Distribution of pre-folding linear indicators of movement direction around the Spring Hill Synform, Vermont: significance for mechanism of folding in this portion of the Appalachians

Bell, T. H.; Hickey, Kenneth A.

doi:10.1016/s0040-1951(97)00008-5

Cited by 43 publications

(30 citation statements)

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“…Although structural geologists routinely attempt to correlate matrix foliations from outcrop to outcrop, they have realized that this is fraught with difficulty (Hobbs and others, 1976). The integration of FIA data, detailed microstructural studies of porphyroblasts and geometrical studies of largescale folds (Bell and Hickey, 1997;Hickey and Bell, 2001) has demonstrated that foliations cannot be correlated from sample to sample within porphyroblasts but FIAs can. However, each FIA develops over a period of time with some samples recording several foliations accompanying FIA development and others only one.…”

Section: Discussionmentioning

confidence: 99%

“…Structural (Hickey and Bell, 2001) and microstructural work (Bell and Hickey, 1997;Bell and others, 1998;Ham, ms, 2001) using newly developed quantitative methods has revealed a more extensive history of deformation and metamorphism than previously recognized for any rocks in the earth's crust. These methods have resulted in the recognition of successions of multiple porphyroblast growth and foliation development and provided a powerful means for establishing relative timing.…”

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confidence: 99%

“…Outline map of bedding across the Spring Hill synform showing the trends of successive FIA sets on each limb (modified fromBell and Hickey, 1997).…”

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confidence: 99%

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Prolonged Acadian orogenesis: Revelations from foliation intersection axis (FIA) controlled monazite dating of foliations in porphyroblasts and matrix

Bell¹

2002

American Journal of Science

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Numerous phases of garnet growth are revealed by detailed studies of foliation inflection/intersection axes preserved in porphyroblasts (FIAs) in Acadian metamorphic rocks in the Chester Dome region of Vermont, in the United States of America. A regionally consistent succession of four different FIA trends in garnet porphyroblasts has been dated by analyzing monazite inclusions with an electron microprobe. These monazite inclusions, which lie within the various foliations that define the FIAs, provide absolute ages for multiple periods of deformation and episodic phases of garnet growth. The monazite inclusions reveal a progression in foliation ages from 431؎2 to 349؎3 million years within porphyroblasts and from 366؎3 to 327؎5 million years in pervasive matrix foliations. Three samples of schist reveal ages ranging through 75 to 70 million years from the cores of porphyroblasts, through the medians to the rims and then into the matrix. Ages determined from monazite grains within garnet porphyroblasts link directly to periods of multiple deformation and episodic garnet growth defined by the FIAs. The four FIA sets began forming prior to 424؎3, 405؎6, 386؎6 and 366؎4 million years ago, respectively. Thus multiple stages of garnet growth occurred throughout Acadian deformation and metamorphism in Vermont, and orogenesis was far more prolonged than previously thought. Invariably, garnet growth occurred early in the accompanying deformation event.Dating of monazite inclusions without careful separation of phases of garnet growth by FIA studies will lead to a spread of ages that will confuse rather than elucidate the metamorphic and structural history. Analysis of monazite grains in the matrix alone will likely only present ages for the youngest events. These ages can be amalgamated from grain to grain to yield apparently precise ages, but such ages reveal nothing about the deformation and metamorphic processes operating during orogenesis, or the overall continuity of the deformation and metamorphism that accompanies plate motion. FIA and microstructural studies of inclusion trail successions allow numerous phases of garnet growth to be distinguished and then dated using monazite that has dissolved and regrown many times as multiple successive foliations developed. This in turn allows very long and involved metamorphic histories to be fully integrated with similarly complex structural histories.

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Section: Discussionmentioning

confidence: 99%

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confidence: 99%

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Prolonged Acadian orogenesis: Revelations from foliation intersection axis (FIA) controlled monazite dating of foliations in porphyroblasts and matrix

Bell¹

2002

American Journal of Science

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“…SSITs appear so obviously a product of rotation of the porphyroblast within a foliation as progressive shearing occurred that no one considered the possibility that this might not be the case until the 1980s. However, a technique for quantitatively measuring the axes of SSITs was developed (Hayward, 1990;Bell, Forde & Wang, 1995) and a large amount of data has now been published on the orientation of spiral axes around folds (Bell & Hickey, 1997;Hickey & Bell, 2001;Bell & Chen, 2002;Timms, 2004), around oroclines (Bell & Mares, 1999;Yeh & Bell, 2004) and along and across orogens (Bell & Bruce, 2006;Bell, Hickey & Upton, 1998;Bell, Ham & Hickey, 2003, Bell, Ham & Kim, 2004Bell et al 2005;Cihan, 2004;Cihan & Parsons, 2005). He also generated a non-coaxial strain field diagram that mimicked asymmetric crenulation cleavages and straight to slightly sigmoidal inclusion trails that are common in so many porphyroblasts.…”

Section: E Alpine Top-to-the-s Shearing During Main Menderes Metammentioning

confidence: 99%

Extensional v. contractional origin for the southern Menderes shear zone, SW Turkey: tectonic and metamorphic implications

Bozkurt

2006

Geol. Mag.

