Rheology modification in a subduction channel due to eclogite facies metasomatism (Rocky Beach Metamorphic Mélange, Port Macquarie, Australia)

IF 2.9 2区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
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Abstract

The rheological properties of the interface between the down-going and overriding plates in subduction zones provides insight into how plate convergence is accommodated and the controls on seismic and aseismic slip. This interface is known as the subduction channel and exhumed examples provide the only direct information on deformation mechanisms and the impact of metamorphism on rheology. The Rocky Beach Metamorphic Mélange in eastern Australia is one such exhumed subduction channel, composed of eclogite, blueschist and greenschist facies blocks within a mélange matrix. Previous phase equilibria modelling indicates that high pressure blocks were subducted to ca. 100 km depth and then retrogressed during return flow and exhumation. We found that the rheology of blocks is modified by metasomatism, consistent with studies on other subduction channels. However, through comparison of blocks from different metamorphic grades we found that the effect of metasomatism on rheology varied depending on the pressure and temperature conditions of metasomatism. While unmetasomatised eclogites behaved as rigid objects in the mélange matrix, rocks with mineral assemblages that equilibrated during eclogite facies metasomatism accumulated significant strain, forming isoclinal folds and refolded folds. Deformation of these blocks began at eclogite facies and continued during return flow and retrogression to blueschist facies. At blueschist facies, metasomatised blocks developed mm-scale isoclinal folds with shearing parallel to fold limbs forming rootless isoclinal folds. At the transition between blueschist and greenschist facies, pressure solution became important, preferentially focusing along layers of lawsonite, dissolving it from the rock. At greenschist facies, dissolution-precipitation processes caused significant mass loss, producing mm-spacing between pressure solution seams and cuspate folds, analogous to dewatering structures in sediments. In the Rocky Beach Metamorphic Mélange eclogite facies metasomatism reduces the competence of rigid blocks, reducing overall subduction channel heterogeneity during return flow. We suggest that subduction channels that experience widespread eclogite facies metasomatism may be less likely to generate seismic slip during return flow, since the proportion of rigid blocks and block strength are both reduced.

斜长岩变质作用导致的俯冲通道流变学改变(澳大利亚麦夸里港岩石滩变质混合区)
俯冲带中下行板块和上行板块之间界面的流变特性,使人们能够深入了解板块聚合是如何进行的,以及对地震和非地震滑动的控制。这一界面被称为俯冲通道,挖掘出的实例提供了有关变形机制和变质作用对流变学影响的唯一直接信息。澳大利亚东部的岩石滩变质混合岩就是这样一个出露的俯冲通道,由混合岩基质中的埃克洛辉石、蓝辉石和绿辉石面块组成。之前的相平衡建模表明,高压块体被俯冲到约 100 千米的深度,然后在回流和吐出过程中发生逆冲。我们发现,块体的流变性受到变质作用的影响,这与对其他俯冲通道的研究结果一致。然而,通过比较不同变质等级的岩块,我们发现变质作用对流变学的影响因变质作用的压力和温度条件而异。在混杂基质中,未发生变质作用的斜长岩表现为刚性物体,而在斜长岩面变质作用期间,矿物组合平衡的岩石则积累了大量应变,形成了等轴褶皱和重褶褶皱。这些岩块的变形始于斜长岩面,并在回流和倒退至蓝斜岩面的过程中持续进行。在蓝斜岩面,变质块体形成了毫米级的等轴褶皱,与褶皱边缘平行的剪切力形成了无根等轴褶皱。在蓝斜岩面与绿斜岩面之间的过渡地带,压力溶蚀变得非常重要,主要集中在岩层沿线,将其从岩石中溶解出来。在绿岩层,溶解-沉淀过程造成了大量的质量损失,在压力溶解缝和尖状褶皱之间产生了毫米间距,类似于沉积物中的脱水结构。在岩石滩变质蜕变层蜕变岩面,变质作用降低了刚性块体的能力,从而减少了回流过程中俯冲通道的整体异质性。我们认为,由于刚性块体的比例和块体强度都会降低,因此经历过广泛的夕闪岩变质作用的俯冲通道在回流过程中产生地震滑移的可能性较小。
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来源期刊
Lithos
Lithos 地学-地球化学与地球物理
CiteScore
6.80
自引率
11.40%
发文量
286
审稿时长
3.5 months
期刊介绍: Lithos publishes original research papers on the petrology, geochemistry and petrogenesis of igneous and metamorphic rocks. Papers on mineralogy/mineral physics related to petrology and petrogenetic problems are also welcomed.
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