, Tectonics, 29, TC4025, doi: 10.1029/2009TC002484.

[1] Long-term crustal flow is computed with a kinematic finite element model based on iterated weighted least squares fits to data and prior constraints. Data include 773 fault traces, 106 fault offset rates, 510 geodetic velocities, 2566 principal stress azimuths, and velocity boundary conditions representing the rigid parts of the Eurasia, Africa, and Anatolia plates. Model predictions include long-term velocities, fault slip rates, and distributed permanent strain rates between faults. One model assumes that geodetic velocities measured adjacent to the Aegean Trench reflect a temporarily locked subduction zone; in this case, longterm subduction velocity averages 45 mm/yr and rapid crustal extension is predicted in the southern Aegean Sea. Another model assumes steady creeping subduction; in this case, subduction velocity averages only 29 mm/yr, and the eastern Aegean Seafloor is predicted to be more nearly rigid. Long-term seismicity maps are computed for each model on the basis of the SHIFT hypotheses and previous global calibrations of plate boundary earthquake production. Retrospective comparisons to seismic catalogs are encouraging: map patterns, spatial distribution functions, and total earthquake counts are all comparable. While neither model accurately predicts earthquake rates at all magnitudes, the creeping subduction model is more accurate for strong m6+ events, which dominate the seismic hazard. Citation: Howe, T. M., and P. Bird (2010), Exploratory models of long-term crustal flow and resulting seismicity across the Alpine-Aegean orogen, Tectonics, 29, TC4023, doi: 10.1029/2009TC002565.

[1] The Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian-Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Bearn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian-early Albian. They are unconformably overlain by the Albian-Cenomanian flysch formations and have experienced high temperature-low pressure mid-Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust-mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low-angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm. Citation: Lagabrielle, Y., P. Labaume, and M. de Saint Blanquat (2010), Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies, Tectonics, 29, TC4012, doi: 10.1029/2009TC002588.

[1] New structural, paleomagnetic, and apatite (U-Th)/ He results from the continental margin inboard of the Queen Charlotte fault (~54 [degrees]N) delineate patterns of brittle faulting linked to transform development since ~50 Ma. In the core of the orogen, ~250 km from the transform, north striking, dip-slip brittle faults and vertical axis rotation of large crustal domains occurred after ~50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until

[1] Passive margins form by rifting and extension of continental crust. Extension of continental crust, associated with subduction elsewhere, decreases ocean basin volume, forcing sea level to rise. The amount that the continental crust has been extended can be quantified using total tectonic subsidence, which is determined using sediment thickness and water depth data coupled with a model of the rifting process. We calculate total tectonic subsidence, the amount of extension or stretching factor, and thickness of extended continental crust at a 5 min grid resolution for passive margins between 77 [degrees] N and 70 [degrees] S. Our modeled crustal thicknesses correlate well with seismic refraction data from North America and Australia and can be used to estimate the change in ocean volume and the impact of extension on sea level. The duration of extension is constrained by estimating the onset of rifting and the breakup age, which is the time when rifting ceased and seafloor spreading began. Our results indicate that extension of continental crust has increased sea level by 20.8 m since the start of Pangea rifting. Citation: Kirschner, J. P., M. A. Kominz, and K. E. Mwakanyamale (2010), Quantifying extension of passive margins: Implications for sea level change, Tectonics, 29, TC4006, doi: 10.1029/2009TC002557.

[1] The Sierra Valle Fertil Range in northwestern Argentina exposes a tilted crustal section through the Ordovician Famatinian arc, from >25 km to shallow crustal paleodepths. Fourteen new U-Pb zircon crystallization ages of magmatic rocks from Sierra Valle Fertil area show that this section of the arc was built over a short time interval during the Ordovician, between 485 and 465 Ma. Zircon rim ages demonstrate that high-grade metamorphism and migmatization were synchronous with magmatic emplacement. Inherited ages in some of the plutonic rocks as well as detrital zircons in the metasedimentary framework suggest that the Famatinian arc was emplaced into a thick miogeoclinal cover to the thinned margin of the proto-South American continent in the Ordovician, which represents a part of Gondwana. Docking of the Precordilleran terrane outboard of proto-South America led to the cessation of arc magmatism in the Valle Fertil area and preservation of the arc in its early stages after

