Marty Grove

A tectonic model for Cenozoic igneous activities in the eastern Indo-Asian collision zone

Geochronologic dating and compilation of existing age data suggest that Cenozoic activities in the eastern Indo^ Asian collision zone of southeast China and Indochina occurred in two episodes, each with distinctive geochemical signatures, at 42^24 Myr and 16^0 Myr. The older rocks are localized along major strike^slip faults such as the Red River fault system and erupted synchronously with transpression. The younger rocks are widely distributed in rift basins and are coeval with east^west extension of Tibet and eastern Asia. Geochemical data suggest that the early igneous phase was generated by continental subduction while the late episode was caused by decompression melting of a metasomatically altered, depleted mantle. The magmatic gap between the two magmatic sequences represents an important geodynamic transition in the evolution of the eastern Indo^Asian collision zone, from processes controlled mainly by crustal deformation to that largely dominated by mantle tectonics. fl 2001 Elsevier Science B.V. All rights reserved.

Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya

The Main Central Thrust (MCT) juxtaposes the high-grade Greater Himalayan Crystallines over the lower-grade Lesser Himalaya Formation; an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust. Garnet-bearing assemblages sampled along the Marysandi River and Darondi Khola in the Annapurna region of central Nepal show striking differences in garnet zoning of Mn, Ca, Mg, and Fe above and below the MCT. Thermobarometry of MCT footwall rocks yields apparent inverted temperature and pressure gradients of ∼18°C km−1 and ∼0.06 km MPa−1, respectively. Pressure-temperature (P-T) paths calculated for upper Lesser Himalaya samples that preserve prograde compositions show evidence of decompression during heating, whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature. In situ (i.e., analyzed in thin section) ion microprobe ages of monazites from rocks immediately beneath the Greater Himalayan Crystallines yield ages from 18 to 22 Ma, whereas late Miocene and Pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence. A Lesser Himalayan sample collected near the garnet isograd along the Marysandi River transect contains 3.3±0.1 Ma monazite ages (P ≈ 0.72 GPa, T ≈ 535°C). This remarkably young age suggests that this portion of the MCT shear zone accommodated a minimum of ∼30 km of slip over the last 3 Ma (i.e., a slip rate of >10 mm yr−1) and thus could account for nearly half of the convergence across the Himalaya in this period. The distribution of ages and P-T histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the MCT formed by the transposition of right-way-up metamorphic sequences during late Miocene-Pliocene shearing.

Tertiary deformation history of southeastern and southwestern Tibet during the Indo-Asian collision

Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.

A model for the origin of Himalayan anatexis and inverted metamorphism

The origin of the paired granite belts and inverted metamorphic sequences of the Himalaya has generally been ascribed to development of the Main Central Thrust (MCT). Although a variety of models have been proposed that link early Miocene anatexis with inverted metamorphism, recent dating studies indicate that recrystallization of elements of the MCT footwall occurred in the central Himalaya as recently as ∼6 Ma. The recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the High Himalayan leucogranites and North Himalayan granites, which differ in age, petrogenesis, and emplacement style. We suggest that their origin is linked to shear heating on a continuously active thrust that cuts through Indian supracrustal rocks that had previously experienced low degrees of partial melting. Numerical simulations assuming a shear stress of 30 MPa indicate that continuous slip on the Himalayan decollement beginning at 25 Ma could trigger partial melting reactions leading to formation of the High Himalayan granite chain between 25 and 20 Ma and the North Himalayan belt between 17 and 8 Ma. The ramp-flat geometry we apply to model the Himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle Miocene. Late Miocene, out-of-sequence thrusting within the broad shear zone beneath the MCT provides a mechanism to bring these rocks to the surface in their present location (i.e., well to the north of the present tectonic front) and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the MCT. We envision that formation of the MCT Zone involved successive accretion of tectonic slivers of the Lesser Himalayan Formations to the hanging wall and incorporate these effects into the model. The model predicts continued anatexis up to 400 km north of the Himalayan range, consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south Tibetan rift.

