Evolution of Neoproterozoic Shillong Basin, Meghalaya, NE India: implications of supercontinent break-up and amalgamation

Fragmentation and amalgamation of supercontinents play an important role in shaping our planet. The break-up of such a widely studied supercontinent, Rodinia, has been well documented from several parts of India, especially the northwestern and eastern sector. Interestingly, being located very close to the Proterozoic tectonic margin, northeastern India is expected to have had a significant role in Neoproterozoic geodynamics, but this aspect has still not been thoroughly studied. We therefore investigate a poorly studied NE–SW-trending Shillong Basin of Meghalaya from NE India, which preserves the stratigraphic record and structural evolution spanning the Neoproterozoic Era. The low-grade metasedimentary rocks of Shillong Basin unconformably overlie the high-grade Archean–Proterozoic basement and comprise a c. 4000-m-thick platform sedimentary rock succession. In this study, we divide this succession into three formations: lower Tarso, middle Ingsaw and upper Umlapher. A NW–SE-aligned compression event later caused the thrusting of these sedimentary rocks over the basement with a tectonic contact in the western margin, resulting in NE–SW-trending fold belts. The rift-controlled Shillong Basin shows a comparable Neoproterozoic evolution with the equivalent basins of peninsular India and eastern Gondwana. The recorded Neoproterozoic rift tectonics are likely associated with Rodinia’s break-up and continent dispersion, which finally ended with the oblique collision of India with Australia and the intrusion of Cambrian granitoids during the Pan-African Orogeny, contributing to the assembly of Gondwana. This contribution is the first to present a complete litho-structural evolution of the Shillong Basin in relation to regional and global geodynamic settings.

Gravitational sliding or tectonic thrusting?: Examples and field recognition in the Miura-Boso subduction zone prism

ABSTRACT Discrimination between gravity slides and tectonic fold-and-thrust belts in the geologic record has long been a challenge, as both have similar layer shortening structures resulting from single bed duplication by thrust faults of outcrop to map scales. Outcrops on uplifted benches within the Miocene to Pliocene Misaki accretionary unit of Miura-Boso accretionary prism, Miura Peninsula, central Japan, preserve good examples of various types of bedding duplication and duplex structures with multiple styles of folds. These provide a foundation for discussion of the processes, mechanisms, and tectonic implications of structure formation in shallow parts of accretionary prisms. Careful observation of 2-D or 3-D and time dimensions of attitudes allows discrimination between formative processes. The structures of gravitational slide origin develop under semi-lithified conditions existing before the sediments are incorporated into the prism at the shallow surfaces of the outward, or on the inward slopes of the trench. They are constrained within the intraformational horizons above bedding-parallel detachment faults and are unconformably covered with the superjacent beds, or are intruded by diapiric, sedimentary sill or dike intrusions associated with liquefaction or fluidization under ductile conditions. The directions of vergence are variable. On the other hand, layer shortening structure formed by tectonic deformation within the accretionary prism are characterized by more constant styles and attitudes, and by strong shear features with cataclastic textures. In these structures, the fault surfaces are oblique to the bedding, and the beds are systematically duplicated (i.e., lacking random styles of slump folds), and they are commonly associated with fault-propagation folds. Gravitational slide bodies may be further deformed at deeper levels in the prism by tectonism. Such deformed rocks with both processes constitute the whole accretionary prism at depth, and later may be deformed, exhumed to shallow levels, and exposed at the surface of the trench slope, where they may experience further deformation. These observations are not only applicable in time and space to large-scale thrust-and-fold belts of accretionary prism orogens, but to small-scale examples. If we know the total 3-D geometry of geologic bodies, including the time and scale of deformational stages, we can discriminate between gravitational slide and tectonic formation of each fold-and-thrust belt at the various scales of occurrence.

A Globally Significant Potential Megascale Geopark: The Eastern Australian Mantle Hotspot Interacting with a North-Migrating Heterogeneous Continental Plate Creating a Variety of Volcano Types, Magmas, Xenoliths, and Xenocrysts

Australia commenced separating from Antarctica some 85 million years ago, finally separating about 33 million years ago, and has been migrating northwards towards the Eurasian plate during that time. In the process, Australia, on its eastern side, progressively passed over a mantle hotspot. A magma plume intersected a variable lithocrust with various lithologic packages such as Phanerozoic sedimentary basins, fold belts and metamorphic terranes, and Precambrian rocks. As such, there was scope for compositional evolution of magmas through melting and assimilation, as well as plucking of host rocks to include xenoliths, and xenocrysts. The volcanic chain, volcanoes, and lava fields that are spread latitudinally along 2000 km of eastern Australia present a globally-significant volcanic system that provides insights into magma and crust interactions, into the variability of xenoliths and xenocrysts, into magma evolution dependent on setting, and into the mantle story of the Earth. The Cosgrove Volcano Chain is an example of this, and stands as a globally-unique potential megascale geopark.

