FIRST PALEOMAGNETIC CONSTRAINTS ON THE LATITUDINAL DISPLACEMENT OF THE WEST BURMA BLOCK

During the collision of India with Asia, southeastern Asia records a complex pattern of deformation with block extrusion and clockwise rotation of part of Indochina. In contrast, the West Burma block presents a very different geodynamic evolution. The very oblique convergence of India with the western margin of Indochina resulted in a strong partitioning of the deformation. Most recent tectonic models propose that the West Burma block moved ~1100 km northward along the dextral Sagaing fault during the Neogene.

Here we report new paleomagnetic data to further quantify the latitudinal displacement of West Burma. Twenty nine sites were drilled in intrusives dioritic rocks and andesites of the Wuntho arc west of the town of Kawlin and Banmauk. A few sites are from sediments. Tilt corrections are not available for the intrusive sites but the sampling sites are distributed over a large area (1000 km2) and a consistent tilting of the arc is unexpected. Paleomagnetic results have been obtained at 17 sites. The mean direction in in situ coordinate is [Dec=69.4°; Inc=-2.9°; α95=17.3°]. After tilt correction, when available, the scatter decreased slightly [Dec:73.0; Inc=3.6°; α95=13.1°]. All sites record the same polarity of magnetization. Our new U-Pb ages, clustering in the range 100-90 Ma, indicate that the Wuntho arc was mainly built during the mid Cretaceous. The magnetization was thus acquired during the Cretaceous normal superchron and the declination demonstrates a large clockwise rotation. The inclination indicates that the Wuntho arc formed near the equator.

Paleomagnetic results were also obtained from a monoclinic section of late Eocene marine shales and sandstones west of the Kalewa town. This section contains also numerous ~10cm-thick iron-rich carbonates that provided stable paleomagnetic results. Normal and reverse polarity magnetizations indicate a primary detrital or a very early diagenetic origin for the acquisition of the magnetization. The mean direction after tilt correction is [Dec=9.6°; Inc=7.9°; α95=10.4°]. The low inclination observed in the Upper Eocene sediments is coherent with an Equatorial position of the West Burma block during the Cretaceous. We will discuss the implication of these results regarding a possible connection of the West Burma block with the Woyla arc and Sumatra.