Structures, flow stresses, and estimated strain rates in metamorphic rocks of the Small Cyclades Islands Iraklia and Schinoussa (Aegean Sea, Greece)

J. Behrmann, C. Seckel
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引用次数: 16

Abstract

Like the larger Cyclades Islands (e.g. Naxos, Paros, Ios) the Small Cyclades Islands form part of the Attic-Cycladic crystalline basement belt. Subduction in the Paleogene was associated with high-pressure metamorphism, followed by a Barroviantype overprint, local partial melting and granitic plutonism in the Neogene. On the islands of Iraklia and Schinoussa Neogene metamorphism did not exceed mid-greenschist grade, and there are some well preserved older high-pressure/low temperature metamorphic assemblages. The rocks and structural geology of both islands had previously not been analysed in detail. Rocks present are calcite and dolomite marbles, in part with preserved paleosoils (metabauxites and metalaterites), and a suite of metapelitic schists with associated glaucophane schists and piemontite quartzites. There is a ductile deformation history comprising four events. The two older ones (D1, D2) are associated with high strains, and have resulted in a subhorizontal foliation, a N-S trending stretching lineation, and recumbent isoclinal folds with variable axial orientations. At least D1 is bracketed by the growth of glaucophane, with uncertain kinematics. D2, syn-blueschist grade on Iraklia, however, has a clear top-to-north sense of shear. D3 and D4 caused upright folds with N-S and E-W trending fold axes, respectively. This is evidence for late, two-phase horizontal shortening. As the deformed rocks show only a slight static metamorphic overprint, we have estimated flow stresses from dynamically recrystallized grain sizes of quartz and calcite. Coincident flow stress estimates are 28-62 MPa in quartzites, and 31-56 MPa in the marbles. Together with syntectonic temperature extimates, and applying published flow laws, this leads to estimated deformation rates on the order of 10−12 to 10−14 sec−1. Even though deformation history and kinematics is very similar to that on Naxos Island, the clear relation of the earlier part to high-pressure metamorphism on Iraklia and Schinoussa speaks in favour of a subduction-related setting here. This is in contrast to the later formation of metamorphic core complexes in the north (Naxos) and south (Ios).
小基克拉迪群岛、伊拉克和希努萨(希腊爱琴海)变质岩的构造、流动应力和估计应变率
像较大的基克拉迪群岛(如纳克索斯岛、帕罗斯岛、伊俄斯岛)一样,小基克拉迪群岛构成了大西洋-基克拉迪水晶基底带的一部分。古近纪俯冲与高压变质作用有关,随后是巴罗维亚型套印作用,新近纪局部熔融作用和花岗质深部成矿作用。在Iraklia岛和Schinoussa岛,新近系变质作用不超过中绿片岩级,有保存较好的古高压/低温变质组合。这两个岛屿的岩石和构造地质以前没有被详细分析过。现存的岩石为方解石和白云石大理岩,部分保存有古土壤(变质长岩和金属岩),一套变质长岩片岩,伴生有蓝闪岩片岩和片铁矿石英岩。延性变形史包括四个事件。两个较老的(D1, D2)与高应变有关,形成了亚水平叶理,N-S向拉伸线理和变轴向的平卧等斜褶皱。至少D1被青绿色生长所包围,具有不确定的运动学。然而,伊拉克的同蓝片岩D2级具有明显的从上至北的剪切感。D3和D4分别形成N-S和E-W走向褶皱轴的直立褶皱。这是后期两阶段水平缩短的证据。由于变形岩石仅表现出轻微的静态变质叠印,我们通过石英和方解石的动态再结晶粒度估计了流动应力。石英岩的一致流变应力估计值为28 ~ 62 MPa,大理岩为31 ~ 56 MPa。结合同构造温度的估计,并应用已发表的流动规律,这导致估计的变形速率在10−12到10−14秒−1之间。尽管变形历史和运动学与纳克索斯岛非常相似,但较早的部分与伊拉克和希努萨岛的高压变质作用的明显关系表明,这里的背景与俯冲有关。这与后来在北部(纳克索斯)和南部(伊俄斯)形成的变质核杂岩形成对比。
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