Elizabeth J. Catlos

A Late Miocene-Pliocene origin for the Central Himalayan inverted metamorphism

Abstract Perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the Himalayan Main Central Thrust (MCT), generally thought to have been active during the Early Miocene. However, in situ 208 Pb/ 232 Th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the MCT footwall occurred in this portion of the central Himalaya at only ca. 6 Ma. The apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the MCT which tectonically telescoped the young metamorphic sequence. This explanation may resolve some outstanding problems in Himalayan tectonics, such why the MCT and not the more recently initiated thrusts marks the break in slope of the present day mountain range. It also renders unnecessary the need for exceptional physical conditions (e.g., high shear stress) to explain the generation of the Himalayan leucogranites.

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.

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.

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.

Kinematic model for the Main Central thrust in Nepal

We present a kinematic model for the Himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late Miocene‐Pliocene geochronologic and thermochronologic ages from rocks in the Main Central thrust zone in central Nepal. At its current exposure level, the Main Central thrust juxtaposes a hanging-wall flat in Greater Himalayan rocks with a footwall flat in Lesser Himalayan rocks of the Ramgarh thrust sheet, which is the roof thrust of a large Lesser Himalayan duplex. Sequential emplacement of the Main Central (early Miocene) and Ramgarh (middle Miocene) thrust sheets was followed by insertion of thrust sheets within the Lesser Himalayan duplex and folding of the Main Central and Ramgarh thrusts during late Miocene‐ Pliocene time. Thorium-lead (Th-Pb) ages of monazite inclusions in garnets from central Nepal record the timing of coeval, progressive metamorphism of Lesser Himalayan rocks in the footwall of the Main Central thrust. Although this model does not rule out minor, late-stage reactivation of the Main Central thrust, major late Miocene reactivation is not required.

Records of the evolution of the Himalayan orogen from in situ Th–Pb ion microprobe dating of monazite: Eastern Nepal and western Garhwal

Abstract In situ Th–Pb monazite ages from rocks collected along two transects (the Dudh Kosi-Everest, eastern Nepal and the Bhagirathi River, Garhwal Himalaya, India) perpendicular to the Main Central Thrust (MCT) suggest a striking continuity of tectonic events across the Himalaya. The youngest age reported in this study, 5.9±0.2 Ma (MSWD=0.4), from matrix monazite grains collected beneath the MCT in the Garhwal region is consistent with several age data from rocks at similar structural levels in central Nepal, providing support for widespread Late Miocene MCT activity. The lateral parallelism of orogenic events is further manifested by the 20.7±0.1 Ma age of a High Himalayan leucogranite from an injection complex that outcrops along the Dudh Kosi-Everest transect, resembling ages of these bodies reported elsewhere. The youngest monazite grain analyzed along the Dudh Kosi-Everest transect is 10.3±0.8 Ma . The absence of 7–3xa0Ma monazite ages in eastern Nepal may reflect a different nappe structure, which obscures the reactivated ramp equivalent exposed in the Garhwal and central Nepal. Garnets from the MCT hanging wall (Greater Himalayan Crystallines) and footwall (Lesser Himalaya) display different major element zoning, and the patterns are useful for constraining the location of the thrust system that separates the two lithologies. Pressure–temperature paths for two upper Lesser Himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone. Along the Dudh Kosi-Everest transect, upper Lesser Himalayan monazite grains from three rocks record a clear signal at 14.5±0.1 Ma (MSWD=8.4), and the ∼23xa0Ma age that characterizes the hanging wall is notably absent. Monazites collected within a large-scale Greater Himalayan Crystallines fold yield the ∼14xa0Ma age, consistent with the structure forming due to MCT-related compression. Paleo-Mesoproterozoic (1407±35 Ma ) matrix monazite grains are found within an augen gneiss unit located beneath the MCT, whereas Cambro-Ordovician (436±8; 548±17 Ma ) inclusions are preserved within garnets of the Greater Himalayan Crystallines. The presence of 45.2±2.1 Ma grains from lower structural levels of the Greater Himalayan Crystallines indicates the unit realized conditions conducive for monazite growth during the Eocene.

