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U-series dating of pedogenic carbonateclast coatings provides a reliable, precise minimum age of 45.1 [+ or -] 0.6 ka (2[sigma]) for the T2 geomorphic surface of the Biskra Palms alluvial fan, Coachella Valley, California. Concordant ages for multiple subsamples from individual carbonate coatings provide evidence that the [sup.238]U-[sup.234]U-[sup.230]Th system has remained closed since carbonate formation. The U-series minimum age is used to assess previously published [sup.10]Be exposure ages of cobbles and boulders. All but one cobble age and some boulder [sup.10]Be ages are younger than the U-series minimum age, indicating that surface cobbles and some boulders were partially shielded after deposition of the fan and have been subsequently exhumed by erosion of fine-grained matrix to expose them on the present fan surface. A comparison of U-series and [sup.10]Be ages indicates that the interval between final alluvial deposition on the T2 fan surface and accumulation of dateable carbonate is not well resolved at Biskra Palms; however, the 'time lag' inherent to dating via U-series on pedogenic carbonate can be no larger than ~10 k.y., the uncertainty of the [sup.10]Be-derived age of the T2 fan surface. Dating of the T2 fan surface via U-series on pedogenic carbonate (minimum age, 45.1 [+ or -] 0.6 ka) and [sup.10]Be on boulder-top samples using forward modeling (preferred age, 50 [+ or -] 5 ka) provides broadly consistent constraints on the age of the fan surface and helps to elucidate its postdepositional development. doi: 10.1130/B30018.1

Tectonic plate motions predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere and slab–upper plate interface. A wide range of observations from active subduction zones and exhumed rocks suggest that subduction interface shear zone rheology is sensitive to the composition of subducting crustal material—for example, sediments versus mafic igneous oceanic crust. Here we use 2-D numerical models of dynamically consistent subduction to systematically investigate how subduction interface viscosity influences large-scale subduction kinematics and dynamics. Our model consists of an oceanic slab subducting beneath an overriding continental plate. The slab includes an oceanic crustal/weak layer that controls the rheology of the interface. We implement a range of slab and interface strengths and explore how the kinematics respond for an initial upper mantle slab stage, and subsequent quasi-steady-state ponding near a viscosity jump at the 660-km-discontinuity. If material properties are suitably averaged, our results confirm the effect of interface strength on plate motions as based on simplified viscous dissipation analysis: a ∼2 order of magnitude increase in interface viscosity can decrease convergence speeds by ∼1 order of magnitude. However, the full dynamic solutions show a range of interesting behaviour including an interplay between interface strength and overriding plate topography and an end-member weak interface-weak slab case that results in slab break-off/tearing. Additionally, for models with a spatially limited, weak sediment strip embedded in regular interface material, as might be expected for the subduction of different types of oceanic materials through Earth's history, the transient response of enhanced rollback and subduction velocity is different for strong and weak slabs. Our work substantiates earlier suggestions as to the importance of the plate interface, and expands the range of quantifiable links between plate reorganizations, the nature of the incoming and overriding plate and the potential geological record. [ABSTRACT FROM AUTHOR]

We combine elastic thermobarometry with oxygen isotope thermometry to quantify the pressure-temperature (P-T) evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ~1.4-1.7 GPa between 500-1550 °C, retrograde epidote growth at ~1.3-1.5 GPa between 400-500 °C, and a second stage of retrograde epidote growth at ~1.0 GPa and 400 °C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist-eclogite facies and subsequent retrogression under blueschist-greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure/low-temperature conditions. These P-T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P-T-deformation conditions of retrograde mineral growth in HP/LT metamorphic terranes. [ABSTRACT FROM AUTHOR]

High strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains-East Humboldt Range, Nevada) suggest the presence of two distinct rheological transitions in the middle crust. (1) The brittle-ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear. (2) The localized-distributed transition or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths. In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. In some examples (e.g. the Whipple Mountains) the lower zone is spatially distinct from the detachment, although high-strain rocks from the lower zone were subsequently exhumed along the detachment. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. [ABSTRACT FROM AUTHOR]

The Betic-Rif arc is one of the smallest and tightest orogenic arcs on Earth, and together with its extensional hinterland, the Alborán Domain, it formed between two colliding continents. The region provides examples of a range of tectonic processes that are not predictable from the rules of rigid-plate tectonics. The Alborán Domain reveals two stages of subduction and accretion, with different thermal histories and mechanisms of exhumation. The external Betic-Rif thrust belt illustrates four processes that create an arcuate orogen and a strongly divergent pattern of slip vectors: (a) the interaction between the westward moving Alborân Domain and the converging African and Iberian margins, (b) divergence in relative motion due to extension within the Alborán Domain, (c) slip partitioning onto strike-slip faults within the arc, and (d) vertical-axis rotations resulting from oblique convergence on the limbs of the arc. [ABSTRACT FROM AUTHOR]

