Abstract: Deccan volcanism was synchronous with rifting along the west coast of India. Pre- and synmagmatic rifting has been widely reported in the Deccan Volcanic Province, but extension post-dating magmatism, and predating India–Eurasia collision, is less well known. A recent study in the Kachchh area of western India documented weathering of basalts to kaolinite at the base of Cenozoic rift basins, with rift flanks relatively less altered to smectites, and this was attributed to post-magmatic rifting. This study models basalt weathering under open- and closed-system conditions to simulate rainwater interacting with basalts either on topographical slopes (within rifts) or on flat-topped hills (flow tops). Both systems were modelled under p CO 2 conditions ranging from low, present-day values to higher values more appropriate for the end-Cretaceous–early Paleocene time, after basalt emplacement. The results show that if p CO 2 exceeded values of 10 −2.5 , basalts would be altered to kaolinite in both open and closed systems. Existing p CO 2 estimates in the aftermath of Deccan volcanism fall below this value, implying that the differential basalt weathering was more likely to have been caused by terminal to post-magmatic rifting. This indicates that extensional tectonics along the Indian west coast in the Kachchh region continued even after cessation of Deccan volcanism.

Abstract Fold and thrust belts are contractional systems created by tectonic plate collisions, creating stress conditions where the maximum principal stress is horizontal and the minimum principal stress is vertical. The stress orientation favours the development of low-angle reverse faults. The trigger mechanism for thrust activation as a recurring seismic event is related to fluctuations in fluid pressure. The fluctuating fluid pressure creates a temporal cycle of elevated and relaxed fluid pressure that shifts the system in and out of failure conditions. The generation of hydrocarbons from organic material is a source of fluids and a means of temporally fluctuating the fluid pressure. In the southern Canadian Rocky Mountains the major detachments follow organic-rich strata that have generated hydrocarbons. The nature of the fluids impacts the size and distributions of faults within the tectonic wedge. The critical taper of the wedge is modified by the presence of high fluid pressure.

Post-Paleozoic magmatism in Angola and Namibia (SW Africa) is widespread along the continental margin (flood tholeiites of the Paraná-Etendeka system), and along transverse lineaments (alkaline and alkaline-carbonatitic complexes; sodic and potassic suites). These different magmatic suites are strictly associated in space and/or time. Variable melting degrees of a veined lithospheric mantle are proposed for the most “primitive” magmas from geochemical modeling and Sr-Nd isotope systematics. A complex evolution emerges for some ultramafic rocks (cumulus processes) and for differentiated rock compositions (assimilation and fractional crystallization, AFC, magma mixing), which may also involve anatexis of the crystalline basement and emplacement of S-type granites and rhyolites. Melting of a lithospheric mantle, without an appreciable contribution of the asthenosphere (thermal input excepted), is consistent with regional thermal anomalies in the deep mantle, mapped by gravity of the geoid, seismic tomography, and paleomagnetic analysis. The Walvis Ridge and Rio Grande “hotspot tracks” are interpreted as stress response in the lithosphere during rifting. A plume-related heat source is not favored by our results.

Dissolution has removed large volumes of rock at low-angle normal faults, i.e., detachment faults, in the Mormon Mountains and the Tule Springs Hills in the eastern Basin and Range Province, southeastern Nevada. Evidence for major dissolution includes widespread solution-collapse breccias, meter-scale stylolite structures, and high-angle accommodation faults that terminate at or merge with dissolution seams. Chemically reactive fluids moving along the fault zones led to a strong depletion of 18 O in the detachment fault breccias (e.g., a δ 18 O decrease of 8‰ relative to the unaltered rocks). These strong chemical shifts, demonstrated by (1) negative oxygen isotope values and (2) steep compositional gradients marked by metal enrichment in elements such as Au, Ag, Ti, Pb, Zn, and Cu, are generally restricted to the narrow (<1 m to 8 m) microbreccia zones. Extensional faulting and fracturing, accompanying regional uplift, opened conduits for the influx of meteoric waters from above and hydrothermal fluids from below. As the largest, most permeable structures that formed during uplift, detachment faults focused the fluid flow. In this deformation and hydrogeologic model, dissolution-caused stratal thinning is a major complement to detachment faulting and is an important process that resolves void space issues in the reconstruction of cross sections.

