Vertical ductile thinning and its contribution to the exhumation of high-pressure rocks: the Cycladic blueschist unit in the Aegean

The contribution of vertical ductile thinning to the exhumation of high-pressure rocks is evaluated by estimating finite strain in 75 exhumed high-pressure rocks of the Cycladic blueschist unit in the Aegean Sea, Greece, and western Turkey. Strain data indicate heterogeneous deformation; principal stretches are 1.24–5.03 for SX, 0.63–2.53 for SY and 0.10–0.81 for SZ, with a tensor average of SX:SY:SZ=1.52:1.28:0.51. A 1D numerical model, which integrates velocity gradients along a vertical flow path with a steady-state orogen, is used to estimate the contribution of ductile thinning of the overburden of the high-pressure rocks to exhumation. Using a strain-rate law that is proportional to depth, averaged results show that ductile flow contributed c. 20% to exhumation. A major implication is that the vertical strain in the exhumed rocks is an overestimate of the contribution that ductile flow makes to the total exhumation. A proportional strain-rate law that scales linearly with depth implies that material points rising towards the surface move quickly out of the more rapidly deforming part of the orogen. Therefore, very large vertical strains >90% in deeply exhumed rocks are needed for vertical ductile thinning to be a major exhumation process.

本研究通过对希腊爱琴海基克拉迪蓝片岩单元（Cycladic blueschist unit）及土耳其西部的75个已折返高压岩石样本开展有限应变（finite strain）估算，评估垂向韧性减薄对高压岩石折返过程的贡献。应变数据表明变形具有非均质性：SX方向主伸长率介于1.24~5.03之间，SY方向为0.63~2.53，SZ方向为0.10~0.81，张量平均（tensor average）比值为SX:SY:SZ=1.52:1.28:0.51。本研究构建了一维数值模型，该模型将沿垂向流动路径的速度梯度与稳态造山带相耦合，用于估算高压岩石上覆层的韧性减薄对岩石折返的贡献。采用与深度成正比的应变速率定律（strain-rate law）进行计算后，平均结果显示韧性流动对岩石折返的贡献约为20%。一项关键推论指出，已折返岩石中的垂向应变值高估了韧性流动对总折返过程的实际贡献。与深度呈线性比例的应变速率定律表明，向地表运移的物质点会快速脱离造山带中变形速率更快的区域。因此，若要使垂向韧性减薄成为主导性的岩石折返过程，深度折返的岩石需要具备超过90%的极大垂向应变。