The recently developed titanium-in-quartz (TitaniQ) geothermobarometer of Wark and Watson (2006) and Thomas et al. (2010) has the potential to be applied to a wide range of igneous and metamorphic rocks. For Ti concentrations > ~10 ppm, the concentrations can be measured using an electron microprobe, but lower concentrations are below detection limits and require techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS). SIMS is ideal for this purpose as it maximizes lateral and depth resolution. We used SIMS to analyze synthetic quartz crystals characterized for Ti concentration by electron probe (18-813 ppm Ti) and compare this calibration to the commonly available NIST 61X glasses using both high mass resolution (HMR) and conventional energy filtering (CEF) techniques. We used a primary beam of 16O- ions and detected positive secondary ions. During HMR sessions, the mass spectrometer was operated at a mass resolving power (M/ΔM) of ~2000 to separate molecular ions from elemental Ti peaks. For CEF analyses, the instrument detected secondary ions sputtered from the sample with excess kinetic energies of 75 ± 20 eV. Titanian quartz measurements reveal general homogeneity and a linear increase in Ti+/Si+ ion ratios with increasing Ti concentrations. Background signals represent 0-1 ppm. The slopes of the calibration curves for the titanian quartz crystals are ~70% of the curves constructed using NIST glasses, indicating a much higher ion yield for Ti from the glasses compared to the simple oxide of silicon. We demonstrate, however, that a simple correction factor allows NIST glasses to be used to quantitatively determine the Ti concentrations of quartz (with 3.8% error using HMR, and 8.2% error using CEF) until homogenous, well-characterized samples of SiO2 become generally available. [ABSTRACT FROM AUTHOR]

Laboratory experiments suggest that water contents and stress magnitudes primarily control olivine lattice preferred orientation (LPO) and associated upper mantle seismic anisotropy. Several natural and experimental datasets, however, suggest that olivine LPO may also be sensitive to pressure, temperature, deformation mechanism, strain geometry, strain magnitude, and/or strain history, and it is unclear to what extent simple water-stress relationships can be extrapolated to nature. We use 65 peridotites deformed by dislocation creep under temperatures ranging from 600-1250°C, and sourced from a wide range of global tectonic settings (62 xenoliths, 2 ophiolitic massifs, 10 continental massifs), to investigate correlations between physical deformation parameters and olivine LPO in natural rocks. Olivine, orthopyroxene, and clinopyroxene water contents were measured using Secondary Ion Mass Spectrometry, and ranged from <1-157 (n = 39), 11-322 (n = 29), and 53-1733 (n = 29) ppm wt%, respectively. Grain transects and comparisons of measured water with published partition coefficients suggest pyroxenes are in equilibrium while olivine likely lost hydrogen during ascent/exhumation; we therefore use pyroxene to approximate olivine H2O. The equilibrium pyroxene contents, and the lack of microstructural annealing or evidence for post-deformation metasomatism, lead us to assume that water in pyroxene reflects the water present during deformation. Stress magnitudes measured using paleopiezometry ranged from 11 to 87 MPa. LPOs measured using electron backscatter diffraction exhibit the full range of fabric types: 35% preserved A-type LPO, 10% B-type, 6% C-type, 8% D-type, 27% E-type, and 15% AG-type. These new data, along with an extensive compilation of published measurements (445 samples from 48 studies), suggest the following:1. Naturally deformed peridotites rarely follow the experimentally derived relationships of water, stress, and LPO, with two exceptions: a) A-type samples always exhibit less than 26 ppm wt% (420 ppm H/10^6Si) water content; and b) the only samples with calculated olivine water contents greater than 90 ppm wt% (1500 ppm H/10^6Si) were C-type samples.2. We find a moderate correlation between olivine LPO and spinel shape constrained using CT-scanning, suggesting an influence of strain geometry. 3. No other systematic trends were observed for LPO types as a function of water, stress, temperature, pressure (i.e. spinel vs. garnet), modal percent, microstructure, olivine aspect ratios, fabric strength, or tectonic setting. 4. 30% of localities preserve three or more different LPO types deformed at the same general conditions, and 20% of localities preserve LPO types in which the olivine seismically fast axes (a-axes) are oriented up to 90 degrees to each other.Collectively, these observations suggest that at the moderate to low stresses preserved in these naturally deformed peridotites (<100 MPa), the strengths of individual olivine slip systems are not as sensitive to changes in external variables as observed in experiments. Instead, the data suggest that olivine LPO in the lithosphere may be more sensitive to strain memory (absent in most experiments) and/or strain magnitude or geometry. These data are consistent with the relative complexity of shear wave splitting measurements observed beneath continental regions where strain memory and strain perturbations are expected to be common. [ABSTRACT FROM AUTHOR]