Ultrahigh-pressure (UHP) metamorphic terranes in contractional orogens reflect descent of continental crust bonded to a dense, dominantly oceanic plate to depths of 90–140 km. All recognized well-documented UHP complexes formed during Phanerozoic time. Rocks are intensely retrogressed to low-pressure assemblages, with rare relict UHP phases retained in tough, refractory host minerals. Resurrected UHP slabs consist chiefly of quartzofeldspathic rocks and serpentinites; dense mafic + ultramafic lithologies comprise <10% of exhumed masses. Associated garnet-bearing ultramafic lenses are of four general origins: type A peridotite + eclogite pods reflect premetamorphic residence in the mantle wedge; type B masses were mantle-derived ultramafic-mafic magmas that rose into the crust prior to subduction; type C tectonic lenses were present in the oceanic lithosphere prior to underflow; and type D garnet peridotites achieved their deep-seated mantle mineralogy long before—and independent of—the subduction event that produced the UHP-phase assemblages in garnet peridotite types A, B, and C. Geochronology constrains the timing of protolith, peak, and retrograde recrystallization of gneissic, ultramafic, and eclogitic rocks. Round-trip pressure-temperature ( P-T ) paths were completed in <5–10 m.y., where ascent rates approximated subduction velocities. Exhumation from profound depth involves near-adiabatic decompression through P-T fields of much lower-pressure metamorphic facies. Many complexes consist of thin, allochthonous sheets, but those in eastern China and western Norway are about 10 km thick. Ductilely deformed nappes generated in subduction zones allow heat to be conducted away as sheet-like UHP complexes rise, cooling across both upper and lower surfaces. Thicker UHP massifs also must be quenched. Ascent along the subduction channel is driven mainly by buoyancy of low-density crustal material relative to the surrounding mantle. Rapid exhumation prevents establishment of a more normal geothermal regime in the subduction zone. Lack of H 2 O impedes back reaction, whereas its presence accelerates transformation to low-P phase assemblages. Late-stage domal uplifts characterize some collisional terranes; erosion, combined with underplating, contraction, tectonic aneurysms, and/or lithospheric plate shallowing, may further elevate mid-crustal UHP terranes toward the surface.

The low-temperature epigenetic and stratabound Pb-Zn-Cu-Ba-F-Sr–bearing ore deposits enclosed within sedimentary columns historically have been major sources of metals. Exploration companies still find these deposits to be a profitable exploration target due to their simple mineralogy as well as the large tonnage that can present, always considering the mineral districts as a whole. In northeastern México, several nonmagmatic, low-temperature Pb-Zn-F-Ba deposits have been systematically considered as magmatic-related (skarns, high-temperature replacement deposits, epithermal deposits, etc.). Recently, these deposits have been restudied and placed within a scenario of deep fluid circulation of basinal brines through the Mesozoic sedimentary series, enriched in Ba, F, and metals during fluid flow and water-rock interactions. These fluids gave rise to a series of strata-bound epigenetic ore deposits scattered throughout the whole Mesozoic carbonate platform and can be shown to be unrelated to any period of magmatism. There is no intense alteration to the host rocks. Commonly there is a close association with organic matter, either liquid hydrocarbons or bitumen; they have a very simple mineralogy of barite, celestine, fluorite, sphalerite, galena, and have low formation temperatures (90–105 °C) combined with variable salinities. These characteristics make these deposits similar to the Mississippi Valley–type deposits, possibly most similar to the Alpine-Appalachian subtype.