The kinematics of the basal allochthon in the SW of the Ordenes Complex is analysed to constraint its evolution during collisional and postcollisional stages of the Variscan orogeny. Two distinct sequences have been identified in the basal allochthon of this sector: the upper and lower sequences, in close correlation with the subdivision of the basal allochthon in the Malpica--Tui Complex. Three main tectonic episodes have been established in the basal allochthon: a high-pressure event related to continental subduction, a mesozonal event of regional exhumation by thrusting and recumbent folding, and a regional-scale extensional episode that resulted in the development of the Bembibre--Pico Sacro detachment system. The Bembibre--Pico Sacro system rejuvenated pre-existing shear zones, whose weakness favoured the nucleation of the detachments. Shear zones associated with the detachments overprinted the previous tectonic fabrics under conditions ranging from lower amphibolite to greenschist facies, and with heterogeneously distributed ductile to brittle deformation. The detachment system is coeval with late orogenic collapse and widespread magmatism, and represents its upper crustal expression. It correlates with mid- and lower crustal flow and the development of gneiss domes at depth in such a way that the basal allochthon can be considered a rheological boundary between the more rigid allochthonous sequences above and the more viscous Schistose Domain and autochthon below. doi: 10.1144/00t6-76492009-160.

An explanation for high-frequency cycles of sea level in non-glacial times has remained elusive, despite more than two centuries of research since Lavoisier's seminal observations were published in 1789. In the development of seismic stratigraphy in the 1970s, putatively global high-frequency changes in relative sea level (Vail third-order cycles) were attributed to an unknown eustatic mechanism, prompting a search for Mesozoic ice ages. Over the last decade, a regional mechanism of sea-level control has been developed from studies of the sedimentary record in high-quality oil-industry data. These geological studies have supported the geophysical prediction that significant regional control of sea level is exercised by mantle-induced vertical motions of the Earth's surface. These vertical motions can occur over time intervals from several tens of million years to less than a million years, with amplitudes of tens of metres or more even at the shorter intervals. The vertical motions are not confined to regions with major hotspots. There are two related controls of surface vertical motion: evolution of mantle-convection cells, and pulsing flow within each cell. The effects are evident in the sedimentary record of North Atlantic basins. Mantle convection provides an alternative, regional, mechanism to eustatic control for explaining medium-frequency to high-frequency sea-level cycles. doi: 10.1144/0016-76492009-127.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.11.020 Byline: A. Doressoundiram (a), F. Leblanc (b), C. Foellmi (c), A. Gicquel (a), G. Cremonese (d), J.-F. Donati (e), C. Veillet (f) Keywords: Mercury, Atmosphere; Abundances, Atmospheres; Spectroscopy Abstract: High-resolution spectroscopy of Mercury has been obtained with two different instruments in 2006: the EMMI instrument at the 3.6-m NTT telescope of ESO La Silla Chile and the ESPADON spectrograph at the 3.6-m CFHT telescope on top of Mauna Kea (Hawaii). The disk of the planet has been scanned for spatial variation of the exospheric species. The large spectral range and high resolution allow simultaneous measurements of the integrated column density of Na and K. We measure Na/K ratio between 80 and 400 with values between 60 and 90 when the telescope was pointed towards the subsolar region of Mercury's disk and much larger value when we looked to other part of the exosphere. Moreover, we observed that the Na and K exospheres display very different spatial distributions. Even if these two species are probably ejected with very similar mechanisms from the surface, their differences in mass and sensitivity to solar pressure acceleration imply very different behavior in Mercury's exosphere. Author Affiliation: (a) LESIA, Observatoire de Paris, F-92195 Meudon Principal Cedex, France (b) LATMOS/IPSL, Universite Versailles Saint Quentin, CNRS, France (c) Laboratoire d'Astrophysique de l'Observatoire de Grenoble, F-38041 Grenoble Cedex 09, France (d) INAF - Osservatorio Astronomico di Padova, Italy (e) CNRS/Universite de Toulouse, LATT/UMR 5572, Observatoire Midi-Pyrenees, F-31400 Toulouse, France (f) Canada-France-Hawaii Telescope Corporation, PO Box 1597, Kamuela, HI 96743, USA Article History: Received 3 March 2009; Revised 7 October 2009; Accepted 15 November 2009 Article Note: (footnote) [star] Based on observations carried out at the European Southern Observatory (ESO), La Silla, Chile, programme id 078.C-0038 and at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

The author emphasizes on the dangerous impact that an asteroid can have on the planet Earth due to the lack of proper surveillance and early-warning technologies. The need for the development of a proper control panel and tracking systems to avoid the damage caused by these space objects is demonstrated.

