Academic literature on the topic 'Geology, structural – Vermont'

In this contribution we review the mineralogical characteristics of five new and rare minerals discovered in the Othrys and Vermion ophiolites located in Greece, with the aim to better understand their origin. Three new minerals, namely tsikourasite Mo3Ni2P(1 + x) (x < 0.25), grammatikopoulosite NiVP and eliopoulosite V7S8, were found in the chromitite from the Agios Stefanos deposit, whereas arsenotučekite Ni18Sb3AsS16 was discovered in the Eretria (Tsangli) chromium mine, located in the Othrys ophiolite complex. The formation of the new phosphides tsikourasite and grammatikopoulosite and the sulfide eliopoulosite from Agios Stefanos took place after the precipitation of the host chromitite. Very likely, they formed at lower pressure in an extremely low fO2 and reducing environment during the serpentinization that affected the host ophiolite. The origin of arsenotučekite in chromitites coexisting with Fe–Ni–Cu-sulfide mineralization and magnetite at the Eretria (Tsangli) mine, is believed to be related to a circulating hydrothermal system. The most salient feature of theophrastite Ni(OH)2 and associated unnamed (Ni,Co,Mn)(OH)2 with a varying compositional range and a concentrating development, as successive thin layers, composed by fine fibrous crystals. The extremely tiny crystals of these hydroxides and the spatial association of mixed layers of Ni-silicides with theophrastite may reflect the significant role of the interaction process between adjacent layers on the observed structural features. The scarcity in nature of the new minerals reviewed in this paper is probably due to the required extreme physical-chemical conditions, which are rarely precipitated.

Petrologic and thermobarometric studies, coupled with geologic mapping and structural analysis, provide critical evaluation of several different models for Acadian (Late Silurian to Middle Devonian) dome evolution in southern Vermont. Previous models considered diapiric uprise and composite nappe-stage crustal thickening and subsequent diapirism as likely causes of dome formation. Both of these previous models result in symmetrical distribution of P-T values about the dome structures with corresponding coreward increases in temperature, and typically, coreward decrease in associated pressures. Thermobarometric calculations made during this study demonstrate that both P and T increase eastward across the entire region and are not symmetrically distributed about dome axes. The P-T data coupled with petrographically derived relative age relationships and available geochronology also suggest that attainment of peak metamorphic conditions and concurrent dome-stage deformation are diachronous and young from west to east. These relationships are consistent with new geologic mapping and structural analysis which show that all of the domes in southern Vermont are low-amplitude fold interference structures. A current tectonic model indicates that Acadian Barrovian metamorphism in this region was a consequence of west-directed crustal thickening of an eastward dipping tectonic wedge, presumably from the Bronson Hill Terrane; an Ordovician arc sequence. The basal surface of this allochthonous mass projects above the present land surface within this area. Accretion of lower-plate rocks (of this study) into the thrust complex and continued west-directed thrusting of the accreted package over a seismically recognizable east dipping ramp structure provided the necessary geometry and mechanism for dome-stage fabric development, calculated uplift rates (1.2 to 1.7 km/m.y. and west to east younging of Acadian structural and metamorphic evolution. Thermobarometric and geochronologic estimates of metamorphic pressure - temperature (P-T) conditions and metamorphic cooling ages were used to constrain the required thermal and tectonic input parameters for use in one-dimensional thermal modeling of an Acadian (Silurian-Devonian} tectonotherma! regime within the pre-Silurian Taconide zone of southern Vermont. This regime includes: 1) garnet-grade rocks from the eastern flank of an Acadian composite dome structure (Sadawga Dome; the western domain); 2) staurolite/kyanite-grade rocks from the western flank of a second composite structure, the Athens dome (eastern domain). Results from thermal modeling include development of P-T paths, temperature-time (T-t) and pressure-time (P-t) curves, related values of maximum temperature and pressure, pressure conditions at maximum temperature, predicted closure ages for radiogenic phases, and integrated uplift and cooling rates. Thermal modeling results are remarkably similar to independently obtained data for Acadian regional metamorphism in western New England, and provide some important constraints on regional thermal evolution: 1) pressure values contemporaneous with peak temperature on P-T paths may be substantially lower than actual maximum pressure (> 2.5 kbars); 2) differences in peak temperature for rocks initially loaded to similar crustal depths (garnetgrade vs. staurolite-grade), differences in calculated uplift rates, and differences in Ar closure ages, are consequences of variations in durations of isobaric heating events (or "residence periods"), and differences in actual tectonic uplift rates. These modeling results are internally consistent with structural model that suggests west to east younging of specific Acadian deformations and resultant diachroneity of peak metamorphic and Ar closure ages. Regional variations in timing and conditions of metamorphism may be controlled by diachronous deformational events coupled with variations in crustal levels to which rocks were initially loaded during the ca. 400 Ma onset of Acadian orogenesis in western New England.
Ph. D.

