No abstract

The Pontides in northern Turkey constituted part of the southern active margin of Eurasia during the Mesozoic. In the Early Cretaceous, a large submarine turbidite fan covered most of the Central Pontides. New U‐Pb detrital zircon data imply that the major source of the turbidites was the East European Craton‐Scythian Platform in the north. This implies that there was no thoroughgoing Black Sea basin between the Pontides and the East European Craton during the Early Cretaceous. The Lower Cretaceous turbidites are bounded in the south by a large metamorphic area, the Central Pontide Supercomplex (CPS). New geological mapping, petrology, and U‐Pb zircon and Ar‐Ar muscovite ages indicate that the northern part of the CPS consists of Lower Cretaceous distal turbidites deformed and metamorphosed in a subduction zone in the Albian. The rest of the CPS is made of Middle Jurassic, Lower Cretaceous, and middle Cretaceous (Albian) metamorphic belts, each constituting distinct subduction‐accretion units. They represent episodes of collision of oceanic volcanic arcs and oceanic plateaus with the Eurasian margin and are marked in the stratigraphy of the hinterland by periods of uplift and erosion. The accretionary complexes are overlain by Upper Cretaceous (Turonian‐Santonian) volcano‐sedimentary sequences deposited in a fore‐arc setting. The detrital zircon data, middle Cretaceous (Albian) metamorphism, and widespread Albian uplift of the Black Sea region suggest that Early Cretaceous (Barremian‐Aptian) nonvolcanic rifting and Late Cretaceous (Turonian‐Santonian) opening of the Black Sea by the splitting of the arc are unrelated events.

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.

Large domes of crystalline, middle to 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‐solidus 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.

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 metamorphismuntil 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.