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Theory suggests the possibility for significant deviations between total pressure (or dynamic pressure) and lithostatic pressure during crustal metamorphism. If such deviations exist, the implications for orogenic reconstruction would be profound. Whether such non-lithostatic pressure conditions during crustal metamorphism are recorded and preserved in the rock record remains unresolved, as direct field evidence for this phenomenon is limited. Here, we investigate the Paleogene Tethyan Himalaya fold-thrust belt in Himachal Pradesh, northwestern India, which is the structurally highest part of the Himalayan orogen and deforms a ~10–15 km thick Neoproterozoic–Cretaceous passive margin stratigraphic section. Field-based kinematic studies demonstrate relatively moderate shortening strain across the Tethyan Himalaya. However, basal Tethyan strata consistently yield elevated pressure-temperature-time (P-T-t) estimates of 7–8 kbar and ~650°C, indicative of deep burial during Himalayan orogeny (ca. 20–45 Ma, 25–30 km depths). These P-T-t conditions can be reconciled by: (1) deep Cenozoic burial along cryptic structures and/or significant flattening of the Tethyan strata; (2) basal Tethyan strata recording metamorphism and deformation related to pre-Himalayan tectonism; or (3) non-lithostatic pressure conditions (i.e., tectonic overpressure). To test these models, we systematically mapped the Tethyan fold-thrust belt along the Pin Valley transect in northwestern India, a classic site for stratigraphic, paleontological, paleoenvironmental, and structural reconstructions. The Pin Valley region provides an opportunity to study a structurally continuous metamorphic field gradient from the near-surface to structural depths between 10–15 km, which should reflect P conditions ≤4 kbar if lithostatic. We integrate a multi-method approach combining detailed geologic mapping with quantitative analytical techniques (e.g., thermometry, finite strain analyses, thermo/geochronology, and thermobarometry) to quantify the magnitude, kinematics, thermal architecture, and timing of regional deformation, metamorphism, and subsequent exhumation. Results show: (1) throw on shortening structures is moderate to low (≤4 km); (2) temperature-depth relationships record a continuous, but regionally elevated, upper-crustal geothermal gradient of ≥40 °C/km, which is inconsistent with deep burial models (≤25 °C/km); (3) minimal flattening of basal Tethyan strata; (4) upper Tethyan strata yield pre-Himalayan low-temperature thermochronology dates, further refuting deep Cenozoic burial; and (5) basal Tethyan P-T-t estimates confirm elevated mid-crustal conditions of ~7 kbar, 630°C at 10–15 km depths during the Cenozoic. Preliminary volume expansion calculations are minimal; therefore, mechanisms involving non-hydrostatic thermodynamics, deviatoric stresses, rock strength contrasts, and tectonic mode switching are being explored. 

One of the most rewarding and enjoyable aspects of teaching in geography and geoscience is field instruction. Designing and implementing field experiences can be both an exciting and overwhelming prospect. The reality is that field instruction requires diligent preparation, both in and outside the classroom, if it is to provide successful and meaningful learning experiences. Many geographers and geoscientists are committed to experiential, field‑based learning and have written about both the practical and theoretical aspects of successfully teaching in the field. This chapter offers a summary of this helpful literature, while also discussing some of the authors’ experiences of “lessons learned” from teaching in the field, and the impact of the recent COVID‑19 pandemic on field instruction. 

