Michael A. Stearns

Synchronous Oligocene–Miocene metamorphism of the Pamir and the north Himalaya driven by plate-scale dynamics

Gneiss domes in the Pamir (Central Asia) and the Himalaya provide key data on mid- to deep-crustal processes operating during the India-Asia collision. Laser ablation split-stream inductively coupled plasma–mass spectrometry (LASS-ICP-MS) data from monazite in these domes yield a time record from U/Th-Pb dates and a petrologic record from rare earth element (REE) abundances. Seven samples from the Pamir and six samples from the north Himalayan gneiss domes yield almost identical monazite dates of ca. 28–15 Ma. Most monazite has invariant heavy REE (HREE) abundances; two samples, however, have older monazite that records progressive HREE depletion and two samples have younger monazite that records progressive HREE enrichment. These variations in HREE are compatible with increasing garnet abundance—prograde metamorphism— until ca. 20 Ma, and decreasing garnet abundance thereafter. The change from HREE depletion to enrichment may record a transition from crustal thickening and heating to dome exhumation and cooling. This documentation of synchronous Barrovian metamorphism within domes of Indian crust along the margin of the orogen (Himalaya) and within domes of Asian crust within the core of the orogen (Pamir) is best explained by a plate-scale driving force rather than by local events. We propose that widespread, synchronous thickening was initiated by the resumption of Indian subduction following slab breakoff and then terminated by a second slab-tearing event—both plate-scale events inferred from tomography.

Titanite petrochronology of the Pamir gneiss domes: Implications for middle to deep crust exhumation and titanite closure to Pb and Zr diffusion

©2015. American Geophysical Union. All Rights Reserved. The Pamir Plateau, a result of the India-Asia collision, contains extensive exposures of Cenozoic middle to lower crust in domes exhumed by north-south crustal extension. Titanite grains from 60 igneous and metamorphic rocks were investigated with U-Pb + trace element petrochronology (including Zr thermometry) to constrain the timing and temperatures of crustal thickening and exhumation. Titanite from the Pamir domes records thickening from ∼44 to 25 Ma. Retrograde titanite from the Yazgulem, Sarez, and Muskol-Shatput domes records a transition from thickening to exhumation at ∼20-16 Ma, whereas titanite from the Shakhadara dome records prolonged exhumation from ∼20 to 8 Ma. The synchronous onset of exhumation may have been initiated by breakoff of the Indian slab and possible convective removal of the Asian lower crust and/or mantle lithosphere. The prolonged exhumation of the Shakhdara and Muztaghata-Kongur Shan domes may have been driven by continued rollback of the Asian lithosphere concurrent with shortening and northwestward translation of the Pamir Plateau.

Building the Pamir-Tibetan Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 1. Geometry and kinematics

Asian deep crust exposed in the Pamir permits determination of the amount, sequence, and interaction of shortening, extension, and lateral extrusion over ~30 km of crustal section during the India-Asia collision. In the Central Pamir, gneiss domes and their hanging walls record Paleogene tripling of the 7–10 km thick Phanerozoic upper crustal strata; total crustal thickness may have amounted to 90 km. Two thrust sheets, comprising Cambro-Ordovician, respectively, Carboniferous to Paleogene strata, straddle the domes. Amphibolite-facies metamorphic rocks within the domes—equivalent to lower grade rocks outside the domes—form fold nappes with dome-scale wavelengths. E-W stretching occurred contemporaneously with top-to- ~ N imbrication and folding. At ~22–12 Ma, bivergent (top-to-N and top-to-S), normal-sense shear zones exhumed the crystalline rocks; most of the extension occurred along the northern dome margins. Shortening resumed at ~12 Ma with opposite-sense thrusting and folding focused along the dome margins. Throughout the building of the Central and South Pamir, dominant ~N-S shortening interacted with ~E-W extension along mostly dextral shear/fault zones. In the Neogene, shear is concentrated along a dextral wrench corridor south of the domes. We interpret the Paleogene shortening to record thickening and northward growth of the Pamir-Tibetan Plateau and short-lived Miocene crustal extension as gravitational adjustment, i.e., collapse, of the thickened Asian crust to Indian slab breakoff. Synconvergent Paleogene lateral extrusion thickened the Afghan Hindu Kush crust west of the India-Asia collision, and the Miocene-Recent dextral shear and ~E-W extension have accommodated collapse of the Pamir Plateau into the Tajik depression.

Building the Pamir‐Tibetan Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 2. Timing and rates

Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before ~35 Ma and peaked at ~23–20 Ma, reflecting top-to- ~N thrust-sheet and fold-nappe emplacement that tripled the thickness of the upper ~7–10 km of the Asian crust. Multimethod thermochronology traces cooling through ~700–100°C between ~22 and 12 Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at ~22–17 Ma. Age-versus-elevation relationships and paleoisotherm spacing imply exhumation at ≥3 km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7–31°C/Myr) starting between ~23 and 12 Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir-wide gravitational collapse of thickened crust starting at ~23–21 Ma during ongoing India-Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at ≥12 Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g., slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.

Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry

Determining early orogenic processes within the Pamir-Tibet orogen represents a critical step toward constructing a comprehensive model on the tectonic evolution of the region. Here we investigate the timing and cause of prograde metamorphism of Cenozoic metamorphic rocks from the Pamir plateau through Lu-Hf geochronology, U-Pb rutile thermochronology, and garnet thermometry. Regional prograde metamorphism and heating to 750–830 °C, as constrained by thermometry, occurred between 37 and 27 Ma. Prograde growth of garnet first occurred in the South Pamir and spread to the Central Pamir during the following 10 m.y. The early metamorphism is attributed to high mantle heat flow following the ca. 45 Ma break-off of the Indian slab south of the Pamir. Our investigation confirms a long-lived thermal history of the Pamir deep crust before the Miocene, and provides a causal link between break-off, enhanced mantle heat flow, and prograde heating of the subduction hanging wall.

Building the Pamir–Tibet Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Pamir: 3. Thermobarometry and Petrochronology of Deep Asian Crust

Large domes of crystalline, mid–deep crustal rocks of Asian provenance make the Pamir a unique part of the India–Asia collision. Combined major-element and trace-element thermobarometry, pseudosections, garnet-zoning deconstruction, and geochronology are used to assess the burial and exhumation history of five of these domes. All domes were buried and heated sufficiently to initiate garnet growth at depths of 15–20 km at 37–27 Ma. The Central Pamir was then heated at ~10–20°C/Myr and buried at 1–2 km/Myr to 600–675°C at depths of 25–35 km by 22–19 Ma. The Shakhdara Dome in the South Pamir was heated at ~20°C/Myr and buried at 2–8 km/Myr to reach 750–800°C at depths of ≥50 km by ~20 Ma. All domes were exhumed at >3 km/Myr to 5–10 km depths and ~300°C by 17–15 Ma. The pressures, temperatures, burial rates, and heating rates are typical of continental collision. Decompression during exhumation outpaced cooling, compatible with tectonic unroofing along mapped large-scale, normal-sense shear zones, and with advection of near- or suprasolidus temperatures into the upper crust, triggering exhumation-related magmatism. The Shakhdara Dome was exhumed from greater depth than the Central Pamir domes perhaps due to its position farther in the hinterland of the Paleogene retrowedge and to higher heat input following Indian slab breakoff. The large-scale thickening and coincident ~20 Ma switch to extension throughout a huge area encompassing the Pamir and Karakorum strengthens the idea that the evolution of orogenic plateaux is governed by catastrophic plate-scale events.

Multistage emplacement of the McDoogle pluton, an early phase of the John Muir intrusive suite, Sierra Nevada, California, by magmatic crack-seal growth

New mapping of the southern McDoogle pluton, an early intrusion in the John Muir suite in the central Sierra Nevada, California, has resolved important details of incremental assembly by magmatic crack-seal. The tabular McDoogle pluton concordantly intruded the steeply dipping mylonitic Sawmill Lake shear zone around 95 Ma. The pluton was previously divided into three phases based on wall-rock screen abundance and lithology. The earliest central phase consists of steep dikes separated by abundant wall-rock screens. These screens are largely absent from border phases located on either side of the central phase. The inferred relative ages of intrusive phases and the wall-rock screen distribution signify a change from antitaxial (addition at the margins of the central phase) to syntaxial (addition at the center of each border phase) assembly during pluton growth. Interaction among the effects of preexisting foliation, older nondeformed intrusions, and synintrusive static recovery of shear zone rocks likely caused the transition from antitaxial to syntaxial growth. The trend from antitaxial to syntaxial growth during emplacement has been observed in other intrusions and intrusive suites and may be a general pattern of incremental intrusion associated with the feedback between preexisting deformation and thermal addition in magmatic arcs.

Controls on Trace Element Uptake in Metamorphic Titanite: Implications for Petrochronology

Petrochronology—the interpretation of isotopic dates with complementary elemental data— requires understanding the relationship between trace elements in chronometers and the petrological evolution of their host rocks. Titanite is a useful petrochronometer for crustal processes, but how titanite records host rock evolution is uncertain. We present an extensive titanite U–Pb and chemical dataset from felsic gneisses and leucosomes in the Western Gneiss Region (WGR) of Norway. Mineral textures, U–Pb dates, and major, minor, and trace element chemistry reveal three titanite populations: (1) Precambrian igneous titanite [high light rare earth elements (LREE), Th, Pb, Zr; low Al, F]; (2) Caledonian recrystallized titanite (low LREE, Th, Pb) that formed from dissolution–reprecipitation of the Precambrian titanite and co-crystallized with allanite; (3) Caledonian neocrystallized titanite (high Al, F and variable REE). Although titanite records multiple igneous and metamorphic events in the WGR, we use a principal components analysis to identify distinct petrological and thermal effects on trace element uptake that hold across all titanite populations. Coupled with textural observations, these data show that different trace element patterns between populations predominantly represent the activity of different rock reactions during continental subduction and exhumation; using correlations between principal component scores and trace element abundances or ratios, we discriminate which phases co-crystallized with titanite. Our results further demonstrate that thermal and fluid partitioning effects can complicate interpretations of rock petrology from titanite trace elements, but these factors can be assessed by measuring specific trace elements (e.g. Al, Zr).