Near Burlington, Vermont, the Champlain Thrust fault placed massive Cambrian dolostones over calcareous shales of Ordovician age during the Ordovician Taconic Orogeny. Although the Champlain Thrust has been studied previously throughout the Champlain Valley, the architecture and structural evolution of its fault zone have never been systematically defined. To document these fault zone characteristics, a detailed structural analysis of multiple outcrops was completed along a 51 km transect between South Hero and Ferrisburgh, Vermont. The Champlain Thrust fault zone is predominately within the footwall and preserves at least four distinct events that are heterogeneous is both style and slip direction. The oldest stage of structures—stage 1—are bedding parallel thrust faults that record a slip direction of top-to-the-W and generated localized fault propagation folds of bedding and discontinuous cleavages. This stage defines the protolith zone and has a maximum upper boundary of 205 meters below the Champlain Thrust fault surface. Stage 2 structures define the damage zone and form two sets of subsidiary faults form thrust duplexes that truncate older recumbent folds of bedding planes and early bedding-parallel thrusts. Slickenlines along stage 2 faults record a change in slip direction from top-to-the-W to top-to-the-NW. The damage zone is ~197 meters thick with its upper boundary marking the lower boundary of the fault core. The core, which is ~8 meters thick, is marked by the appearance of mylonite, phyllitic shales, fault gouge, fault breccia, and cataclastic lined faults. In addition, stage 3 sheath folds of bedding and cleavage are preserved as well as tight folds of stage 2 faults. Stage 3 faults include thrusts that record slip as top-to-the-NW and -SW and coeval normal faults that record slip as top-to-the-N and -S. The Champlain Thrust surface is the youngest event as it cuts all previous structures, and records fault reactivation with any top-to-the-W slip direction and a later top-to-the-S slip. Axes of mullions on this surface trend to the SE and do not parallel slickenlines. The Champlain Thrust fault zone evolved asymmetrically across its principal slip surface through the process of strain localization and fault reactivation. Strain localization is characterized by the changes in relative age, motion direction along faults, and style of structures preserved within the fault zone. Reactivation of the Champlain Thrust surface and the corresponding change in slip direction was due to the influence of pre-existing structures at depth. This study defines the architecture of the Champlain Thrust fault zone and documents the importance of comparing the structural architecture of the fault zone core, damage zone, and protolith to determine the comprehensive fault zone evolution.

The Silurian-Devonian metasedimentary rocks of the Connecticut Valley-Gaspé trough (CVGT) were subjected to multiple deformational and metamorphic events during the Acadian orogeny in the Middle-Late Devonian. Plutons intruding the Devonian Waits River and Gile Mountain Formations have been considered post-tectonic, but microstructural studies of the intrusions and their metamorphic aureoles indicate some of these plutons intruded syntectonically. This study investigates the relationship between Acadian deformation and intrusion of the Knox Mountain pluton (KMP) of central Vermont. Structural and geochronological data were collected along a c. 15 km transect from the western limit of the CVGT, where the unconformable Richardson Memorial Contact coincides with the Dog River Fault Zone, into the margin of the KMP in the east. Field and microstructural observations indicate the KMP intruded syntectonically. Evidence for Acadian deformation post-dating intrusion includes folded and boudinaged granitic dikes at the margin of the KMP, and microstructures such as flame perthite, myrmekite, deformation twins, and textures associated with grain-boundary migration recrystallization in the granite. In the metamorphic aureole, biotite porphyroblasts overgrow S3, the earliest Acadian secondary foliation, and were deformed during S4 crenulation cleavage development. The KMP intruded at 377±5.2 Ma based on a U-Th-total Pb monazite crystallization age, which is concordant with the published age of the nearby Barre granite. The timing of S4 foliation development in the CVGT is constrained locally by 40Ar/39Ar geochronology at ~365 Ma, consistent with the microstructurally-inferred relative-age relationships. Plateau/weighted mean 40Ar/39Ar ages from across the transect and minimum ages from argon-loss profiles show a general trend of younging towards the east, suggesting these rocks have been affected by Alleghanian and Mesozoic deformation and exhumation.