Thermo-tectonic development of the Wandel Sea Basin, North Greenland

The Carboniferous–Palaeogene Wandel Sea Basin of eastern North Greenland (north of 80°N, east of 40°W) is an important piece in the puzzle of Arctic geology. It is particularly important for understanding how the Paleocene–Eocene convergence between Greenland, the Canadian Arctic and Svalbard relates to the compressional tectonics in the High Arctic, collectively known as the Eurekan Orogeny. In this study, we present apatite fission-track analysis (AFTA) data and review published vitrinite reflectance data combined with observations from the stratigraphic record to place firmer constraints on the timing of key tectonic events. This research study reveals a long history of episodic burial and exhumation since the collapse of the Palaeozoic fold belts in Greenland. Our results define pre-Cenozoic exhumation episodes in early Permian, Late Triassic, Late Jurassic and mid-Cretaceous times, each involving the removal of kilometre-scale sedimentary covers. Mid-Paleocene exhumation defines the timing of compression along the major fault zones during the first stage of the Eurekan Orogeny, after the onset of sea-floor spreading west of Greenland. Regional exhumation that began at the end of the Eocene led to the removal of most of a kilometre-thick cover that had accumulated during Eocene subsidence and involved a major reverse movement along the Harder Fjord Fault Zone, northern Peary Land. These events took place after the end of sea-floor spreading west of Greenland, and thus, represent post-Eurekan tectonics. Mid–late Miocene exhumation is most likely a consequence of uplift and incision across most of the Wandel Sea Basin study area. The preserved sedimentary sequences of the Wandel Sea Basin represent remnants of thicker strata that likely extended substantially beyond the present-day outline of the basin. We find that the present-day outline of the basin with scattered sedimentary outliers is primarily the result of fault inversion during Eurekan compression followed by deposition and removal of a kilometre-thick overburden.

Age of volcanic-sedimentary complex from Сape Svyatoi Nos (Eastern Arctic)

Geological structure and age of the volcanogenic-sedimentary complex of the Cape Svyatoi Nos (Svyatonosskaya formation) are presented. The rocks of Cape Svyatoi Nos are located on the border of the Novosibirsk-Chukotka and Verkhoyansk–Kolyma fold belts, on the coast of the Laptev and East- Siberian Seas. Field studies indicate that the rocks belong to a single volcanogenic-sedimentary complex. The maximum thickness of individual sections reaches up 700 m. Coarse-grained pyroclastic rocks with rare lava flows prevail on the north (on the coast of Laptev Sea). The proportion and dimension of volcanics and pyroclastic rocks decrease in the south, terrigenous rocks appear. In the modern structure, the rocks are deformed.Zircons of several populations were separated from the flow of basalts. Two, the most representative zircon populations are characterized by close subconcordant ages. The structure and U-Pb ages of zircons from the first population suggest their formation during magmatic crystallization with a superimposed postmagmatic thermal event. Zircons of the second population have a xenomorphic appearance, which is typical of zircons formed at the late or postmagmatic phases. The weighted average age (MSWD = 3) of the first two populations is 149.3 ± 1.2 Ma (Tithonian age). It corresponds to the age of crystallization of basalts and the superimposed (close in time) postmagmatic thermal event.The third population of zircons is represented by two rounded grains with Archean U-Pb ages. It is assumed that these grains were trapped by magmatic melt from pre-Jurassic clastic rocks.Late Jurassic-Early Cretaceous radiolarians were identified from different horizons of tuff-terrigenous and terrigenous rocks. This is confirm the obtained U-Pb ages and the coeval of all the studied sections. The Titonian age of volcanic-sedimentary rocks allows us to classify them as suprasubduction complexes of the Late Jurassic - Early Cretaceous, widespread in the Verkhoyansk-Chukotka Mesozoids.