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.

通过对希腊爱琴海（Aegean Sea）与土耳其西部的基克拉迪蓝片岩单元（Cycladic blueschist unit）中75块已折返高压岩石（high-pressure rocks）的有限应变（finite strain）进行估算，本研究评估了垂直韧性减薄（vertical ductile thinning）对高压岩石折返（exhumation）的贡献。应变数据显示变形为非均匀变形（heterogeneous deformation）：三个主伸长量（principal stretches）SX分别为1.24~5.03、SY为0.63~2.53、SZ为0.10~0.81，应变张量的平均比值为SX:SY:SZ=1.52:1.28:0.51。本研究采用一维数值模型（1D numerical model），该模型将垂直流动路径（vertical flow path）上的速度梯度（velocity gradients）与稳态造山带（steady-state orogen）相结合，用于估算高压岩石上覆岩层（overburden）的韧性减薄对折返的贡献。采用与深度成正比的应变速率定律（strain-rate law）进行计算，平均结果显示韧性流动对折返的贡献约为20%。一项关键推论指出，折返岩石所测得的垂直应变实际上高估了韧性流动在总折返过程中的实际贡献。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（material points）会快速脱离造山带的强变形区域。因此，若要使垂直韧性减薄成为主导折返过程，则深度折返的岩石需要产生大于90%的极高垂直应变。