The Himalayan crystalline core zone exposed along the Sutlej Valley (India) is composed of two high‐grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded, as a consequence of the foreland‐directed propagation of crustal deformation in the Indian plate margin. The High Himalayan Crystalline Sequence (HHCS) is composed of amphibolite facies to migmatitic paragneisses, metamorphosed at temperatures up to 750°C at 30 km depth between Eocene and early Miocene. During early Miocene, combined thrusting along the Main Central Thrust (MCT) and extension along the Sangla Detachment induced the rapid exhumation and cooling of the HHCS, whereas exhumation was mainly controlled by erosion since middle Miocene. The Lesser Himalayan Crystalline Sequence (LHCS) is composed of amphibolite facies para‐ and orthogneisses, metamorphosed at temperatures up to 700°C during underthrusting down to 30 km depth beneath the MCT. The LHCS cooled very rapidly since late Miocene, as a consequence of exhumation controlled by thrusting along the Munsiari Thrust and extension in the MCT hanging wall. This renewed phase of tectonic extrusion at the Himalayan front is still active, as indicated by the present‐day regional seismicity, and by hydrothermal circulation linked to elevated near‐surface geothermal gradients in the LHCS. As recently evidenced in the Himalayan syntaxes, active exhumation of deep crustal rocks along the Sutlej Valley is spatially correlated with the high erosional potential of this major trans‐Himalayan river. This correlation supports the emerging view of a positive feedback during continental collision between crustal‐scale tectono‐thermal reworking and efficient erosion along major river systems.

Fission-track and 40 Ar/ 39 Ar ages place time constraints on the exhumation of the North Himalayan nappe stack, the Indus Suture Zone and Molasse, and the Transhimalayan Batholith in eastern Ladakh (NW India). Results from this and previous studies on a north-south transect passing near Tso Morari Lake suggest that the SW-directed North Himalayan nappe stack (comprising the Mata, Tetraogal and Tso Morari nappes) was emplaced and metamorphosed by c. 50-45 Ma, and exhumed to moderately shallow depths (c. 10 km) by c. 45-40 Ma. From the mid-Eocene to the present, exhumation continued at a steady and slow rate except for the root zone of the Tso Morari nappe, which cooled faster than the rest of the nappe stack. Rapid cooling occurred at c. 20 Ma and is linked to brittle deformation along the normal Ribil-Zildat Fault concomitant with extrusion of the Crystalline nappe in the south. Data from the Indus Molasse suggest that sediments were still being deposited during the Miocene.

[1] At the latitude of the Thor-Odin dome (British Columbia) the Columbia River Detachment defines the eastern margin of the Shuswap metamorphic core complex and localizes in a 1 km thick muscovite-bearing quartzite mylonite. We present a combined 40 Ar/ 39 Ar, (micro)structural, and oxygen isotope study of the deformation history in the detachment and evaluate the spatial and temporal relationships between microstructure formation and localization of strain. Ar/ 39 Ar geochronology from different levels in the mylonite delineates a pattern of increasingly younger (49.0 to 47.9 Ma) deformation ages in deeper levels of the mylonitic footwall. The correlation of 40 Ar/ 39 Ar ages with decreasing deformation temperatures ($550°-400°C) in the top 200 m of the mylonite indicates that deformation migrated downward from the contact with the hanging wall. Strain localization was diachronous in progressively deeper levels of the footwall and was likely controlled by fluid-assisted strain hardening due to advective heat removal and contemporaneous reaction weakening due to dissolutionreprecipitation of white mica. The observed constant high-stress microstructures across the entire detachment indicate that flow stress was buffered by the interplay of strain rate and temperature, where high strain rates at elevated temperature produced the same microstructure as lower strain rates under decreasing temperature conditions. The combined data suggest that the complex interplay among temporally nonuniform rates of footwall exhumation, heat advection, and embrittlement by meteoric fluids strongly determines the thermomechanical behavior of extensional detachments.

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The hydrogen isotope composition of Eocene muscovite in mylonitic quartzite from the Kettle and Shuswap metamorphic core complexes (Washington and British Columbia) permits estimates of paleoaltimetry of the North American Cordillera at the onset of post‐collisional lithospheric extension. Coupled oxygen, hydrogen, and 40Ar/39Ar isotope data indicate that meteoric water penetrated to significant depths during normal faulting along the Columbia River Detachment bounding both Kettle and Shuswap metamorphic core complexes. Synkinematic muscovite attains δDmuscovite values as low as −135‰ and −157‰, respectively, consistent with δDwater values of −115 ± 5‰ and −135 ± 5‰. In context with stable isotope data from Eocene sedimentary basins of continental North America, these data constrain the isotopic composition of precipitation from which paleoelevation estimates can be made. Stable isotope paleoaltimetry indicates that during detachment formation (49–47 Ma), orogen‐parallel variations in topography existed, and mean elevations decreased from ≥4000 m at the latitude of the Shuswap core complex to ≥3000 m in the Kettle core complex. In addition, these data indicate a net decrease in mean surface elevation by ∼1000 m since the Eocene. Our results for the Kettle core complex are consistent with paleoelevation estimates based on fossil floral physiognomy in the Eocene Republic basin (Washington), indicating that high elevations characterized not only the frontal escarpment near the crustal‐scale detachment but continued into the Okanogan highlands further west. Collectively, the stable isotope, geochronological, and paleofloral data support tectonic models of high Eocene surface elevations contemporaneous with magmatism and lithospheric extension.