Suture zones located across the Tibetan region clearly demarcate the rift-and-drift and continental accretion history of the region. However, the intraplate responses to these marginal plate-tectonic events are rarely quantified. Our understanding of the Paleo-Tethyan orogenic system, which involved ocean opening and closing events to grow the central Asian continent, depends on the tectonic architecture and histories of major late Paleozoic−early Mesozoic orogenic belts. These opening and collision events were associated with coupled intracontinental deformation, which has been difficult to resolve due to subsequent overprinting deformation. The late Paleozoic−early Mesozoic Zongwulong Shan−Qinghai Nanshan belt in northern Tibet separates the Qilian and North Qaidam regions and is composed of Carboniferous−Triassic sedimentary materials and mantle-derived magmatic rocks. The tectonic setting and evolutional history of this belt provide important insight into the paleogeographic and tectonic relationships of the Paleo-Tethyan orogenic system located ∼200 km to the south. In this study, we integrated new and previous geological observations, detailed structural mapping, and zircon U-Pb geochronology data from the Zongwulong Shan−Qinghai Nanshan to document a complete tectonic inversion cycle from intraplate rifting to intracontinental shortening associated with the opening and closing of the Paleo-Tethyan Ocean. Carboniferous−Permian strata in the Zongwulong Shan were deposited in an intracontinental rift basin and sourced from both the north and the south. At the end of the Early−Middle Triassic, foreland molasse strata were deposited in the southern part of the Zongwulong Shan during tectonic inversion in the western part of the tectonic belt following the onset of regional contraction deformation. The Zongwulong Shan−Qinghai Nanshan system has experienced polyphase deformation since the late Paleozoic, including: (1) early Carboniferous intracontinental extension and (2) Early−Middle Triassic tectonic inversion involving reactivation of older normal faults as thrusts and folding of pre- and synrift strata. We interpret that the Zongwulong Shan−Qinghai Nanshan initiated as a Carboniferous−Early Triassic intracontinental rift basin related to the opening of the Paleo-Tethyan Ocean to the south, and it was then inverted during the Early−Middle Triassic closing of the Paleo-Tethyan Ocean. This work emphasizes that pre-Cenozoic intraplate structures related to the opening and closing of ocean basins in the Tethyan realm may be underappreciated across Tibet. 

The northwest-trending Altai Mountains of central Asia expose a complex network of thrust and strike-slip faults that are key features accommodating intracontinental crustal shortening related to the Cenozoic India-Asia collision. In this study, we investigated the Quaternary slip history of the Fuyun fault, a right-lateral strike-slip fault bounding the southwestern margin of the Altai Mountains, through geologic mapping, geomorphic surveying, and optically stimulated luminescence (OSL) geochronology. At the Kuoyibagaer site, the Fuyun fault displaces three generations of Pleistocene–Holocene fill-cut river terraces (i.e., T3, T2, and T1) containing landslide and debris-flow deposits. The right-lateral offsets are magnified by erosion of terrace risers, suggesting that river course migration has been faster than slip along the Fuyun fault. The highest Tp2 terrace was abandoned in the middle Pleistocene (150.4 ± 8.1 ka uppermost OSL age) and was displaced 145.5 +45.6/–12.1 m along the Fuyun fault, yielding a slip rate of 1.0 +0.4/–0.1 mm/yr since the middle Pleistocene. The lower Tp1 terrace was abandoned in the late Pleistocene and aggraded by landslides and debris flows in the latest Pleistocene–Holocene (36.7 ± 1.6 ka uppermost OSL age). Tp1 was displaced 67.5 +14.2/–6.1 m along the Fuyun fault, yielding a slip rate of 1.8 +0.5/–0.2 mm/yr since the late Pleistocene. Our preferred minimum slip rate of ~1 mm/yr suggests the Fuyun fault accommodates ~16% of the average geodetic velocity of ~6 mm/yr across the Altai Mountains. Integration of our new Fuyun slip rate with other published fault slip rates accounts for ~4.2 mm/yr of convergence across the Chinese Altai, or ~70% of the geodetic velocity field. 

