Late Jurassic-Early Cretaceous closure of the Mongol-Okhotsk Ocean demonstrated by new Mesozoic palaeomagnetic results from the Trans-Baikal area (SE Siberia)

We present new palaeomagnetic results from the Transbaikal area (SE Siberia), from the Mongol-Okhotsk suture zone, the boundary between the Amuria and Siberia blocks. In order to better constrain the time of closure of the Mongol-Okhotsk Ocean in the Mesozoic, we collected 532 rock samples at 68 sites in six localities of basalts, trachy-basalts and andesites, from both sides of the Mongol-Okhotsk suture: at Unda river (J(3); 51.7 degrees N, 117.4 degrees E), Kremljevka peak (K-1; 51.8 degrees N, 117.5 degrees E) and Torey lakes (K-1; 50.1 degrees N, 115.9 degrees E) on the southern side of the suture, and at Monostoy river (J(1); 51.1 degrees N, 106.8 degrees E), Ingoda river (K-1; 51.2 degrees N, 112.2 degrees E) and Bichura town (K-1; 50.6 degrees N, 107.6 degrees E) on the northern side. Progressive thermal demagnetization enabled us to resolve low (LTC) and high (HTC) temperature components of magnetization at most sites. Jurassic palaeopoles computed from the HTCs show a large discrepancy with respect to the Apparent Polar Wander Path of Eurasia, which we interpret in terms of 1700-2700 km of post-Late Jurassic northward movement of Amuria with respect to Siberia. Although geological data suggest a middle Jurassic closure of the Mongol-Okhotsk Ocean in the west Trans-Baikal region, our data give evidence of a large remaining palaeolatitude difference between the Amuria and Siberia blocks. In contrast, Early Cretaceous sites cluster remarkably well along a small-circle, which is centred on the average site location. This implies the absence of post-Early Cretaceous northward motion of Amuria relative to Siberia, and demonstrates the pre-Early Cretaceous closure of the Mongol-Okhotsk Ocean. Finally, we interpret the very large tectonic rotations about local vertical axes, evidenced by the small-circle distribution of poles, as arising both from collision processes and from left-lateral shear movement along the suture zone, due to the eastward extrusion of Amuria under the effect of the collision of India into Asia.