Exhumation of the west-central Alborz Mountains, Iran, Caspian subsidence, and collision-related tectonics

Crystallization and thermal histories of two plutons in the west-central Alborz (also Elburz, Elburs) Mountains, northern Iran, are combined with crosscutting relations and kinematic data from nearby faults to determine the Cenozoic tectonic evolution of this segment of the youthful Euro-Arabian collision zone. U/Pb, ^(40)Ar/^(39)Ar, and (U-Th)/He data were obtained from zircon, biotite, K-feldspar, and apatite. The Akapol pluton intruded at 56 ± 2 Ma, cooled to ∼150 °C by ca. 40 Ma, and stayed near that temperature until at least 25 Ma. The nearby Alam Kuh granite intruded at 6.8 ± 0.1 Ma and cooled rapidly to ∼70 °C by ca. 6 Ma. These results imply tectonic stability of the west-central Alborz from late Eocene to late Miocene time, consistent with Miocene sedimentation patterns in central Iran. Elevation-correlated (U- Th)/He ages from the Akapol suite indicate 0.7 km/m.y. exhumation between 6 and 4 Ma, and imply ∼10 km of Alborz uplift that was nearly synchronous with rapid south Caspian subsidence, suggesting a causal relation. Uplift, south Caspian subsidence and subsequent folding, reversal of Alborz strike-slip (from dextral to sinistral) and(?) eastward extrusion of central Iran, coarse Zagros molasse deposition, Dead Sea transform reorganization, Red Sea oceanic spreading, and(?) North and East Anatolian fault slip all apparently began ca. 5 ± 2 Ma, suggesting a widespread tectonic event that we infer was a response to buoyant Arabian lithosphere choking the Neo-Tethyan subduction zone.

Thermal evolution and slip history of the Renbu Zedong Thrust, southeastern Tibet

Cretaceous granitoids of the Gangdese batholith, southeastern Tibet, were overthrust by upper greenschist to epidote-amphibolite facies Tethyan rocks derived from the Indian shelf along the north directed (∼30° dip) Renbu Zedong Thrust (RZT). Thermochronological results obtained from a NE-SW transect near Lian Xian show evidence of thermal effects related to thrusting. Granitoids immediately beneath the RZT exhibit considerable recrystallization to greenschist facies assemblages. Biotite and K-feldspar 40Ar/39Ar ages measured along the traverse into the footwall increase systematically away from the RZT. The timing of initial upward displacement of the RZT hanging wall is constrained to have occurred at ∼18 Ma from the hornblende ages and the two K-feldspar samples closest to the fault. K-feldspars up to 15 km from the thrust yield 9–12 Ma ages for the initial ∼20% of 39Ar release. Distal samples yield 40Ar/39Ar ages nearly as old as the 70–110 Ma ion microprobe 206Pb*/238U ages determined for coexisting zircons. We have integrated our thermal history results with numerical heat flow models and found that while reheating to 320–280°C at shallow (∼7 km) levels due to rapid (>15 mm/yr) slip along the RZT at ∼10 Ma is capable of explaining the initial portion of the K-feldspar age spectra, a prior common thermal history experienced by all samples cannot satisfactorily account for all the 40Ar/39Ar results. Instead, we find our thermal history constraints to be more completely explained by a numerical model in which (1) rocks currently at the surface originated from different depths,(2) footwall samples in close proximity to the RZT experienced fault drag from 19 to 15 Ma and (3) postthrusting denudation of the region involving localized tilting occurred at ∼10 Ma. The minimum average slip rate and displacement along the ramp during this period are 2 mm/yr and 12 km, respectively, but are likely to have been greater. The cooling episode recorded in all the K-feldspar age spectra beginning at ∼10 Ma may either reflect denudation following regional uplift due to displacement along the ramp of the Main Himalayan Thrust or topographic collapse following cessation of RZT thrusting.

Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea

As lithospheric plates are subducted, rocks are metamorphosed under high-pressure and ultrahigh-pressure conditions to produce eclogites and eclogite facies metamorphic rocks. Because chemical equilibrium is rarely fully achieved, eclogites may preserve in their distinctive mineral assemblages and textures a record of the pressures, temperatures and deformation the rock was subjected to during subduction and subsequent exhumation. Radioactive parent–daughter isotopic variations within minerals reveal the timing of these events. Here we present in situ zircon U/Pb ion microprobe data that dates the timing of eclogite facies metamorphism in eastern Papua New Guinea at 4.3 ± 0.4 Myr ago, making this the youngest documented eclogite exposed at the Earths surface. Eclogite exhumation from depths of ∼75 km was extremely rapid and occurred at plate tectonic rates (cm yr-1). The eclogite was exhumed within a portion of the obliquely convergent Australian–Pacific plate boundary zone, in an extending region located west of the Woodlark basin sea floor spreading centre. Such rapid exhumation (> 1 cm yr-1) of high-pressure and, we infer, ultrahigh-pressure rocks is facilitated by extension within transient plate boundary zones associated with rapid oblique plate convergence.