Here, we use multi-method thermobarometric analyses (thermodynamic modelling, quartz in garnet barometry, Raman spectroscopy of carbonaceous material (RSCM) thermometry, and titanium in biotite thermometry) from samples throughout two transects in the Northwestern Tethyan Himalaya (TH) to constrain the pressure-temperature conditions of the basal TH. Peak metamorphic conditions from the basal TH indicate anomalously high pressures relative to the paleogeothermal gradients recorded along the two transects, suggesting non-lithostatic pressure conditions at the base of the Tethyan Himalaya. The TH fold-thrust belt comprises a deformed Neoproterozoic-Cretaceous section of sedimentary rocks that record the early stages of deformation of the Himalayan orogen. In the northwestern Himalaya, rocks at the base of the TH are metamorphosed and are useful for reconstructing the thermal evolution of the Himalaya during initial stages of crustal thickening. RSCM thermometry on samples along the Pin Valley and Sutlej Valley transects of the TH suggest a continuous ~1500 °C/GPa thermobarometric gradient through the entire TH section. These samples are from a continuous ~10-12 km-thick TH section in which the stratigraphically highest units are undeformed, fossil-bearing sedimentary rocks. Assuming lithostatic pressure, the basal TH is expected to record peak pressure-temperature (P-T) conditions of ~0.4-0.5 GPa and ~600 °C. However, quartz-in-garnet (QuiG) barometry and titanium-in-biotite thermometry of samples from the basal TH indicate peak P-T conditions of 0.94 ± 0.25 GPa and ~600°C, suggesting a paleo-geothermal gradient of 870-500 °C/GPa. These data constitute unexpectedly high peak pressure conditions along the basal TH. Possible explanations for these anomalously high basal TH pressures include pre-Himalayan metamorphic assemblages preserved in the TH resulting in erroneous Himalayan peak P-T estimates, or regional non-lithostatic pressure along the basal TH during Himalayan orogenesis. Thermobarometric work on samples from different stratigraphic levels of the basal TH in the Sutlej Valley is in progress to determine paleo-geothermal gradient continuity both across- and along-strike of the orogen. 

The Tethyan Himalayan sequence (THS) is the structurally highest lithotectonic unit of Indian affinity within the Cenozoic Himalayan orogen. In the NW Himalaya of the Himachal Pradesh, India, the Neoproterozoic–Cretaceous THS is thought to have relatively modest deformation despite the unit commonly recording early collision-related shortening. This lack of significant deformation contrasts that of other Himachal lithotectonic units closer to the foreland. In addition, burial depths of the Himachal THS estimated from structural reconstructions (~10 km) and basal metamorphic pressures (7–8 kbar, ~28 km lithostatic burial) conflict. To address these issues, we performed geologic mapping, thermochronology, and restored new balanced cross sections along two transects across the Himachal THS to better constrain its deformation state and timing, stratigraphic thickness, and burial extent. Along the Spiti and Pin valleys, the THS is shortened by seven NE-dipping thrusts and one SW-dipping thrusts that mostly form fault-propagation folds. The Mata Nappe region (NE of Spiti Valley) has been reinterpreted as a thrust pop-up structure, consistent with structural observations. Along this transect, the estimated THS thickness measured from the basal Akpa granite and Haimanta Group to the uppermost-exposed Tandi Group is ~12.3 km. Restoration of one cross section along this transect yields a minimum shortening of ~30 km (~22% strain). Farther SE along Sutlej Valley, the THS is cut by three SW-dipping thrusts and several S-dipping normal faults. The estimated thickness of the exposed Akpa granite and Haimanta Group is ~8.5 km. Restoration of one cross section along this transect yields a minimum shortening of ~8 km (~21% strain). Thrusts mapped along both transects are interpreted to branch from a single decollement formed by the South Tibet detachment and Great Counter thrust. Our THS shortening estimates added to those for other Indian rocks in the Himachal Himalaya (Webb, 2013) yields a total minimum estimate of ~515–537 km. Preliminary zircon (U-Th)/He dates along Spiti and Pin valleys generally young towards the SW from ca. 42–5 Ma. These results confirm: (1) relatively minor shortening of the Himachal THS that was likely compensated by duplexing of other units; and (2) the discrepancy between THS burial estimates, which may be a product of non-lithostatic pressure. 

