Geology of the Gobi and Mongol Altai junction enhanced by gravity analysis: a key for understanding of the Mongolian Altaides

The geological map L-47-V at a scale 1:500,000 covers part of Mongolian Altaids with ophiolite fragments in southern Central Asian Orogenic Belt in SW Mongolia. This region has a basin and range topography with Neoproterozoic and Palaeozoic units exposed at NW–SE trending ranges rising along major intracontinental faults and with intermontane basins filled by Mesozoic and Cenozoic sediments in between. The map shows clear N–S tectonic zonation featuring the northerly Precambrian Baidrag microcontinent, the lower Palaeozoic Lake Zone in the centre and the southerly Palaeozoic Gobi-Altai and Trans-Altai zones. Gravity highs are located in the SW part of the map and low to intermediate Bouguer anomalies in the NE part. NW–SE trends of gravity anomalies correlate well with the contact between the Trans-Altai and the Gobi-Altai zones but the important first-order geological boundary between the Lake and Gobi-Altai zones cannot be delineated by the gravity gradients.比例尺为1:500000的地质图L-47-V，覆盖蒙古国西南部中亚造山带（Central Asian Orogenic Belt）南部的部分蒙古阿尔泰造山带（Mongolian Altaids）区域，区内发育蛇绿岩碎块（ophiolite fragments）。该区域呈现盆山地貌格局：沿大陆内部大型断裂带隆起的北西-南东向展布山脉，出露新元古代与古生代地层；山脉之间的山间盆地则充填有中生代与新生代沉积地层。该地质图清晰展现了南北向构造分带：北部为前寒武纪贝德拉格微陆块（Baidrag microcontinent），中部为下古生界湖带（Lake Zone），南部则为古生代戈壁-阿尔泰带（Gobi-Altai）与跨阿尔泰带（Trans-Altai）。布格重力异常（Bouguer anomalies）高值区分布于图幅西南部，东北部则以低-中等强度的布格重力异常为主。布格重力异常的北西-南东向展布特征，与跨阿尔泰带和戈壁-阿尔泰带之间的接触带吻合度较高，但湖带与戈壁-阿尔泰带之间的重要一级地质边界，无法通过重力梯度进行厘定。