09:00 〜 11:00
[SIT20-P06] Characteristics of Late-stage Deformation and Exhumation of a High-Pressure Metamorphic Belt in Taiwan
キーワード：exhumation, high pressure
Geologic processes that exhume metamorphic rocks to Earth’s surface include erosion, normal faulting, and ductile deformation processes that thin the Earth’s crust. Erosion is relatively well understood and documented in many orogenic systems. Observations over the last few decades from a number of orogens have highlighted, however, the importance of ductile thinning and normal faulting, and raise a number of key questions about the relative contribution of tectonic versus erosional processes in orogenesis.
The orogenic belt in Taiwan is well suited to better understand tectonic exhumation because it is accessible, features normal faults and a low-dipping ductile fabric, and it is one of the few active orogens in the world where rocks that once experienced high-pressure metamorphism (>50 km) are exposed at the Earth’s surface. The nature of ongoing and rapid exhumation of the metamorphic core of Taiwan, by either erosion or structural processes, is, therefore, fertile ground for inquiry (e.g., Lee et al., 2015; Malavieille and Trullenque, 2009; Mirakian et al., 2013; Mondro et al., 2017).
Previous studies of the structural geology of the Taiwan metamorphic core, including the Miocene high-pressure Yuli Belt and the adjacent Mesozoic, greenschist facies Tailuko Belt, suggest three episodes of deformation, generalized as D1, D2, and D3 (Lin 1984, Faure 1991; Clark, 1995; Ho and Lo, 2015). Each event included the development of an associated penetrative foliation, respectively, S1, S2, and S3, and a variety of outcrop-scale folds.
Here, we focus on D3 and present new structural and paleotemperature data interpreted to occur across the brittle/ductile transition. D3 is characterized by a low-angle, northward-dipping, penetrative cleavage, S3, and associated, late-stage, mode-I fractures, and records vertical shortening and northeast-southwest extension across the brittle/ductile transition. Fluid inclusion microthermometry on quartz and adularia (K-spar) crystals from the mode-I fractures suggest maximum temperatures during vein precipitation between 200°C to 450°C. This would imply a brittle/ductile transition to at least ~8 km. The age of D3 was constrained to ~1.1-1.6 Ma by dating associated brittle deformation on pseudotachylite-bearing fault surfaces (Chen et al., 2018) and adularia phases within associated mode-I fractures (Wang, 1997, see Chen et al., 2018).
We integrate the new structural and paleotemperature data with previously published thermochronological data (Hsu et al., 2016; Liu 2001, 1982; Shen et al., 2020), and a preliminary analysis of unusual surfaces of low relief perched at high elevations (Ouimet et al., 2018; Lin, 1957) to argue that tectonic exhumation played a critical role in exhuming the metamorphic core of the orogen until very recently, probably <800 ka. We propose that the combination of tectonic thinning and erosional denudation limited the growth of the orogen, generating a low-relief surface or peneplain that covered much of the metamorphic core exposed in the modern eastern Backbone Range. This surface is currently perched along many of the ridge crests and mountain flanks in Taiwan, reflecting a recent acceleration in the rate of uplift, and resulting in one of the few places in the world where extreme rates of erosion are juxtaposed against high elevation, low relief surfaces in the same orogen (Ouimet et al., 2018).
The orogenic belt in Taiwan is well suited to better understand tectonic exhumation because it is accessible, features normal faults and a low-dipping ductile fabric, and it is one of the few active orogens in the world where rocks that once experienced high-pressure metamorphism (>50 km) are exposed at the Earth’s surface. The nature of ongoing and rapid exhumation of the metamorphic core of Taiwan, by either erosion or structural processes, is, therefore, fertile ground for inquiry (e.g., Lee et al., 2015; Malavieille and Trullenque, 2009; Mirakian et al., 2013; Mondro et al., 2017).
Previous studies of the structural geology of the Taiwan metamorphic core, including the Miocene high-pressure Yuli Belt and the adjacent Mesozoic, greenschist facies Tailuko Belt, suggest three episodes of deformation, generalized as D1, D2, and D3 (Lin 1984, Faure 1991; Clark, 1995; Ho and Lo, 2015). Each event included the development of an associated penetrative foliation, respectively, S1, S2, and S3, and a variety of outcrop-scale folds.
Here, we focus on D3 and present new structural and paleotemperature data interpreted to occur across the brittle/ductile transition. D3 is characterized by a low-angle, northward-dipping, penetrative cleavage, S3, and associated, late-stage, mode-I fractures, and records vertical shortening and northeast-southwest extension across the brittle/ductile transition. Fluid inclusion microthermometry on quartz and adularia (K-spar) crystals from the mode-I fractures suggest maximum temperatures during vein precipitation between 200°C to 450°C. This would imply a brittle/ductile transition to at least ~8 km. The age of D3 was constrained to ~1.1-1.6 Ma by dating associated brittle deformation on pseudotachylite-bearing fault surfaces (Chen et al., 2018) and adularia phases within associated mode-I fractures (Wang, 1997, see Chen et al., 2018).
We integrate the new structural and paleotemperature data with previously published thermochronological data (Hsu et al., 2016; Liu 2001, 1982; Shen et al., 2020), and a preliminary analysis of unusual surfaces of low relief perched at high elevations (Ouimet et al., 2018; Lin, 1957) to argue that tectonic exhumation played a critical role in exhuming the metamorphic core of the orogen until very recently, probably <800 ka. We propose that the combination of tectonic thinning and erosional denudation limited the growth of the orogen, generating a low-relief surface or peneplain that covered much of the metamorphic core exposed in the modern eastern Backbone Range. This surface is currently perched along many of the ridge crests and mountain flanks in Taiwan, reflecting a recent acceleration in the rate of uplift, and resulting in one of the few places in the world where extreme rates of erosion are juxtaposed against high elevation, low relief surfaces in the same orogen (Ouimet et al., 2018).