Theory suggests the possibility for significant deviations between the total pressure (or dynamic pressure) and lithostatic pressure throughout Earth’s crust. Whether such non-lithostatic pressure conditions are recorded and preserved in the rock record remains unresolved, as direct field confirmation is limited, yet the implications for orogenic reconstruction are profound. Here we investigate the Paleogene Tethyan Himalaya fold-thrust belt in Himachal Pradesh, NW India, which is the structurally highest part of the Himalayan orogen and deforms a ~10–15 km thick Neoproterozoic–Cretaceous passive margin stratigraphic section. Field-based kinematic studies demonstrate relatively moderate shortening strain estimates across the Tethyan Himalaya, yet basal Tethyan strata consistently yield elevated pressure-temperature-time (P-T-t) estimates of 7–8 kbar and ~650°C, indicative of deep burial during Himalayan orogeny (25–30 km depths). These P-T-t conditions can be reconciled by: (1) deep Cenozoic burial along cryptic structures and/or significant flattening of the Tethyan strata; (2) basal Tethyan strata recording pre-Himalayan deformation related to Pan-African orogeny; or (3) non-lithostatic pressure conditions (i.e., tectonic overpressure). To test these models, we systematically mapped the Tethyan fold-thrust belt along the Bhaba Pass-Pin Valley transect in NW India, a classic site for stratigraphic, paleontological, paleoenvironmental, and structural reconstructions. We integrate a multi-method approach combining detailed geologic mapping with quantitative analytical techniques (e.g., finite strain analyses, thermometry, thermobarometry, thermochronology, and geochronology) to quantify the magnitude, kinematics, thermal architecture, and timing of regional deformation, metamorphism, and subsequent exhumation of the Tethyan fold-thrust belt. Our preliminary observations refute deep Cenozoic burial of the Tethyan Himalaya, suggesting either the preservation of non-lithostatic pressures in the rock record or relicts of pre-Himalayan metamorphism. Either scenario demonstrates that caution is required in using Himalayan P-T-t estimates to reconstruct the Cenozoic Himalayan orogeny. 

Tectonic models for the development of the Himalaya, Earth's largest active collisional mountain belt, have been developed and tested through pressure, temperature, and time (P-T-t) information collected from exposed metamorphic rocks. Inferred deep burial and subsequent exhumation of these rocks are usually justified by observable structures (e.g., Main Central thrust) and mapping relationships. However, regions where pressure estimates are at odds with field-based reconstructions are reconciled with hypothesized cryptic structures that have since been completely eroded. Such field versus thermobarometric discrepancies significantly impact interpretations on the geometry, magnitude, and distribution of deformation. Here, we conducted detailed field mapping of the Paleogene Tethyan fold-thrust belt in the Himachal Himalaya, NW India, which is the structurally highest part of the Himalayan orogen and deforms a ~10–15 km thick Neoproterozoic–Cretaceous passive margin section. In this region, P-T estimates yield 6–8 kbar and ~650°C, which suggests burial to depths of ~25–30 km. To assess the viability of this deep burial, we constructed a 1:200,000 scale geologic map of the Bhaba Pass-Pin Valley region. Geologic mapping was focused on the stratigraphy, structural configuration, and metamorphic isograds of the basal Tethyan strata. Detailed field mapping aided the construction of balanced cross sections, which guided subsequent multi-method analytical approaches that fit into a coherent structural framework. Our field observations and map relationships show no major structures, abrupt changes in metamorphic grade or composition that would suggest deep burial of the stratigraphically continuous basal Tethyan group. Balanced cross sections throughout the study area suggest moderate amounts of shortening strain (~30–36%). This contribution highlights the importance of detailed field mapping to interpret P-T estimates. Ongoing analytical methods are being conducted to constrain the thermal architecture and metamorphic history of the Tethyan fold-thrust belt. 

