Bradley R. Hacker

U/PB ZIRCON AGES CONSTRAIN THE ARCHITECTURE OF THE ULTRAHIGH-PRESSURE QINLING-DABIE OROGEN, CHINA

New SHRIMP and TIMS zircon ages, 40Ar/39Ar ages, and eclogite locations contribute significantly to our understanding of the ultrahigh-pressure Dabie Shan. (1) The geographic extent of the Yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the Dabie Shan. (2) The northern half of the Dabie Shan is a magmatic complex, intruded over a 10-Myr interval between 137 and 126 Ma, that accommodated ∼100% N–S stretching of the pre-existing collisional architecture. (3) Granitic orthogneisses and enclosing ultrahigh-pressure paragneisses have indistinguishable zircon populations. The population of Triassic zircon ages ranges from ∼219 to ∼245 Ma, leading us to question the prevailing assumption that 219 Ma zircons formed at ultrahigh pressure, and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation.

Subduction factory 2. Are intermediate‐depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?

[1] New thermal-petrologic models of subduction zones are used to test the hypothesis that intermediate-depth intraslab earthquakes are linked to metamorphic dehydration reactions in the subducting oceanic crust and mantle. We show that there is a correlation between the patterns of intermediate-depth seismicity and the locations of predicted hydrous minerals: Earthquakes occur in subducting slabs where dehydration is expected, and they are absent from parts of slabs predicted to be anhydrous. We propose that a subductingoceanicplatecanconsistoffourpetrologicallyandseismicallydistinctlayers:(1) hydrated, fine-grained basaltic upper crust dehydrating under equilibrium conditions and producing earthquakes facilitated by dehydration embrittlement; (2) coarse-grained, locally hydrated gabbroic lower crust that produces some earthquakes during dehydration but transformschieflyaseismicallytoeclogiteatdepthsbeyondequilibrium;(3)locallyhydrated uppermost mantle dehydrating under equilibrium conditions and producing earthquakes; and (4) anhydrous mantle lithosphere transforming sluggishly and aseismically to denser minerals. Fluid generated through dehydration reactions can move via at least three distinct flowpaths:percolationthroughlocal,transient,reaction-generatedhigh-permeabilityzones; flow through mode I cracks produced by the local stress state; and postseismic flow through fault zones. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7230 Seismology: Seismicity and seismotectonics; 8123 Tectonophysics: Dynamics, seismotectonics; 8135 Tectonophysics: Evolution of the Earth: Hydrothermalsystems (8424); 3660 Mineralogyand Petrology: Metamorphicpetrology;

Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H 2 O contents

[1] We present a new compilation of physical properties of minerals relevant to subduction zones and new phase diagrams for mid-ocean ridge basalt, lherzolite, depleted lherzolite, harzburgite, and serpentinite. We use these data to calculate H2O content, density and seismic wave speeds of subduction zone rocks. These calculations provide a new basis for evaluating the subduction factory, including (1) the presence of hydrous phases and the distribution of H2O within a subduction zone; (2) the densification of the subducting slab and resultant effects on measured gravity and slab shape; and (3) the variations in seismic wave speeds resulting from thermal and metamorphic processes at depth. In considering specific examples, we find that for ocean basins worldwide the lower oceanic crust is partially hydrated (<1.3 wt % H2O), and the uppermost mantle ranges from unhydrated to � 20% serpentinized (� 2.4 wt % H2O). Anhydrous eclogite cannot be distinguished from harzburgite on the basis of wave speeds, but its � 6% greater density may render it detectable through gravity measurements. Subducted hydrous crust in cold slabs can persist to several gigapascals at seismic velocities that are several percent slower than the surrounding mantle. Seismic velocities and VP/VS ratios indicate that mantle wedges locally reach 60–80% hydration. INDEX TERMS: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 3660 Mineralogy and Petrology: Metamorphic petrology; 3919 Mineral Physics: Equations of state; 5199 Physical Properties of Rocks: General or miscellaneous; 8123 Tectonophysics: Dynamics, seismotectonics; KEYWORDS: subduction, seismic velocities, mineral physics, H2O

Tectonics of the Qinling (Central China): tectonostratigraphy, geochronology, and deformation history

