Magnetic anomaly modeling at the Indo Eurasian collision zone

Abstract

One of the consistent features of satellite crustal magnetic anomaly maps is the large negative anomaly in the Indo Eurasian collision zone. This region is modeled by the introduction of magnetic blocks representing the Tibetan, Asian and Himalayan mountain regions. The crustal thickness of these blocks is based on the principle of isostasy. It has been shown that the removal of the core field from the Magsat data removes most of the long-wavelength anomalies associated with the ocean-continent susceptibility contrast, and that some of the observed anomalies, after processing of the satellite data, are a manifestation of this contrast. To account for this, the final model is a superposition of the above block model on a global model of typical continent and ocean susceptibility, called a Standard Earth Magnetization Model (SEMM). A combination of the three blocks superposed on the SEMM reproduces the major features of the observed Magsat anomalies. Block model susceptibilities of the Asian and Himalayan regions are the same as for the continental portion of the SEMM, albeit with a greater crustal thickness. Thus, the crust in these regions is typical. By contrast, the satellite magnetic data require an integrated susceptibility in the Tibetan region which is considerably lower than typical. Two possible explanations are suggested. One, that the origin and tectonic history of the Tibetan region differs from the neighboring regions in such a way as to result in lower susceptibility. This is consistent with current thinking that the Tibetan plateau is made up of several fragments or micro-continents which were attached to Asia prior to the collision of India with Asia. The other possible explanation is that the Curie isotherm in the Tibetan region is elevated compared to the Asian and Himalayan regions.