Geological studies along a transect across the Himalaya in eastern Ladakh and Lahul provide new insights into the Tertiary structural evolution of this region. The initiation of the Nyimaling-Tsarap Nappe is related to an early phase of underthrusting of India below Asia. In Lahul, an opposite vergent intra-continental underthrusting develops immediately after continental collision (NE-vergent Tandi Syncline and Shikar Beh Nappe). This NE-vergent nappe stack is responsible for the amphibolite-facies regional metamorphism of the lower Chandra Valley. The subsequent phase corresponds to the main thrusting of the SW-vergent Nyimaling-Tsarap Nappe, developed by ductile shear (87 km Eocene shortening). This nappe pile is responsible for the regional metamorphism of SE Zanskar (kyanite-staurolite near Sarchu). The root zone and the frontal part of the Nyimaling-Tsarap Nappe are subsequently overprinted by two NW-SE-orientated dextral transpressional shear zones. To the south of the investigated area, the Main Central Thrust has been developed as a shear zone in the regional metamorphic ductile crustal rocks below the older nappes to the north. In the Sarchu and Nyimaling regions, the following tectonic phase corresponds to NE-vergent ‘backfolding’ (Miocene). Normal faults in the Sarchu area record a late extension of approximately 14 km.
In the Tethyan sedimentary series and the High-Himalayan ‘crystalline’ units of NW India, two generations of granite have previously been described. These rocks include large Cambro-Ordovician batholiths and small Tertiary bodies. A third type, the Yunam granite, has been discovered in the SE Zanskar — Upper Lahul region. It crops out as discordant dykes in the Ordovician Thaple Fm. Radiometric (U-Pb) measurements on zircons give an Early Permian crystallization age of 284 ± 1 Ma. Tertiary metamorphic cooling ages of 33.5, 20.8 ± 0.4, 22.8 ± 0.5 and 19.3 ± 0.2 Ma have been obtained by 40 Ar/ 39 Ar and K-Ar methods on amphiboles, muscovites and biotites of the granite and associated rocks. In contrast to the crustal affinity of the Cambro-Ordovician and Tertiary Himalayan granites, the Yunam granite is associated with an anorogenic, possibly thinned lithosphere context. This magmatic event is most probably related to the Permo-Mesozoic rifting of Neo-Tethys.
Karst reservoirs in the Chumphon Basin of the Gulf of Thailand have produced oil at well rates exceeding 10 000 BBL/d. Meteorically karstified buried hills were recognized as a potential exploration play. The Nang Nuan discovery well appeared to confirm such a play, and the concept prevailed despite the accumulation of contrary and unusual data. By the time a subsequent well had produced nearly 4 × 10 6 BBL oil, there was a desire to better understand the prospectivity of the concession. The accumulated data indicate that the highs are probably syn-rift horsts and inversion features. Karst reservoirs occur in Ratburi carbonates, and Mesozoic and Tertiary clastics, apparently unrelated to subaerial exposure. The karstification appears to be primarily of deep-burial origin, as indicated by the nature of the karst, substantial pore volumes that are difficult to account for, and temperature and flow anomalies consistent with active geothermal circulation. There are granites and hot springs in the vicinity, and abundant CO 2 in this and neighbouring basins. Such deep-burial karst reservoirs have different implications for reserves estimation, prospect ranking and well completions.
The intrusive continental basalts of the Tethyan Himalaya are not necessarily comagmatic feeder dykes or the hypabyssal equivalents of the middle Permian Panjal Traps flows associated with the Permo-Mesozoic rifting of the Tethys, as is generally considered. A comparative study of basic rocks from SE Zanskar, NW Spiti and Upper Lahul shows the coexistence of geochemically different basalts in this region. The Panjal Traps of SE Zanskar and NW Spiti are tholeiitic continental flood basalts (CFB). They originated from an ‘enriched’ P-MORB-type magma which underwent a limited evolution by fractional crystallization and probable crustal contamination. In the adjacent Upper Lahul area, the Baralacha La Dyke Swarm shows a close spatial association with Lower Carboniferous transtensional synsedimentary faults. These dykes consist of tholeiitic to alkalic basalts, belonging to a comagmatic suite that evolved mainly by fractional crystallization. Geochemical data indicate that the Panjal Traps and the dykes are not comagmatically related. These characteristics imply either the coexistence, in the studied area, of contemporaneous but chemically different magmas or temporally unrelated magmatic events. The Baralacha La Dyke Swarm could be associated with a Lower Carboniferous transtensional tectonic event, related to an early phase of Permian rifting of Neo-Tethys.
In the southern part of the Indian Tethys Himalaya between SE Zanskar and NW Lahul, nappe tectonics with transport toward the WSW during the main deformation phase has been ascertained. If the stratigraphy of the different units considered as nappes is combined, it results in a composite section ranging from lower Cambrian to Middle Cretaceous. A metamorphism of amphibolite facies in the lowest unit is associated with the nappe stacking. The Lower Miocene cooling ages of metamorphic biotite and muscovite suggest that the nappe emplacement starts at the Oligocene, or at the Oligocene‐Eocene boundary. After the compressional phases, an extension provokes the reactivation of older main thrusts as normal fault and the formation of a steeply dipping normal fault. This interpretation is based on detailed mapping.
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