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The southern Menderes Massif in southwest Turkey consists mainly of orthogneisses and overlying Palaeozoic-Middle Paleocene schists and marbles. The contact between the two distinct rock types is almost everywhere structural, herein named the southern Menderes shear zone: a S-facing, high-angle ductile shear zone that separates metamorphic rocks of differing grade. Although there is a consensus that the shear zone was associated with top-to-the-S-SSW shearing and is of Tertiary age, its origin and nature have been highly debated over the last decade. Some claim the contact is a thrust fault, while others have argued for an extensional shear zone. Integration of field and microstructural data (the identification of different fabrics, associated kinematics and overprinting relationships) with fissiontrack thermochronology and the P-T paths of the rocks above and below the shear zone, supports the conclusion that the southern Menderes shear zone is an extensional shear zone and not a thrust. The data are consistent with a model that the exhumation and cooling of the southern Menderes Massif occurred after a period of extensional deformation. Pervasive top-to-the-N-NNE high-temperature-mediumpressure ductile shear structures (D 2 deformation) overprint an early HP event (D 1 deformation). The subsequent top-to-the-S-SSW greenschist shear band foliation (D 3 deformation) developed mostly around the orthogniess-schist contact and forms the most characteristic features of the massif. The topto-the-N-NNE structures are attributed to the main Alpine constructional deformation that developed during back-thrusting of the Lycian nappes during the latest Palaeogene collision between the Sakarya continent and the Anatolide-Tauride platform across the Neotethyan Ocean. The top-to-the-S-SSW structures are interpreted to be the result of the exhumation of the massif during the activity of the southern Menderes shear zone. The presence of these two distinct fabrics with differing kinematics suggests that the southern Menderes shear zone operated as a top-to-the-N-NNE thrust fault during early Alpine contractional deformation but was later reactivated with an opposite sense of movement (top-to-the-S-SSW) during subsequent Oligocene-Miocene extensional collapse.

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“…9). The trails are continuous with S5 in the matrix, Bell et al, 1997). The principal point of contention has been whether the spiral inclusion trails result from indicating growth subsequent to the development of S5.…”

Section: Timing Of Porphyroblast Nucleation and Growthmentioning

confidence: 99%

Episodic porphyroblast growth in the Fleur de Lys Supergroup, Newfoundland: timing relative to the sequential development of multiple crenulation cleavages

Stallard

1998

Journal Metamorphic Geology

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Inclusion trails in garnet and albite porphyroblasts in the Fleur de Lys Supergroup preserve successive generations of microstructures, some of which correlate with equivalent microstructures in the matrix. Microstructure-porphyroblast relationships provide timing constraints on a succession of seven crenulation cleavages (S1-S7) and five stages of porphyroblast growth. Significant destruction and alteration of early fabrics has occurred during the microstructural development of the rock mass. Garnet porphyroblasts grew episodically through four growth stages (G1-G4) and preserve a succession of five fabrics (S1-S5) as inclusion trails. Garnet growth during each of the four growth phases did not occur on all pre-existing porphyroblasts, resulting in contrasting growth histories between individual garnet porphyroblasts from the same outcrop. Albite porphyroblasts grew during a single stage of growth and have overgrown microstructures continuous with the matrix. The garnet and albite porphyroblast inclusion trails record a succession of crenulation cleavages without any rotation of the porphyroblasts relative to other porphyroblasts in the population.Complex microstructural histories are best resolved by preparing multiple oriented thin sections from a large number of samples of different rock types within the area of study. The succession of matrix foliations must be understood, as it provides the most useful time-frame against which to measure the relative timing of phases of porphyroblast growth. Comparable microstructures must be identified in different porphyroblasts and in the rock matrix.

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Distribution of pre-folding linear indicators of movement direction around the Spring Hill Synform, Vermont: significance for mechanism of folding in this portion of the Appalachians

Cited by 43 publications

References 13 publications

Prolonged Acadian orogenesis: Revelations from foliation intersection axis (FIA) controlled monazite dating of foliations in porphyroblasts and matrix

Prolonged Acadian orogenesis: Revelations from foliation intersection axis (FIA) controlled monazite dating of foliations in porphyroblasts and matrix

Extensional v. contractional origin for the southern Menderes shear zone, SW Turkey: tectonic and metamorphic implications

Episodic porphyroblast growth in the Fleur de Lys Supergroup, Newfoundland: timing relative to the sequential development of multiple crenulation cleavages

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