Garnet porphyroblasts from southern Vermont and northwestern Massachusetts typically preserve multiple generations of inclusion trails, reflecting a potentially complex deformation and metamorphic growth history. Textural discontinuities, such as inclusion trail truncations or deflection planes, are commonly preserved within these complex inclusion trails and they are dominantly sub-vertically and sub-horizontally oriented. This observation cannot be adequately explained using the rotational model of spiral inclusion trail formation, leading to the conclusion that the trails were formed by the inclusion of multiple sub-vertical and sub-horizontal foliations during episodic garnet growth without porphyroblast rotation. Foliation inflection/intersection axes preserved within porphyroblasts (FIAs) provide important information about the kinematics of deformation, particularly the direction of bulk shortening at the time they formed. Samples from this study preserve a succession of six FIA sets resulting from a progressive change in the direction of bulk shortening through time. The distribution of these FIA sets across the field area indicates that the deformation was heterogeneously partitioned and occurred at different scales throughout orogenesis. Localization of deformation is an important control on garnet growth and repartitioning during successive deformation events resulted in a heterogeneous spatial distribution of garnet growth through time. The episodic nature of garnet growth is reflected in compositional zoning anomalies in garnet porphyroblasts from the Hoosac Formation. Zones of manganese enrichment, accompanied by calcium depletion, reveal pauses in garnet growth that may have been accompanied by fluid infiltration, garnet dissolution-reprecipitation and metasomatism at crystal or greater scales. The metamorphic history of these samples was further investigated using P-T pseudosections constructed via THERMOCALC. The mineral assemblages predicted are in good agreement with the observed mineralogy of the samples and estimates of P-T conditions at the time of garnet core growth were made using compositional isopleths based on microprobe analyses of garnet core composition. The samples do not show a clear relationship between the P-T data and the FIA data, suggesting that the preservation of different FIAs is not simply a function of P-T conditions. In samples where initial garnet appears to have occurred at temperature and/or pressure conditions higher than the minimum P-T conditions predicted for garnet stability, deformation probably played an essential role in garnet nucleation and growth. These “overstepped” samples indicate a progressive increase in pressure through the different phases of orogenesis. This study reveals that garnet porphyroblasts in southern Vermont and northwestern Massachusetts grew during a complex history of deformation involving the production of multiple sub-horizontal and sub-vertical foliations with garnet growth primarily controlled by deformation partitioning, resulting in a heterogeneous spatial distribution through time. FIAs allow the relative age of different phases of garnet growth to be established and provide a framework for interpreting the relationship between deformation and metamorphism. Compositional zoning anomalies reflect the episodic nature of garnet growth and P-T modelling suggests that deformation took place under conditions of increasing pressure.

Garnet porphyroblasts from southern Vermont and northwestern Massachusetts typically preserve multiple generations of inclusion trails, reflecting a potentially complex deformation and metamorphic growth history. Textural discontinuities, such as inclusion trail truncations or deflection planes, are commonly preserved within these complex inclusion trails and they are dominantly sub-vertically and sub-horizontally oriented. This observation cannot be adequately explained using the rotational model of spiral inclusion trail formation, leading to the conclusion that the trails were formed by the inclusion of multiple sub-vertical and sub-horizontal foliations during episodic garnet growth without porphyroblast rotation. Foliation inflection/intersection axes preserved within porphyroblasts (FIAs) provide important information about the kinematics of deformation, particularly the direction of bulk shortening at the time they formed. Samples from this study preserve a succession of six FIA sets resulting from a progressive change in the direction of bulk shortening through time. The distribution of these FIA sets across the field area indicates that the deformation was heterogeneously partitioned and occurred at different scales throughout orogenesis. Localization of deformation is an important control on garnet growth and repartitioning during successive deformation events resulted in a heterogeneous spatial distribution of garnet growth through time. The episodic nature of garnet growth is reflected in compositional zoning anomalies in garnet porphyroblasts from the Hoosac Formation. Zones of manganese enrichment, accompanied by calcium depletion, reveal pauses in garnet growth that may have been accompanied by fluid infiltration, garnet dissolution-reprecipitation and metasomatism at crystal or greater scales. The metamorphic history of these samples was further investigated using P-T pseudosections constructed via THERMOCALC. The mineral assemblages predicted are in good agreement with the observed mineralogy of the samples and estimates of P-T conditions at the time of garnet core growth were made using compositional isopleths based on microprobe analyses of garnet core composition. The samples do not show a clear relationship between the P-T data and the FIA data, suggesting that the preservation of different FIAs is not simply a function of P-T conditions. In samples where initial garnet appears to have occurred at temperature and/or pressure conditions higher than the minimum P-T conditions predicted for garnet stability, deformation probably played an essential role in garnet nucleation and growth. These “overstepped” samples indicate a progressive increase in pressure through the different phases of orogenesis. This study reveals that garnet porphyroblasts in southern Vermont and northwestern Massachusetts grew during a complex history of deformation involving the production of multiple sub-horizontal and sub-vertical foliations with garnet growth primarily controlled by deformation partitioning, resulting in a heterogeneous spatial distribution through time. FIAs allow the relative age of different phases of garnet growth to be established and provide a framework for interpreting the relationship between deformation and metamorphism. Compositional zoning anomalies reflect the episodic nature of garnet growth and P-T modelling suggests that deformation took place under conditions of increasing pressure.