Abstract The Qinling orogen preserves a record of late mid-Proterozoic to Cenozoic tectonism in central China. High-pressure metamorphism and ophiolite emplacement (Songshugou ophiolite) assembled the Yangtze craton, including the lower Qinling unit, into Rodinia during the ∼1.0 Ga Grenvillian orogeny. The lower Qinling unit then rifted from the Yangtze craton at ∼0.7 Ga. Subsequent intra-oceanic arc formation at ∼470–490 Ma was followed by accretion of the lower Qinling unit first to the intra-oceanic arc and then to the Sino-Korea craton. Subduction then imprinted a ∼400 Ma Andean-type magmatic arc onto all units north of the northern Liuling unit. Oblique subduction created Silurian–Devonian WNW-trending, sinistral transpressive wrench zones (e.g., Lo-Nan, Shang-Dan), and Late Permian–Early Triassic subduction reactivated them in dextral transpression (Lo-Nan, Shang-Xiang, Shang-Dan) and subducted the northern edge of the Yangtze craton. Exhumation of the cratonal edge formed the Wudang metamorphic core complex during dominantly pure shear crustal extension at ∼230–235 Ma. Post-collisional south-directed shortening continued through the Early Jurassic. Cretaceous reactivation of the Qinling orogen started with NW–SE sinistral transtension, coeval with large-scale Early Cretaceous crustal extension and sinistral transtension in the northern Dabie Shan; it presumably resulted from the combined effects of the Siberia–Mongolia—Sino-Korean and Lhasa–West Burma—Qiangtang–Indochina collisions and Pacific subduction. Regional dextral wrenching was active within a NE–SW extensional regime between ∼60 and 100 Ma. An Early Cretaceous Andean-type continental magmatic arc, with widespread Early Cretaceous magmatism and back-arc extension, was overprinted by shortening related to the collision of Yangtze–Indochina Block with the West Philippines Block. Strike–slip and normal faults associated with Eocene half-graben basins record Paleogene NNE–SSW contraction and WNW–ESE extension. The Neogene(?) is characterized by normal faults and NNE-trending sub-horizontal extension. Pleistocene(?)–Quaternary NW–SE extension and NE–SW contraction comprises sinistral strike–slip faults and is part of the NW–SE extension imposed across eastern Asia by the India–Asia collision.

Normal faulting in central Tibet since at least 13.5 Myr ago

Tectonic models for the evolution of the Tibetan plateau interpret observed east–west thinning of the upper crust to be the result of either increased potential energy of elevated crust or geodynamic processes that may be unrelated to plateau formation. A key piece of information needed to evaluate these models is the timing of deformation within the plateau. The onset of normal faulting has been estimated to have commenced in southern Tibet between about 14 Myr ago and about 8 Myr ago and, in central Tibet, about 4 Myr ago. Here, however, we report a minimum age of approximately 13.5 Myr for the onset of graben formation in central Tibet, based on mineralization ages determined with Rb–Sr and 40Ar–39Ar data that post-date a major graben-bounding normal fault. These data, along with evidence for prolonged activity of normal faulting in this and other Tibetan grabens, support models that relate normal faulting to processes occurring beneath the plateau. Thinning of the upper crust is most plausibly the result of potential-energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath Tibet. This is supported by recent geophysical and geological data, which indicate that spatial heterogeneity exists in both the Tibetan crust and lithospheric mantle.

When continents collide : geodynamics and geochemistry of ultrahigh-pressure rocks

Preface. 1. Active Crustal Subduction and Exhumation in Taiwan C.H. Lin, S.W. Roecker. 2. Melting of Crustal Rocks During Continental Collision and Subduction A.E.P. Douce, T.C. McCarthy. 3. Rheology of crustal Rocks at Ultrahigh Pressure B. Stockhert, J. Renner. 4. Thermal Controls on Slab Breakoff and the Rise of High-Pressure Rocks During Continental Collisions J.H. Davies, F. von Blanckenburg. 5. Exhumation of Ultrahigh-Pressure Rocks: Thermal Boundary Conditions and Cooling History B. Grasemann, et al. 6. Active Tectonics and Ultrahigh-Pressure Rocks A.E. Blythe. 7. K-Ar (40Ar/39Ar) Geochronology of Ultrahigh Pressure Rocks S. Scaillet. 8. Geochemical and Isotopic Characteristics of UHP Eclogites and Ultramafic Rocks of the Dabie Orogen: Implications for Continental Subduction and Collisional Tectonics B.M. Jahn. 9. Stable Isotope Geochemistry of Ultrahigh-Pressure Rocks D. Rumble. 10. Tracing the Extent of a UHP Metamorphic Terrane: Mineral-Inclusion Study of Zircons in Gneisses from the Dabie Shan H. Tabata, et al. 11. H2O Recycling During Continental Collision: Phase-Equilibrium and Kinetic Considerations W.G. Ernst, et al. 12. Influence of Fluid and Deformation on Metamorphism of the Deep Crust and Consequences for the Geodynamics of Collision Zones H. Austrheim.

