TY - Generic T1 - Age-constraints on fabric reactivation in the Tusas Range, northern New Mexico, using electron-microprobe monazite geochronology; implications for the nature of regional approximately 1400 Ga deformation T2 - Abstracts with Programs - Geological Society of America Y1 - 2002 A1 - Joseph P Kopera A1 - Williams, Michael L. A1 - Jercinovic, Michael J. KW - #StaffPubs KW - dates KW - deformation KW - electron probe data KW - fabric KW - folds KW - geochronology KW - Geochronology 03 KW - geometry KW - in situ KW - Laurentia KW - Mesoproterozoic KW - metamorphism KW - monazite KW - New Mexico KW - northern New Mexico KW - orogeny KW - Ortega Group KW - overgrowths KW - phosphates KW - Precambrian KW - preferred orientation KW - proterozoic KW - reactivation KW - Southwestern U.S. KW - strain KW - structural analysis KW - Structural geology 16 KW - synclines KW - tectonics KW - Tusas Mountains KW - United States KW - upper Precambrian KW - zoning AB - A key issue in constructing models for the southward growth of Laurentia during the Proterozoic is distinguishing the effects of approximately 1650 Ma and approximately 1400 Ma tectonism. These events share similar styles of deformation and metamorphism, making it difficult to assign structures, fabrics, and metamorphic phases to a particular event. The fundamental geometry of this orogen in the southwestern United States is defined in many areas by fold-fault pairs and isolated synclines of thick approximately 1700 Ma quartzite. In-situ EMP chemical dating of monazite, combined with detailed structural analysis, indicates that such synclines within the Tusas Range of northern New Mexico (locally F (sub 3) ) were substantially modified, if not developed, during approximately 1400 Ma tectonism. Monazite grains from the Ortega quartzite in the central Tusas Range display a shape preferred orientation parallel to the axial-planar fabric of these folds (S (sub 3) ), with overgrowth rims preferentially developed in the X direction of strain. These monazite grains have either >1700 Ma cores or approximately 1650 Ma cores with approximately 1400 Ma overgrowth rims, or are entirely approximately 1400 Ma in age. Field and microstructural observations show that the upright, east-west trending F (sub 3) and S (sub 3) are reactivations of older, northwest-trending fabrics and structures. The presence of approximately 1650 Ma overgrowth rims on monazite grains from the central and northern Tusas Range implies that these folds and fabrics may have nucleated prior to approximately 1400 Ma tectonism. Previous studies have shown an increase in approximately 1400 Ma monazite ages from north to south within the range, consistent with a similar increase in metamorphic grade. This gradient suggests that the central and northern Tusas may have been at progressively shallower crustal levels during approximately 1400 Ma tectonism, thus increasing the preservation of older fabrics, structures, and metamorphic monazite from south to north within the range. These observations support the hypothesis that approximately 1400 Ma tectonism locally reactivated and utilized pre-existing structures and fabrics, but had also profoundly shaped the geometry and metamorphic character of the orogen. JF - Abstracts with Programs - Geological Society of America PB - Geological Society of America (GSA) : Boulder, CO, United States CY - United States VL - 34 SN - 00167592 UR - http://silk.library.umass.edu/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=geh&AN=2004-044516&site=ehost-live&scope=site IS - 66 N1 - Accession Number: 2004-044516; Conference Name: Geological Society of America, 2002 annual meeting; Denver, CO, United States; Conference Date: 20021027; Language: English; Coden: GAAPBC; Collation: 1; Collation: 180; Publication Types: Abstract Only; Serial; Conference document; Updated Code: 200413; Monograph Title: Geological Society of America, 2002 annual meeting; Monograph Author(s): Anonymous; Reviewed Item: Analytic JO - Abstracts with Programs - Geological Society of America ER - TY - Generic T1 - A structural framework for the Nashoba Terrane in eastern Massachusetts. T2 - Abstracts with Programs - Geological Society of America Y1 - 2013 A1 - Joseph P Kopera A1 - Matthew A Massey KW - #StaffPubs KW - Cambrian KW - eastern Massachusetts KW - exhumation KW - fabric KW - fault zones KW - faults KW - foliation KW - massachusetts KW - metamorphism KW - Nashoba terrane KW - Ordovician KW - Paleozoic KW - Structural geology KW - tectonics KW - United States AB - The exhumation and tectonic significance of the migmatitic Cambro-Ordovician arc-complex of the Nashoba terrane, located between lower-grade rocks of the Avalon and Merrimack terranes in Massachusetts, has historically presented an enigma, in part, due to a lack of detailed analysis of internal structure. We propose a new terrane-scale structural model based on nearly a decade of detailed geologic mapping to provide a framework for future study. A subvertical NE-striking composite fabric (S (sub n/n-1) ) forms the dominant structural grain of the terrane. S (sub n) commonly deforms an older layer-parallel foliation (S (sub n-1) ) about meter- to kilometer-scale, upright to steeply inclined, NE- and SW-plunging, tight disharmonic folds (F (sub n) ). In the Nashoba Formation migmatites, S (sub n) commonly transposes a subhorizontal S (sub n-1) enveloping surface into spaced meter-scale subvertical shear bands that are absent in the dominantly metavolcanic Marlborough Formation. Fold axis-parallel mineral stretching and intersection lineations (L (sub n) ) are locally overprinted on S (sub n) by subhorizontal peak metamorphic to retrograde mineral lineations (L (sub n/n+1) ). Ambiguous D (sub n) kinematics in the NE transition SW along strike to top-to-NW normal fold vergence and drag along steep north-dipping S (sub n) axial planes and S (sub n) - S (sub n+1) shear bands. Later strain (S (sub n+1) - S (sub n+2) ) appears to be progressively partitioned at lower grade to pre-existing S (sub n) shear bands and discrete internal and terrane bounding fault zones which display early high-grade top-to-SE dextral or sinistral motion (S (sub n-1) - S (sub n) ) broadly overprinted by lower-grade top-NW movement (S (sub n+1) ). We propose a tentative tectonic history incorporating sparse existing geochronologic and petrologic studies: Top SW D (sub n-1) motion coeval with approximately 425 Ma sill-grade metamorphism and possible accretion. D (sub n) initiating syn approximately 395 Ma peak metamorphism with migmatite generation along S (sub n) and progressive bulk fabric development largely complete by the intrusion of the relatively undeformed approximately 349 Ma Indian Head Hill granite. Exhumation can be accommodated by well-documented syn-to-post D (sub n) regional sinistral motion combined with progressively lower grade top-NW extension along discrete structures continuing through deposition and deformation of presumed Carboniferous basin sediments along the terrane boundary. JF - Abstracts with Programs - Geological Society of America VL - 45 UR - https://gsa.confex.com/gsa/2013NE/webprogram/Paper215867.html ER -