Fault zones that slip when oriented at large angles to the maximum compressive stress, i.e., weak faults, represent a significant mechanical problem. Here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite + abundant talc along a low-angle normal fault. Within the fault core, talc forms an interconnected foliated network that deforms by frictional sliding along 50-200-nm-thick talc lamellae. The low frictional strength of talc, combined with dissolution-precipitation creep, can explain slip on low-angle normal faults. In addition, the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures. The development of phyllosilicates such as talc by fluid-assisted processes within fault zones cutting Mg-rich carbonate sequences may be widespread, leading to profound and long-term fault weakness.

All exposed rocks on Earth's surface experience erosion; the fastest rates are documented in rapidly uplifted monsoonal mountain ranges, and the slowest occur in extreme cold or warm deserts--millennial submeterscale erosion may be approached only in the latter. The oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface, and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings; thus, these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape, while the surrounding landscape may erode faster and be younger. We present erosion rate and exposure age data from the Paran Plains, a typical environment in the Near East where vast abandoned alluvial sur faces ([10.sup.2]-[10.sup.4] [km.sup.2]) are covered by well-developed desert pavements. These surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m.y. Major factors that reduce erosion converge in these regions: extreme hyperaridity, tectonic stability, flat and horizontal surfaces (i.e., no relief), and effective surface armoring by a clast mosaic of highly resistant lithology. The [sup.10]Be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern Negev, Israel (representing the Sahara-Arabia Deserts), indicate simple exposure ages of 1.5-1.8 Ma or correspond to maximum erosion rates of 0.25-0.3 m [m.y..sup.-1]. The [sup.36]Cl in carbonate clasts, from the same pavement, weathers faster than the chert and yields simple exposure ages of 430-490 ka or maximum erosion rates of 0.7-0.8 m [m.y..sup.-1]. These ages and rates are exceptional because they represent an extensive landform. The [sup.10]Be concentrations from samples collected at depth and optically stimulated luminescence (OSL) dating reveal a two-stage colluvial deposition history followed by eolian addition of 40 cm of silt during the past 170 k.y. Our results highlight the efficiency of desert pavement armor in protecting rocks from erosion and preserving such geomorphic surfaces for millions of years.

Oceanic arcs and deep-sea trenches are the surface expressions of oceanic plates subducting into the Earth's mantle. We use a new numerical technique for simulating the dynamical evolution of the lithosphere-mantle interaction in order to assess the causes of arc curvature. We group the possible causes into two classes, external feedback between the migrating lithosphere and the secondary induced mantle flow, and internal heterogeneities within the lithosphere, e.g., owing to differences in cooling ages of the plate at the trench. We statistically assess that almost all arcs on the Earth can be described by these hypotheses. The method is also directly applied to the Tonga and Aleutian arcs, bringing new insights on the origin of their shapes. Keywords: island arcs, subduction, lithosphere, rheology.

Surface uplift of the southeast margin of the Tibetan Plateau is interpreted to have progressed from the northwest, near the Tibetan border, to the southeast, in the Red River region of the central Yunnan Province, China. This interpretation is based on existing thermochronologic data and new mapping and sedimentologic and paleobotanic data demonstrating incision in the headwaters of the Red River in Pliocene time or later. Together with previously published data demonstrating surface uplift and a gradient in crustal thickness in the absence of upper crustal shortening, this is strong evidence for growth of the southeast margin of the Tibetan Plateau through lower crustal flow. Displacement along the Ailao Shan--Red River shear zone slowed or ceased in early Pliocene time, and the Xianshuihe-Xiaojiang fault system initiated, accommodating diffuse deformation and rotation around the Eastern Himalayan syntaxis. We suggest a kinematic link between the change in mode of deformation and the introduction of a weak crustal layer through lower crustal flow. Keywords: Tibetan Plateau, lower crustal flow, continental extrusion, Red River fault, crustal deformation.

Evidence for the existence of continental crust older than 4.06 Ga has so far been obtained only from zircons in the Yilgarn Craton of Western Australia. In this paper we report the first occurrence of a very old zircon with a U-Pb age of 4.2 Ga in the Acasta Gneiss Complex of northwestern Canada, based on a laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe study. The U-Pb data reveal that the 4.2 Ga zircon occurs as a xenocryst in a 3.9 Ga granitic rock. Trace element compositions of the xenocryst suggest that it crystallized from a granitic magma. Our results, suggesting the existence of granitic rocks outside the Yilgarn Craton at 4.2 Ga, imply that granitic continental crust was more widespread than previously thought, and that it was reworked into Early Archean continental crust. Keywords: Acasta Gneiss Complex, Hadean, ancient zircon, crustal reworking, laser ablation-inductively coupled plasma-mass spectrometry, sensitive high-resolution ion microprobe, U-Pb dating.

