Analysis of the geometry and kinematics of structural fabrics in the pre-Tertiary metamorphic basement of east-central Taiwan provides evidence for transpression and lateral extrusion during oblique collision. Four structural domains are recognized on the basis of systematic variations in the dip direction of S1 and the trend of L1. Domains I, IIa, and IIb contain a northeast-striking S1 and a northeast-or southwestplunging L1, raking 30Њ-45Њ on S1. In contrast, domain III, at the western edge of the pre-Tertiary metamorphic basement, has an approximately downdip L1, similar to the Slate Belt. Microscale kinematic indicators from domains I, IIa, and IIb yield a consistent, left-lateral, strike-slip component of oblique motion. Analyses of microstructures in the Chipan granitic gneiss in domain I and the geometry of mesoscale folds in phyllitic rocks in domain IIa provide qualitative evidence for constrictional strain. Evaluation of potential primary configurations of structural domains, combined with geologic and tectonic information, suggests that the pre-Tertiary metamorphic basement and the Slate Belt formed as a southeast-dipping package; post-S1 block rotation of domain IIa is interpreted to have produced the present arrangement of domains. The pre-Tertiary metamorphic basement was deformed within a zone of left-lateral transpression. The shallow plunge of the L1 lineation and the evidence for constrictional strain in domains I, IIa, and IIb suggest that the orogenperpendicular component of oblique convergence was accommodated by lateral extrusion. Significant vertical thickening may have been prevented by the presence of a shallowly east-dipping rigid arc lid overlying the pre-Tertiary metamorphic basement.
Rock fabrics that result from displacement in extensional orogens provide a means of identifying geometric models responsible for extension of continental crust. Strain compatibility arguments indicate that a finite extension can be accommodated by displacements across (1) planar, nonrotating faults or ductile shear zones, (2) shear zones which rotate above a horizontal detachment (the domino model), or (3) shear zones which rotate as a result of a horizontally oriented, pure shear stretching component (the plastic model). Listric, normal shear zone geometries may develop as the result of a depth dependent change in the pure shear component (in the plastic model) or the area loss component (in the planar shear zone model). Within the geometric framework of these various models, the effects of superposed simple shear, pure shear extension, and area change on the rock fabric are investigated. These displacement components, which may be superposed sequentially or simultaneously, determine the state of finite strain associated with a given magnitude of tectonic extension. The relationships between displacement and strain are expressed as graphs of foliation dip (or strain ratio) versus shear strain and as graphs of foliation dip (or strain ratio) versus tectonic extension. The shear strain graphs illustrate the effects of the different displacement components on the rock fabric in a single shear zone, whereas the tectonic extension graphs better illustrate these effects on the regional scale. The shear strain and tectonic extension graphs can be used by field geologists (1) to determine tectonic extension across a region from measurements of strain ratio or orientation and fluctuation of foliation, (2) to distinguish between possible extension models from field data, and (3) to evaluate the influence of shear zone attitude and thickness upon the amount of displacement. Applications of these methods are illustrated for field areas in metamorphic core complexes of Arizona and in the Basin and Range province of the North America Cordillera.
Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high‐spatial‐resolution 40Ar/39Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2, S3, and S4) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40Ar/39Ar geochronology. Mean 40Ar/39Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.
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