Paleomagnetism in the San Bernardo foldbelt, Patagonia, and the Cretaceous geomagnetic field on South America

ZAFFARANA C. B.

Fort Lauderdale, Florida, Estados Unidos

Congreso; 2008 American Geophysical Union Joint Assembly; 2008

American Geophysical Union

The apparent polar wander curves for major continents constitute powerful tools for testing paleogeographic reconstructions, evaluation of global tectonics, and analyzing the evolution of deformed belts. On the other hand, the mid-Cretaceous is an interesting interval in the annals of global change, characterized by the occurrence of major geodynamic events such as fast sea-floor spreading, increased mantle plume activity, and an about 37 m.y. interval of uniform geomagnetic polarity. During the mid-Cretaceous occurred the final dismembering of Gondwana leading to consolidation of present-day continents and oceanic basins. This contribution presents new paleomagnetic data from mid-Cretaceous (92 Ma from averaging available 40Ar/39Ar ages) sedimentary rocks in the San Bernardo foldbelt, Patagonia. Paleomagnetic routines allowed observing high unblocking temperature, high-coercivity magnetizations of normal polarity carried by magnetite. A positive tilt-test constraints the age of the remanences to be older than folding (middle Miocene). This, together with the presence of normal and reversed polarity zones in conformably overlying Paleogene units suggest that the observed remanences were acquired during the Cretaceous Normal Superchron, and then close to the time of deposition. Samples from paleosols show scattered magnetic foliations, suggesting that the last important modification to their primary sedimentary fabric is related to bioturbation associated with pedogenesis instead to post-paleosol compaction linked to burial, which in turn argues for negligible post-paleosol inclination flattening. The obtained paleomagnetic pole is indistinguishable from those deriving from 125 to 100 Ma magmatic rocks in the Brazilian platform, indicating 1) integrity of Patagonia and stable South America since the mid-Cretaceous, and 2) that the continent was essentially motionless with respect to the paleomagnetic axis during the 125-95 Ma time span. This behavior resembles the coeval polar standstill previously recognized for North America, Eurasia, and possibly also the Pacific.