10:45 〜 12:15
[SMP26-P05] Constraints on exhumation models of regional high-pressure rocks in the Del Puerto Canyon region, Franciscan Belt, USA
キーワード：上昇テクトニクス、フランシスカン帯、変形解析
The Franciscan Belt of the western USA represents an accretionary complex that underwent regional LT-HP subduction-type metamorphism. The metamorphic domains are commonly divided into the Coastal, Central, and Eastern belts from west to east. The eastern boundary of the Eastern belt is marked by the east-dipping Coast Range Fault (CRF) which marks the boundary with very low-grade rocks of the Great Valley Group (GVG), a forearc sedimentary unit, and the Coast Range Ophiolite, a forearc ophiolite. LT-HP metamorphism in the lawsonite-albite or blueschist facies is widely developed in the Eastern belt. Peak metamorphism reaches eclogite facies.
The tectonics of exhumation of regional Franciscan HP metamorphic rocks has been studied extensively since the 1970s. There are currently two contrasting models for the exhumation. The first is an extension model, where exhumation is largely explained by tectonic extension driven by mechanical instability due to underplating at the rear of the wedge, and the action of gravity on an overthickened accretionary wedge. In this model, extension is expressed as ductile flow at depth and normal faulting at shallower levels. Ductile deformation structures are widely developed throughout the higher-grade parts of the Franciscan terrane, mainly due to solution mass transfer (SMT), and the CRF is interpreted by most workers to be top-to-E normal fault. In contrast, an erosion model has also been proposed in which surface erosion dominates exhumation of HP rocks. The main evidence for this model comes from estimates of the absolute strain related to SMT and the resulting conclusion that wedge extension is very small. The main method used in this study was the mode-PDS method based on microstructural observations of deformed quartz grains in metasedimentary rocks [1]. The data show negligible net elongation due to SMT deformation and a large regional volume decrease (> 20 vol.%), suggesting that underplating did not induce wedge extension. In this model, the CRF is interpreted as a top-to-W thrust that does not contribute to exhumation. This model has been noted to have difficulties mainly from a sedimentological perspective, but has not been explicitly refuted against the strain data that are more fundamental to tectonics.
In this study, we re-examined the absolute strain and shear sense due to SMT using a new approach to strain analysis based on the analysis of sets of deformed veins [2]. The results suggest that the length after deformation in the direction of maximum elongation is about twice that before deformation and that this was accompanied by a volume increase of 7ｰ21 vol.%. In addition, the shear sense was top-to-west. This result is in agreement with the extension model. The discrepancy between the results of this method and the mode-PDS method can be explained by considering grain boundary sliding and rotation. The mode-PDS method assumes that the orientation of the long axis of mineral grains before deformation is random and that the grains deform by SMT without rotation. However, this assumption is likely false, since the long axis direction of the grains shows a significant preferred orientation in Franciscan HP rocks which were nearly undeformed by SMT, and this original alignment may have caused the underestimation of the absolute strain in this method. In addition, new field observations confirm the shear sense indicators that clearly indicate top-to-E in the CRF. Therefore, in this study, as in many previous studies, we consider the CRF as a top-to-E normal fault.
We conclude the exhumation of regional HP rocks in the Franciscan Belt can be adequately explained by the extension model. The top-to-W shear within the Franciscan high-P rocks and the top-to-E shear in the CRF could be explained by a corner-flow type exhumation path.
[1] Ring & Brandon (1999) GSL, Special Publications, 154(1), 55–86.
[2] Soejima & Wallis (2022), JGR: Solid Earth, 127(6), e2022JB024197.
The tectonics of exhumation of regional Franciscan HP metamorphic rocks has been studied extensively since the 1970s. There are currently two contrasting models for the exhumation. The first is an extension model, where exhumation is largely explained by tectonic extension driven by mechanical instability due to underplating at the rear of the wedge, and the action of gravity on an overthickened accretionary wedge. In this model, extension is expressed as ductile flow at depth and normal faulting at shallower levels. Ductile deformation structures are widely developed throughout the higher-grade parts of the Franciscan terrane, mainly due to solution mass transfer (SMT), and the CRF is interpreted by most workers to be top-to-E normal fault. In contrast, an erosion model has also been proposed in which surface erosion dominates exhumation of HP rocks. The main evidence for this model comes from estimates of the absolute strain related to SMT and the resulting conclusion that wedge extension is very small. The main method used in this study was the mode-PDS method based on microstructural observations of deformed quartz grains in metasedimentary rocks [1]. The data show negligible net elongation due to SMT deformation and a large regional volume decrease (> 20 vol.%), suggesting that underplating did not induce wedge extension. In this model, the CRF is interpreted as a top-to-W thrust that does not contribute to exhumation. This model has been noted to have difficulties mainly from a sedimentological perspective, but has not been explicitly refuted against the strain data that are more fundamental to tectonics.
In this study, we re-examined the absolute strain and shear sense due to SMT using a new approach to strain analysis based on the analysis of sets of deformed veins [2]. The results suggest that the length after deformation in the direction of maximum elongation is about twice that before deformation and that this was accompanied by a volume increase of 7ｰ21 vol.%. In addition, the shear sense was top-to-west. This result is in agreement with the extension model. The discrepancy between the results of this method and the mode-PDS method can be explained by considering grain boundary sliding and rotation. The mode-PDS method assumes that the orientation of the long axis of mineral grains before deformation is random and that the grains deform by SMT without rotation. However, this assumption is likely false, since the long axis direction of the grains shows a significant preferred orientation in Franciscan HP rocks which were nearly undeformed by SMT, and this original alignment may have caused the underestimation of the absolute strain in this method. In addition, new field observations confirm the shear sense indicators that clearly indicate top-to-E in the CRF. Therefore, in this study, as in many previous studies, we consider the CRF as a top-to-E normal fault.
We conclude the exhumation of regional HP rocks in the Franciscan Belt can be adequately explained by the extension model. The top-to-W shear within the Franciscan high-P rocks and the top-to-E shear in the CRF could be explained by a corner-flow type exhumation path.
[1] Ring & Brandon (1999) GSL, Special Publications, 154(1), 55–86.
[2] Soejima & Wallis (2022), JGR: Solid Earth, 127(6), e2022JB024197.