@proceedings {268,
	title = {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},
	volume = {34},
	year = {2002},
	note = {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},
	month = {2002/10/01/},
	pages = {180 - 180},
	publisher = {Geological Society of America (GSA) : Boulder, CO, United States},
	address = {United States},
	abstract = {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.},
	keywords = {$\#$StaffPubs, dates, deformation, electron probe data, fabric, folds, geochronology, Geochronology 03, geometry, in situ, Laurentia, Mesoproterozoic, metamorphism, monazite, New Mexico, northern New Mexico, orogeny, Ortega Group, overgrowths, phosphates, Precambrian, preferred orientation, proterozoic, reactivation, Southwestern U.S., strain, structural analysis, Structural geology 16, synclines, tectonics, Tusas Mountains, United States, upper Precambrian, zoning},
	isbn = {00167592},
	url = {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},
	author = {Joseph P Kopera and Williams, Michael L. and Jercinovic, Michael J.}
}
@proceedings {319,
	title = {The Nashoba Terrane: A new tectonostratigraphy and shared structural styles with the Merrimack belt in Massachusetts},
	volume = {47},
	year = {2015},
	month = {03/2015},
	pages = {42},
	publisher = {Geological Society of America (GSA) : Boulder, CO, United States},
	edition = {3},
	address = {Northeastern Section - 50th Annual Meeting (23{\textendash}25 March 2015), Bretton Woods, NH},
	abstract = {Recent STATEMAP-sponsored geologic mapping of several 7.5{\textquoteright} quadrangles in east-central Massachusetts describe a new tectonostratigraphy and structural history for migmatitic gneisses of the Cambro-Ordivician arc complex represented, in part, by the Nashoba Formation. While preserved sedimentary structures are absent, four discrete subunits can be mapped based on distinct lithologies. From structurally lowest to highest, they are: A felsic magnetite-bearing bt gneiss, a magnetite and silliminite rich mu-bearing bt paragneiss, gt-bearing sulfidic bt-gneisses interlayered with sulfidic schist, amphibolite, and marble, and a calc-silicate rich bt-gneiss.
The Nashoba terrane exhibits near-identical early and late styles of deformation to those in the adjacent Nashua sub-belt of the Merrimack terrane: Early amphibolite-facies isoclinal folding (D1) overprinted by tight upright folding (D2) and associated thrust faults define the map-scale geometry of tectonostratigraphic units between the two terranes. D2 occurred syn-peak upper amphibolite facies metamorphism in the Nashoba terrane while defined by retrograde greenschist facies fabrics in the Merrimack terrane. These are overprinted by outcrop-scale sinistral strike-slip motion (D3) in the Nashoba terrane progressively transitioning to late chlorite-grade NW-side down oblique extension (D4) exhibited in both terranes.
Correlation of the above sequence with existing geochronology, structural petrology, and tectonic studies by other workers in the area suggest that D1 in both terranes occurred prior to and/or during ~370 Ma Acadian orogenesis. Existing geochronology shows D2 fold and fabric development to be diachronous both within and across terranes: occurring both before and after ~363 Ma, with progressive development possibly as late as ~325 Ma, in the Nashoba terrane. D2 in the Nashua sub-belt of the Merrimack terrane occurs after ~330 Ma in the Carboniferous, as late at ~293 Ma in the Permian, during Alleghenian orogenesis. NW-side-down extension and associated rotation of M1 metamorphic isograds in the Merrimack Terrane, and exhumation of the Nashoba terrane, are generally accepted to have occurred during later stages of Alleghenian orogenesis in the Permian.
},
	keywords = {$\#$StaffPubs, acadian, alleghenian, amphibolite facies, chlorite, deformation, eastern Massachusetts, fabric, folds, Harvard Conglomerate, Hudson, merrimack, Merrimack Belt, nashoba, Nashoba terrane, nashua, Nashua Trough, pin hill, Structural geology, tectonic history, tectonostratigraphy},
	url = {https://gsa.confex.com/gsa/2015NE/webprogram/Paper253009.html},
	author = {Joseph P Kopera}
}