Rapid emplacement of the Oman ophiolite: Thermal and geochronologic constraints

In understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting, and the rate and duration of that thrusting. Hornblende 4oAr/39Ar ages demonstrate that the igneous oceanic crust in Oman crystallized and cooled to -825 K in 1-2 m.y. Hornblende ages from metamorphic rocks and from cross-cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below -825 K in 1-2 m.y. Motion along the sole thrust accounted for 200 km of displacement at a rate of 100-200 mm/yr. On the basis of age relationships and thermal considerations, we favor a two- stage model for the initial stages of Samall ophiolite emplacement: intraoceanic thrusting over < 2-m.y.-old lithosphere at 150 km/m.y. parallel to a spreading ridge for 1-2 m.y., followed by equally rapid and brief thrusting over cold and old lithosphere. Preservation of the Samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure, and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting.

Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean

Cretaceous Neotethyan ophiolites occur in four east-west–trending subparallel zones within the Tauride tectonic belt in southern Turkey. The ophiolites of the Inner, Intermediate, and Outer zones tectonically overlie the Mesozoic platform carbonates of the Tauride belt and are commonly underlain by a Cenomanian ophiolitic melange. These ophiolites consist mainly of tectonized mantle rocks, mafic-ultramafic cumulates, and gabbros, and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence. Metamorphic soles that are several hundred meters thick occur as thrust-faulted slices beneath these ophiolites and show well-developed metamorphic field gradients. Ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit. Dike rocks are made of subalkalic basalt to andesite typical of evolved island-arc tholeiites; they display large compositional variations, with SiO 2 content between 50 and 60 wt% and MgO between 8 and 4 wt%, and contain higher Ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates. The majority of the analyzed dike rocks show a slight depletion in light rare earth elements (REE) with low La/SmN ratios and are depleted in both high-field strength (HFS) and heavy REEs, while enriched in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalt (MORB). These characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by LILE- and light REE-enriched fluids. Geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window, in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field, or in a transitional zone between the garnet and spinel stability fields at about 60 km depth. This asthenospheric window was probably created during subduction of a Neotethyan ridge system; magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere. The new 40 Ar/ 39 Ar hornblende dates of 92–90 Ma and 90–91 Ma from the metamorphic soles and dike swarms, respectively, show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the Inner Tauride seaway. These data suggest that the Tauride ophiolites within the three zones to the north originated from the same root zone situated north of the Tauride carbonate platform, and that they constitute remnants of a single ophiolitic nappe sheet derived from the Inner Tauride seaway within the Neotethyan ocean.

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite-in, staurolite-in, garnet-in, and chloritoid-in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E-W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N-S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post-D1 and pre- to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end-member metamorphic core complex-type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

Sedimentary record and tectonic implications of Mesozoic rifting in southeast Mongolia

The East Gobi basin of Mongolia is a poorly described Late Jurassic–Early Cretaceous extensional province that holds great importance for reconstructions of Mesozoic tectonics and paleogeography of eastern Asia. Extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southwest of Saynshand, Mongolia, where outcrop and subsurface relationships permit recognition of prerift, synrift, and postrift Mesozoic stratigraphic megasequences. Within the synrift megasequence, three sequences developed in response to climatic and rift-related structural controls on sedimentation. Where best exposed along the northern margin of the Unegt subbasin, each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone. Resedimented ashes and basalt flows punctuate the synrift megasequence. Rifting began in the Unegt subbasin prior to 155 Ma with coarse alluvial filling of local fault depressions. Subsidence generally outstripped sediment supply, and fresh to saline lacustrine environments, expanding southward with time, dominated the Unegt- Zuunbayan landscape for much of latest Jurassic–Early Cretaceous time. Episodic faulting and volcanism characterized the basin system for the balance of the Early Cretaceous. A brief period of compressional and/or transpressional basin inversion occurred at the end of the Early Cretaceous, prior to deposition of a widespread Upper Cretaceous overlap sequence. The driver(s) of Late Jurassic–Early Cretaceous extension remain uncertain because southeast Mongolia occupied an intraplate position by the beginning of the Cretaceous. Extension in the East Gobi basin was coeval with collapse and extension of early Mesozoic contractional orogenic belts along the northern and southern borders of Mongolia and probably was a linked phenomenon. Strike-slip faulting associated with collisions on the southern Asian and Mongol- Okhotsk margins likely also played a role in late Mesozoic deformation of the East Gobi region, perhaps partitioning the Gobi from apparently coeval large-magnitude contractional deformation in the Yinshan- Yanshan orogenic belt south of the study area in Inner Mongolia.