Crustal thickening along the Indus-Yarlung suture zone during the India-Asia collision was primarily accommodated by slip along the north-dipping Gangdese thrust (ca. 27–18 Ma) and south-dipping Great Counter thrust (ca. 25–10 Ma). However, the along-strike continuities, geometries, and timings of these thrusts remain unclear, resulting in an inadequate understanding of Himalayan-Tibetan orogenesis. In this study, we performed geologic mapping, strain analyses, geo/thermochronology, and thermobarometry across the easternmost Himalayan orogen (i.e., the northern Indo-Burma thrust belt), specifically: (1) the Tidding thrust and easternmost Indus-Yarlung suture zone (i.e., Tidding mélange complex) in its hanging wall; and (2) the Lohit thrust and Jurassic–Cretaceous Gangdese batholith and Mesoproterozoic basement (i.e., Lohit Plutonic Complex) in its hanging wall. The Tidding thrust is a north-dipping, top-south mylonitic shear zone that was active by ca. 36–30 Ma, during which hanging-wall mélange rocks were exhumed from ~33–38 km depth. The geometry, kinematics, and initiation age of the Tidding thrust contrast those of the top-north Great Counter thrust at the same structural position to the west. North of the Tidding thrust, the Lohit thrust is a ~5-km-wide, subvertical, north-side-up mylonitic shear zone that contains a basal, discrete “Lohit thrust fault”. Results of electron backscatter diffraction analyses across the Lohit thrust shear zone show that deformation fabric intensity and finite strain magnitudes decrease southwards toward the discrete thrust fault. This spatial relationship may be the result of transient peak strain during the lifespan of the shear zone. The Lohit thrust was active by ca. 25–23 Ma, during which hanging-wall basement and batholithic root rocks were exhumed to mid-crustal depths. The Lohit thrust and Gangdese thrust to the west are located at the same structural position and have comparable geometries, kinematics, and timings. Based on these similarities and previous findings, we interpret that the Lohit and Gangdese thrusts are correlative segments of a single, orogen-wide thrust system that accommodated crustal thickening along the Indus-Yarlung suture zone during the Oligocene–Miocene. 

The Tethyan Himalaya (TH) fold-thrust belt comprises a deformed Neoproterozoic-Cretaceous section of sedimentary rocks that record the early stages of deformation of the Himalayan orogen. In the northwestern Himalaya, rocks at the base of the TH are metamorphosed and are useful for reconstructing the thermal evolution of the Himalaya during initial stages of crustal thickening. Here, we present results of multi-method thermobarometry (thermodynamic modelling, Si in white mica barometry, quartz in garnet barometry, raman spectroscopy of carbonaceous material (RSCM) thermometry) on metasedimentary samples from two transects across the TH, with apparently continuous stratigraphy separated along strike of the orogen by ~40 km. Samples from the Pin Valley region record peak pressure-temperature (P-T) conditions of 0.4-0.5 GPa, 600 °C, suggesting a paleo-geothermal gradient of 30-40 °C/km. These samples are from the base of a continuous ~10-12 km-thick TH section in which the stratigraphically highest units are undeformed, fossil-bearing sedimentary rocks. RSCM thermometry on samples from stratigraphically higher levels of the TH suggest a continuous ~40 °C/km geothermal gradient through the entire TH section in the Pin Valley region. In contrast, previous thermobarometric studies from the Sutlej Valley ~40 km to the east report peak P-T conditions of 0.7-0.8 GPa, 600-650 °C, suggesting a paleo-geothermal gradient of 20-25 °C/km. Our new data indicate significant along-strike variation in peak P-T conditions and paleo-geothermal gradients at the base of the TH. Possible explanations for this along-strike thermobarometric discrepancy include: 1) pre-Himalayan metamorphic assemblages preserved in the TH resulting in erroneous Himalayan peak P-T estimates, 2) along-strike structural differences that resulted in differential burial and exhumation during Himalayan orogenesis, or 3) non-lithostatic pressure during orogenesis. Thermobarometric work on samples from different stratigraphic levels of the basal TH in the Sutlej Valley is in progress to determine paleo-geothermal gradient continuity both across- and along-strike of the orogen.