Subcontinental lithospheric mantle xenoliths from beneath the French Massif Central contain clinopyroxene with high Lu/Hf and Hf isotope ratios ([sup.176]Lu/[sup.177]Hf = 0.1-10.8; [[epsilon].sub.Hr] = +40 to +2600). These Lu-Hf isotope systematics yield model ages of 313-377 Ma and apparently date melt extraction in a mantle wedge during Variscan subduction, with the residual mantle subsequently forming the stabilizing mantle keel of young European continental crust. The extremely depleted Lu-Hf mantle systematics contrast with other isotopic signatures (Sr, Pb, Nd) that have been overprinted by mantle metasomatism. The high Lu/Hf values developed in mantle clinopyroxene by melt extraction, coupled with the robustness of this system to some types of metasomatism, provide a new chronometer for directly dating crust-mantle differentiation in both young and ancient continental regions. The process of metasomatic decoupling of Nd and Hf isotopes, which has been previously recognized in Hawaiian oceanic mantle and in French Massif Central subcontinental mantle, may be responsible for the prominent displacement of terrestrial rocks to lower [[epsilon].sub.Nd] at a given [[epsilon].sub.Hf] as compared to Bulk Earth reference values obtained from analysis of chondritic meteorites. Keywords: Lu-Hf isotopes, subcontinental lithosphere, mantle xenoliths, geochronology, Variscan orogeny, Europe.

The planet is technically in an ice age, a condition that Earth has experienced in different ways many times throughout in history and a deep time geologic record archives a nearly-billion-year-long record of several ice houses. The icehouse periods was associated with abrupt climate change over a range of timescales and provides insight into the climatically fickle transitions from icehouse to greenhouse states.

We present analytical results from four dredge locations across the eastern Zealandia continental margin and adjacent ocean crust. The 115 Ma dacites dredged from the West Wishbone Ridge (WWR) are isotopically primitive, weakly adakitic, slab-derived lavas. The 97 Ma A-type granites and a basalt from the easternmost Chatham Rise enlarge the known area of postsubduction Gondwana magmatism. Amphibolite-grade schists from a fault block south of the Chatham Rise provide a critical bridge between the Zealandia and West Antarctica belts of Jurassic--Cretaceous accretionary prism rocks. The new recognition of the WWR as a remnant of a 115 Ma intraoceanic subduction system means that previous hypotheses of the WWR as a fracture zone or spreading ridge require modification. The dacite ages constrain the start of Osbourn Trough spreading, which caused breakup of the Hikurangi-Manihiki igneous plateau, to before 115 Ma. We speculate that, after 115 Ma, the WWR was rifted by an intraoceanic spreading center that developed along its southeast side. Impingement of this spreading center against the Gondwana margin led to widespread 95-100 Ma postsubduction magmatism, variable lithospheric stretching, and ultimately continental splitting of Zealandia and West Antarctica across basement trends. Keywords: Cretaceous, southwest Pacific, Gondwana, dating, geochemistry, tectonics.

Unusual compositions of some oceanic basalts have been attributed to their sources containing continental lithosphere detached during the breakup of Gondwana. However, the processes of how such continental lithospheric material is detached and transported into the ocean basin have not been constrained. Here we identify Walvis Ridge, where it has been argued that Deep Sea Drilling Project (DSDP) Site 525A contains continental material, as a unique location to constrain these processes. Absolute plate motion (relative to the Tristan mantle plume) and relative plate motion (between Africa and South America) of the African plate are oblique to one another, such that tectonic detachment versus hotspot-related thermal erosion should sample spatially separated continental units of different age. We present isotopic compositions of xenoliths representing the neoProterozoic lithosphere at the inferred site for tectonic detachment during continental breakup and show that this process does not explain the Walvis Ridge DSDP Site 525A mantle source. Rather, thermal erosion of ancient cratonic mantle by the Tristan mantle plume is indicated. A convective return flow is required to transport the eroded subcontinental lithospheric mantle to the site of plume activity some ~50 m.y. later and provides constraints on the direction and velocity of mantle flow in the upper mantle. Keywords: mantle plume, lithosphere, recycling, craton, geodynamics.

