Tuesday, December 24, 2013

Orosirian/Rhyacian Paleoproterozoic Evidence From India Concerning the Columbia Supercontinent

Paleoproterozoic mafic dyke swarms from the Dharwar craton; paleomagnetic poles for India from 2.37-1.88 Ga and rethinking the Columbia supercontinent


Belica et al


Here we report new paleomagnetic and geochronologic results from the Dharwar craton (south India) from 2.37-1.88 Ga. The presence of a ∼85,000 km2 radiating dyke swarm with a fanning angle of 65° is confirmed within Peninsular India at 1.88 Ga. North of the Cuddapah basin the dykes are oriented NW-SE and progress to an E-W orientation further south, converging at a focal point southeast of the basin. The Grand Mean dual polarity paleomagnetic pole falls at 36.5°N and 333.5°E (D = 129.1°, I = 4.2°, α95 = 4.5°, λ=2.1°) for 29 sites from the present study combined with previously published sites. Our continental reconstruction for India at ∼1.9 Ga conflicts with the archetypal Columbia model, suggesting that the exact configuration needs modification. We also report two separate paleomagnetic directions from NW-SE (D = 3.2°, I = 56.4°, α95 = 17.9°, λ=37°) and N-S (D = 240.1°, I = -65.5°, α95 = 10.9°, λ=47.7°) trending ∼2.2 Ga dykes. We attribute this difference in directions to the separate magmatic pulses at 2.18 and 2.21 Ga identified by French and Heaman (2010). Our results place India at intermediate latitudes from 2.21-2.18 Ga and are supported by a positive baked contact test. New paleomagnetic results from E-W and NW-SE trending 2.37 Ga dykes, combined with previous work in the Dharwar craton, yields a Grand Mean dual polarity paleomagnetic pole at 15.1°N and 62.2°E (A95 = 4.0°), placing India at polar latitudes (D = 88.7°, I = -81.7°, α95 = 4.8°, λ=73.7°). Here we also report a shallow NE direction (D = 52.2°, I = -1.5°, α95 = 6.3°) previously classified as a secondary magnetization from three dykes near the Cuddapah basin. A baked contact test and petrophysical analysis of two cross-cutting dykes supports a primary remanence. Finally we present a Paleoproterozoic Apparent Polar Wander Path (APWP) for the Dharwar craton, and examine paleogeographic relationships between India and other cratonic blocks for the 2.37-1.88 Ga time interval

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