摘要
超慢速扩张的北冰洋Gakkel洋中脊具有六个沿扩张方向的线性基底隆起(本文编号为A—F).这些线性基底隆起在中轴两侧的地球物理场和地壳结构呈现不同程度的非对称性.本文利用Gakkel洋中脊的地形、空间重力异常(FAA)和航空磁力数据,计算了它的扩张速率、剩余地幔布格重力异常(RMBA)、地壳厚度和非均衡地形.根据中轴两侧地形和地壳厚度的对称关系,我们将六个基底隆起分为对称型和非对称型两种类型.整体上,B、D和F区基底隆起在中轴两侧的地形和地壳厚度的非对称幅值(两侧差值的绝对值)较小,其中地形的非对称幅值分别为～157m、～125m、～208m,地壳厚度的非对称幅值分别为～1km、～0.06km、～0.3km;而A、C和E区的非对称幅值较大,其中地形的非对称幅值分别为～510m、～410m、～673m,地壳厚度的非对称幅值分别为～2km、～2.5km、～1.1km.我们因此推断B、D和F区具有相对对称的地壳结构,而A、C和E区具有非对称的地壳结构.根据A、C和E区中轴两侧非均衡地形的对称关系和非对称地形的补偿状态,推测A区的非对称性可能是由岩浆分配不均所导致;而C区和E区的非对称性可能是由构造断层作用使断层下盘向上抬升变薄所导致.我们进一步推测洋中脊走向的改变可能使得构造作用更易集中于基底隆起的一侧.
The ultraslow-spreading Gakkel Ridge has six linear basement ridges(A-F)along the spreading direction.They show different degrees of asymmetric geophysical signatures andcrustal structures.The spreading rates,residual mantle bouguer anomaly,crustal thickness,and non-isostatic topography of Gakkel Ridge were calculated based on topographic data,free-air gravity anomaly data,and aeromagnetic data.Based on the comparison of topography and crustal thickness on conjugated flanks,we divide basement ridges into symmetric and asymmetric types.In general,the asymmetric amplitude(the absolute value of the difference between conjugated flanks)of topography and crustal thickness of basement ridges in B zone,D zone,and F zone are relatively small,and the asymmetric amplitude of topography is～157 m,～125 m,～208 m,the asymmetric amplitude of crustal thickness is ～1 km,～0.06 km,～0.3 km.However,the asymmetric amplitude of topography and crustal thickness of basement ridges in A zone,C zone and E zone are relatively large,and the asymmetric amplitude of topography is～510 m,～410 m,～673 m,the asymmetric amplitude of crustal thickness is～2 km,～2.5 km,～1.1 km.We therefore suggest that B,D,and F have a relatively symmetric crustal structures,while A,C and E have an asymmetric crustal structures.According to the asymmetry of non-isostatic topography on conjugated flanks and compensation state of asymmetric topography,the asymmetry of A zone may due to uneven distribution of magma on conjugated flanks.The asymmetry of C zone and E zone may be caused by concentrated tectonism on one flank of basement ridge.We speculate that changes in the strike of ridge may facilitated the concentration of tectonism on one flank of the Gakkel Ridge.