Flood lavas are major geological features on all the major rocky planetary bodies. They provide important insight into the dynamics and chemistry of the interior of these bodies. On the Earth, they appear to be associated with major and mass extinction events. It is therefore not surprising that there has been significant research on flood lavas in recent years. Initial models suggested eruption durations of days and volumetric fluxes of order [10.sup.7] [m.sup.3] [s.sup.-1] with flows moving as turbulent floods. However, our understanding of how lava flows can be emplaced under an insulating crust was revolutionized by the observations of actively inflating pahoehoe flows in Hawaii. These new ideas led to the hypothesis that flood lavas were emplaced over many years with eruption rates of the order of [10.sup.7] [m.sup.3] [s.sup.-1]. The field evidence indicates that flood lava flows in the Columbia River Basalts, Deccan Traps, Etendeka lavas, and the Kerguelen Plateau were emplaced as inflated pahoehoe sheet flows. This was reinforced by the observation of active lava flows of [greater than or equal to] 100 km length on Io being formed as tube-fed flows fed by moderate eruption rates ([10.sup.2] [10.sup.3] [m.sup.3] [s.sup.-1]). More recently it has been found that some flood lavas are also emplaced in a more rapid manner. New high-resolution images from Mars revealed 'platy-ridged' flood lava flows, named after the large rafted plates and ridges formed by compression of the flow top. A search for appropriate terrestrial analogues found an excellent example in Iceland: the 1783-1784 Laki Flow Field. The brecciated Laki flow top consists of pieces of pahoehoe, not aa clinker, leading us to call this 'rubbly pahoehoe'. Similar flows have been found in the Columbia River Basalts and the Kerguelen Plateau. We hypothesize that these flows form with a thick, insulating, but mobile crust, which is disrupted when surges in the erupted flux are too large to maintain the normal pahoehoe mode of emplacement. Flood lavas emplaced in this manner could have (intermittently) reached effusion rates of the order of [10.sup.6] [m.sup.3] [s.sup.-1]

The dynamical processes that drive intraseasonal equatorial atmospheric angular momentum (EAAM) fluctuations are examined with the National Centers for Environmental Prediction--National Center for Atmospheric Research (NCEP-NCAR) reanalysis data. The primary methodology involves the regression of relevant variables including the equatorial bulge, mountain, and friction torques, surface pressure, streamfunction, and outgoing longwave radiation, against the time derivative of the two components and the amplitude of the EAAM vector. The results indicate that the observed 10-day westward rotation of the EAAM vector corresponds to the propagation of a zonal wavenumber-1, antisymmetric, Rossby wave normal mode. Additional findings suggest that fluctuations in the amplitude of the EAAM vector are driven by poleward-propagating Rossby waves excited by the latent heating within equatorial mixed Rossby--gravity waves and also by wave--wave interaction among planetary waves. Both of these processes can induce surface pressure anomalies that amplify the EAAM vector via the equatorial bulge torque. The Antarctic and Greenland mountain torques were found to drive large fluctuations in the amplitude of the EAAM vector. Both the friction torque and wave--zonal-mean flow interaction were shown to dampen the EAAM amplitude fluctuations. A comparison of the EAAM dynamics in the atmosphere with that in an aquaplanet GCM suggests that the mountain torque also drives fluctuations in the phase speed of the atmospheric wave field associated with the EAAM vector, and it confines the wave--wave interaction to planetary scales.

[1] The distribution of mass anomalies within the crust of the Archean Western Superior Province has been investigated with forward and inverse gravity modeling routines. The gravity models indicate that in most of the Western Superior Province, significant mass anomalies occur only within the top 10 km of the crust, where they are generally related to dense metavolcanic rocks and low-density granitoid plutons, and at the crust-mantle boundary, where they are linked to undulations of this interface. An exception is the region encompassing the central Wabigoon Subprovince and the segments of the Quetico and Wawa belts south of it, where implied large intracrustal mass anomalies are associated with a segment of denser lower crust, and with overlying depressions of the interfaces between the main crustal layers. We interpret the predominant lateral mass homogeneity observed at deep crustal levels as evidence of mass redistribution processes associated with a major postaccretionary episode of thermal softening of the Western Superior crust. The incongruent presence of a denser lower crustal segment may reflect the subsequent modification of the lower crust through magmatic intra and underplating, possibly in relation with the Mesoproterozoic midcontinent rifting event. Citation: Nitescu, B., A. R. Cruden, and R. C. Bailey (2006), Crustal structure and implications for the tectonic evolution of the Archean Western Superior craton from forward and inverse gravity modeling. Tectonics, 25. TC1009, doi:10.1029/ 2004TC001717.