40Ar* diffusion in Fe-rich biotite

Abstract Hydrothermal bulk-loss experiments employing radiogenic Ar (40Ar∗) were performed to determine whether 40Ar∗ diffusivity in biotite increases with Fe content. Diffusion laws determined for intermediate and Fe-rich biotite assuming single-domain diffusion (infinite-cylinder geometry) are remarkably similar: Fe-mica biotite (Xanitee = 0.71) D = 0.40+096-0.28exp[-(50500 ± 2.2)/RT] and Cooma biotite (Xanite = 0.54) D = 0.075+0.049-0.021exp [-(47100 ± 1.5)/RT]. The nearly identical results for Fe-mica biotite and Cooma biotite and their similarity to those from previous studies indicate that most biotite grains of intermediate composition possess comparable 40Ar∗ diffusion properties. Because limited grain breakage and volumetrically minor recrystallization is unavoidable during hydrothermal heating in bulk diffusion experiments, these diffusion laws necessarily provide upper limits to 40Ar∗ loss by intercrystalline diffusion. The measured rates of 40Ar∗ loss from biotite agree reasonably well with expectations based on single-domain volume diffusion using infinite-cylinder geometry when experimental uncertainties are taken into account. However, lack of information regarding 40Ar∗ gradients within the hydrothermally treated mica prevents us from precluding more complex diffusion mechanisms involving high diffusivity pathways. In this paper we consider the significance of bulk-loss 40Ar∗ diffusion experiments and discuss how diffusion parameters determined in the laboratory may be applied to thermochronology provided suitable constraints are available.

Thermal structure and exhumation history of the Lesser Himalaya in central Nepal

The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last ~20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°–550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°–50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e., an increase from about 3–4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid-Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle-ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m^2/yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units.

Systematic analysis of K-feldspar 40Ar39Ar step heating results: I. Significance of activation energy determinations

To better understand the argon retention properties of basement K-feldspars, 40Ar39Ar step-heating results for 115 specimens representing a wide range of temperature-time evolution from a diverse array of geologic environments have been systematically evaluated. In carrying out the measurements, we instituted nonconventional analysis routines including: (1) duplicate isothermal steps; (2) multiple isothermal steps at 1100°C to extract as much gas as possible prior to melting; and (3) temperature cycling. To maintain a self-consistent approach, we systematically applied a weighted least square regression to determine the activation energies (E) and log(D0/r02) values. Activation energies were found to define a normal distribution (46 ± 6 kcal/mol) spanning over 30 kcal/mol. Corresponding log(D0/r02) values (5 ± 3 s−1) were highly correlated with E, yielding a slope that reproduces the previously documented feldspar compensation relationship. Numerical analysis of this correlation permits us to rule out large systematic laboratory errors in our data. In applying the multiple diffusion domain (MDD) model, we observed general tendencies in the domain distribution of basement K-feldspars. For the majority of samples, most 39Ar resides in the larger domains. The smallest domains generally constitute <5% volume fraction of the sample and tend to plot ∼2 orders of magnitude above log (D0/r 02) Alternatively, the largest domains constrained by 39Ar released below melting have log(D/r2) values that are typically ∼3 orders of magnitude smaller than log(D0/r02). Systematic analysis of the database demonstrates that the diffusion behavior predicted by the MDD model prevails for virtually all samples. However, two kinds of anomalous degassing behavior were observed. The first appears to be due to our inability to isolate effects resulting from distributions characterized by very small domains while the second may result from inhomogeneous K-distributions. Although these phenomena are capable of producing a wide range of calculated diffusion parameters, close inspection reveals that important systematics of the K-feldspar results cannot be accounted for these factors and remain best explained by real intrasample differences in diffusion properties. Finally, while uncertainties in E determined from 40Ar39Ar step-heating experiments can produce significant dispersion in calculated temperatures (∼10°C/kcal/mol), the overall form of the cooling history, usually the most important result for tectonic applications, is preserved.