Microstructural insights into the tectonic history of the southeastern New England Appalachians; porphyroblast-matrix structural analysis and insitu geochronology of rocks from the Merrimack Terrane, Connecticut and the Narragansett Basin, Rhode Island

Rich, Benjamin Heath (2005) Microstructural insights into the tectonic history of the southeastern New England Appalachians; porphyroblast-matrix structural analysis and insitu geochronology of rocks from the Merrimack Terrane, Connecticut and the Narragansett Basin, Rhode Island. PhD thesis, James Cook University.

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Abstract

Chapter 1 Pennsylvanian age sediments within the Narragansett Basin allow the effects of Alleghanian deformation to be distinguished from the Taconic and Acadian orogenies that affected the rocks to the west. Three dimensional microstructural analyses of both matrix foliations and structures preserved in porphyroblasts define two extended periods of deformation and metamorphism about two differently trending foliation intersection/ inflection axes in porphyroblasts (FIAs). SSW-NNE trending FIAs (Set 1) formed first followed by WSW-ENE trending FIAs (Set 2). The SSW-NNE FIA trends from the south-central zone lie parallel to regional folds in the entire southern graben. The changes in FIA trends reveal that the direction of maximum bulk shortening changed from WNW-ESE to NNW-SSE during amphibolite facies metamorphism within the Central Zone of the basin. They reveal a more varied history than the simple kinematic framework of solely W-E directed bulk shortening that was previously suggested.

Chapter 2 Foliation intersection/inflection axes preserved in porphyroblasts (FIAs), from the Rhode Island Formation in the central zone of the Narragansett Basin, have two distinct trends oriented NNE and ENE. Both trends occur in some samples and reveal a consistent succession from NNE to ENE. Each FIA trend is defined by asymmetric curved inclusion trails that in every case have an anticlockwise asymmetry looking NE. The two distinct trends and their consistent relative timing cannot be explained if the porphyroblasts had rotated. Therefore, a deformation history of progressive bulk inhomogeneous shortening with a consistent non-coaxial component of shear of top to the WNW followed by top to the NNW is required. This correlates with the macroscopic regional relationships suggested by the folds which verge WNW and provides a convincing demonstration that shear on cleavage corresponds with the differentiation asymmetry (curvature of a crenulated cleavage into differentiated crenulation cleavage).

Chapter 3 High grade Pennsylvanian metamorphism appears pervasive throughout the Scotland Schist of the Merrimack Terrane in SE New England. This contrasts with previous work that suggested post-Acadian metamorphism within the cover sequence was limited to narrow fault controlled zones. A three dimensional microstructural analysis of both matrix foliations and structures preserved in porphyroblasts defined four periods of deformation and metamorphism about differently trending foliation/ inflection axes in porphyroblasts (FIAs). FIA sets 1 to 4 are oriented WNW-ESE, SW-NE, NNW-SSE and WNW-ESE, respectively. The SSW-NNE, NW-SE, WSW-ENE and SSW-NNE directions of bulk shortening that accompanied the development of this succession of FIA sets produced dominantly non-coaxial deformation in the rocks. NNE directed bulk orogenic flow accompanied high grade metamorphism during the earliest determined period of porphyroblast growth whereas WSW to NNE directed bulk orogenic flow accompanied high grade Pennsylvanian (310 Ma) metamorphism. Consistent NNE to WSW directed bulk orogenic flow in eastern Connecticut and southwest Rhode Island is not compatible with current models for loading of the Bronson Hill -Central Maine -Merrimack terranes by stacking thrust nappes towards the S and E.

Chapter 4 Late Palaeozoic U-Th-Pb ages of monazite from rocks of the Merrimack Terrane in eastern Connecticut reflect an Alleghanian overprint on Acadian metamorphic rocks. The period of monazite growth was dated using an electron microprobe and has identified high grade metamorphism occurring in association with compressional deformation at 311 ± 3 Ma. High grade metamorphism was not restricted to shear controlled zones but affected the bulk of the vertical rock pile within the Merrimack Terrane during the Pennsylvanian. However, the late Alleghanian (Permian) deformation and metamorphism that affected the Narragansett Basin to the east had no impact on the Merrimack Terrane. This is strongly supported by the lack of correlation between the FIA sets for the two regions. As metamorphism increased to sillimanite grade within the Merrimack Terrane, deformation became increasingly coarsely partitioned until the Early Permian when the strain became localised along ductile shear zones at the Putnam-Avalon boundary. That is, collision related compressional deformation within the Narragansett Basin was partitioned entirely into the Honey Hill fault system during the early Permian.

Item ID: 2084
Item Type: Thesis (PhD)
Keywords: tectonics, New England, Appalachians, porphyroblast, structural analysis, geochronology, Merrimack Terrane, Connecticut, Narragansett Basin, Rhode Island, Pennsylvania, sediments, taconic orogeny, acadian orogeny, orogenies, foliations, deformations, metamorphism, grabens, amphilbolite rocks, faults, crenulations, folds, Scotland Schist, nappes, Maine, Palaeozoic age, monazite minerals, Alleghenian orogeny, sillimanites, compression, Honey Hill, garnets, metasediment
Date Deposited: 12 Feb 2009 05:35
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 0%
04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 0%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 0%
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