Interpretation of monazite ages obtained via in situ analysis

Abstract Monazite grains from Nepal and Vietnam were compositionally analyzed with an electron microprobe and dated (Th–Pb) with an ion microprobe. Five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population: (1) Pb loss due to diffusion, (2) dissolution/reprecipitation reactions along a retrograde path, (3) analytical uncertainties, (4) analyses of overlapping age domains, and (5) episodic monazite growth. The influence of these factors is sample-dependent, but can be evaluated: (1) using peak metamorphic conditions and X-ray element maps to assess potential polymetamorphism or retrogression, (2) obtaining other geochronologic data including previous work or dating other minerals in the sample, (3) evaluating any method-related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses, and (4) ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element (REE) oxide or allanite. Chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms. Instead of relying on monazite chemical composition, major (Mn, Fe, Mg, Ca) and minor (Y) element garnet-zoning patterns and peak P – T conditions should be used to facilitate age interpretation. This thermobarometric data records the samples thermal history, changes in garnet growth rate and mechanisms, and accessory mineral breakdown.

Pressure-temperature-time path discontinuity in the Main Central thrust zone, central Nepal

Metapelites collected in central Nepal reveal a discontinuity in metamorphic pressuretemperature-time (P-T-t) paths near the base of the Main Central thrust zone, despite an absence of obvious structural breaks. Garnets in the structurally lowest rocks grew with increasing T and P (loading), whereas garnets 1‐3 km upsection grew with increasing T, but decreasing P (exhumation). Monazite grains in structurally lower rocks yield ionmicroprobe Th-Pb ages of 8‐9 Ma. Structurally higher monazite grains range from 10 to 22 Ma. The P-T-t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca. 8 Ma, but also reflect thermal relaxation following older (20 Ma or older) thrust movement. The Main Central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate.

Th-Pb ion-microprobe dating of allanite

Abstract Allanite, which is a common accessory mineral in a wide variety of rock types, typically contains high concentrations of Th and U; thus, an in-situ method of U-Th-Pb dating of this phase would have broad application. We describe a method to permit Th-Pb ages of allanite to be determined with approximately ±10% accuracy using a high-resolution ion microprobe. Knowledge of the composition and substitution mechanisms of this complex mineral is key to understanding the relative ionization efficiencies of Th+ and Pb+. The chemical compositions of three allanite samples used as age standards (Cima d’Asta Pluton, 275.5 ± 1.5 Ma; Atesina Volcanic Complex, 276.3 ± 2.2 Ma; La Posta Pluton, 94 ± 2 Ma) were determined using an electron microprobe, permitting an assessment of matrix effects on ionization. An ion-microprobe calibration curve involving elemental and oxide species of Th and Pb (i.e., 208Pb*/Th+ vs. ThO2+/Th+) yields highly scattered apparent ages when allanite age standards with different Fe contents are used. However, a three-dimensional plot of 208Pb*/Th+ vs. ThO2+/Th+ vs. FeO+/SiO+ improves the accuracy of the calibration to about ±10%. Even though this level of uncertainty is substantially greater than that expected for U-Th-Pb ionmicroprobe analyses of zircon or monazite, Th-Pb ages of allanite can still be used to address important geologic questions. We used this method to date two metamorphic allanite grains from the footwall of the Main Central Thrust, Nepal Himalaya, and an allanite grain from the Pacoima Canyon pegmatite, California. Allanite inclusions in garnet from Nepal yield significantly older ages than the coexisting monazite, indicating that allanite formation in these rocks records a previous metamorphic cycle that predates slip along the fault. The Pacoima Canyon allanite grain yields a younger age than that reported for zircon, implying Pb loss during cooling of the pegmatite.

The origin of Himalayan anatexis and inverted metamorphism: Models and constraints

Abstract The key to comprehending the tectonic evolution of the Himalaya is to understand the relationships between large-scale faulting, anatexis, and inverted metamorphism. The great number and variety of mechanisms that have been proposed to explain some or all of these features reflects the fact that fundamental constraints on such models have been slow in coming. Recent developments, most notably in geophysical imaging and geochronology, have been key to coalescing the results of varied Himalayan investigations into constraints with which to test proposed evolutionary models. These models fall into four general types: (1) the inverted metamorphic sequences within the footwall of the Himalayan thrust and adjacent hanging wall anatexis are spatially and temporally related by thrusting; (2) thrusting results from anatexis; (3) anatexis results from normal faulting; and (4) apparent inverted metamorphism in the footwall of the Himalayan thrust is produced by underplating of right-way-up metamorphic sequences. We review a number of models and find that many are inconsistent with available constraints, most notably the recognition that the exposed crustal melts and inverted metamorphic sequences not temporally related. The generalization that appears to best explain the observed distribution of crustal melts and inverted metamorphic sequences is that, due to specific petrological and tectonic controls, episodic magmatism and out-of-sequence thrusting developed during continuous convergence juxtaposing allochthonous igneous and metamorphic rocks. This coincidental juxtaposition has proven to be something of a red herring, unduly influencing attention toward finding a causal relationship between anatexis and inverted metamorphism.