• V S HEGDE

Articles written in Journal of Earth System Science

• Zircon SHRIMP U–Pb geochronology, geochemical and Nd isotope systematics of Neoarchean granitoids, Gadag Greenstone Belt, Dharwar Craton, southern India: Petrogenesis and tectonic significance

Coupling of the geological processes in the arc-magmatic and back-arc provinces of accretionary orogens in Neoarchean plate tectonic setting is a subject of current research all over the world. The Dharwar Craton of southern India is an example of such an accretionary orogen, with an arc-magmatic province in the east and a back-arc province in the west, referred as the Eastern Dharwar Craton and the Western Dharwar Craton, respectively. The boundary between the two provinces is considered to be marked by a 400-km long shear zone along the eastern margin of the Gadag–Chitradurga–Karighatta greenstone belt which is called as the Chitradurga Boundary Shear Zone. Potassic, metaluminous, I type, calc-alkalic to alkali-calcic, arc-magmatic granitoids are widespread in the EDC. But they are also found to occur along the western margin of the Chitradurga–Gadag greenstone belt. SHRIMP U–Pb zircon ages of the granitoids in the western back-arc province in the Gadag region occurring near Srimant Gudda, Mulgund and Chabbi have been determined as 2565  $\pm$   24 to 2591  $\pm$   64 Ma old. Within errors, the ages of these granitoids are the same as the Lakundi and Turchihal granitoids occurring to the east of the Gadag Greenstone Belt in the arc-magmatic province. Nd isotope systematics of the granitoids suggest that they were formed from magmatic melts that were produced by remelting of 3200–3500 Ma old heterogenous continental crust. Rare inherited zircons support this antiquity of the protoliths. Occurrence of granitoids of similar age and origin, in the western back-arc province and eastern arc-magmatic province in the Gadag area was attributed to thrust duplex structure in the Gadag region. However, elsewhere, along the western margin of the Chitradurga greenstone belt near Harpanhalli, Hosdurga–Nagamangala–Pandavapura sector, or away from it in the Arsikere–Banavara, where repetition by thrusting is not obvious, late potash arc-magmatic type granitoids of similar age as the Gadag arc-magmatic granitoids are observed. The arc-magmatic type granitoids appear to have overstepped the boundary shear into the back-arc province at several places. Arc-magmatic and back-arc boundary may be diffuse rather than sharp, as also suggested by some earlier workers.

$\bf{Highlights}$

$\bullet$ The ages and petrogenesis of the granitoids around GGB of both the sides of EDC (i.e., eastern magmatic-arc province) and WDC (i.e., western back-arc provinces) are very much similar.

$\bullet$ The geochemistry and isotope systematics (i.e., Nd TDM2 ages and εNdT values at 2.5–2.6 Ga) of the granitoids exposed one both the sides are also similar.

$\bullet$ This shows not only a similar antiquity of both the provinces, but also gives evidence for a possible diffusive nature of boundary between the EDC and the WDC around GGB.

• Sediment dispersal pattern along an engineered micro-tidal tropical estuarine beach

To evaluate the annual sediment dispersal pattern, a year-long beach profile monitoring and granulometric analysis were adopted along the Kundapura estuarine beaches of coastal Karnataka. The river mouth is bounded by breakwaters and the coast is armoured by seawalls and groins. Beaches adjacent to the breakwater showed progradation as a result of the shadow zone created due to the sheltering effect from incident waves. Evidence also comes from the replacement of coarser mode sediments by finer. Those away from the shadow zone, however, experienced extensive erosion. They are subjected to along-shore drift and minimal replacement from the same. It was also observed that despite being tropical estuarine beaches, sediments did not show any affinity towards a Cuvial-derived character. These points towards transport of fine sediments further offshore and retention of coarser fraction in the surf zone that is to be carried shoreward. Breakwaters in turn facilitate this by channelising the river discharge farther in the sea. Evidence towards the same is also provided by the well-sorted to moderately well-sorted nature of the sediments. Breakwaters were thus observed to highly influence the sediment dispersion and distribution along an estuarine coast. This study will provide knowledge on the role of coastal structures in sediment dispersion while framing future coastal protection strategies for the conservation of the coast.

$\bf{Highlights}$

$\bullet$ Progradation of beaches adjacent to the breakwaters due to sheltering from direct waves whereas erosion of beaches away from the influence of these structures is observed.

$\bullet$ Extensive engineered coast has resulted in the narrowing of the natural beach width making it reflective with a high-energy environment thus accelerating further erosion. Due to this, the beach has lost its recreational value.

$\bullet$ The along-shore drift directions derived considering beach morphology, beach volume change and beach sedimentology, along with the mode of transport and environmental affinity of sediments provide a comprehensive understanding of sediment dispersal pattern along the coast.

• # Journal of Earth System Science

Volume 131, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019