336 research outputs found
Tectonic implications of fault-scarp-derived volcaniclastic deposits on Macquarie Island: sedimentation at a fossil ridge-transform intersection?
No full textUpper Miocene to lower Pliocene sedimentary rocks on Macquarie Island are dominantly volcaniclastic breccia, sandstone, and siltstone produced by the physical disintegration and tectonic abrasion of oceanic crust in fault zones and mass wasting of these tectonic features. They represent small debris fans and small-scale turbidites deposited at the base of active fault scarps, related to Late Miocene to Early Pliocene seafloor spreading. Most of the sediment is derived from basalts, but diabase and gabbro clasts in some sedimentary rocks indicate that middle and lower oceanic crust was exposed to erosion on the sea floor. A lack of exotic clasts and a low degree of clast roundness are consistent with a local source for the sediment and no input from continental rocks. Spatial relationships between sedimentary rocks and major faults associated with seafloor spreading on the island and correlation between sedimentary clast and adjacent up-thrown block compositions allow us to infer paleotectonic relief for Macquarie Island crust during deposition. Our data support a model involving the deposition of these rocks at the inside corner of a ridge-transform intersection. Furthermore, a tectonic reconstruction of the Australian-Pacific plate boundary for the approximate time that Macquarie Island crust formed (10.9 Ma) also shows that Macquarie Island crust most likely formed near a ridge-transform intersection. This paper describes sedimentation associated with active faulting at a ridge-transform intersection that has been uplifted in situ above sea level along with the surrounding oceanic crust, and demonstrates that high-angle faults have the most pronounced influence, compared with low-angle faults, on sedimentation in this tectonic environment.<br/
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Mesoproterozoic structural evolution and lithologic investigation of the western Llano Uplift, Mason County, Central Texas
No full texttextThe Llano Uplift of central Texas contains the largest exposure of Mesoproterozoic rocks along southern Laurentia and is thus crucial to the understanding of orogenesis and plate reconstructions along a portion of one of the largest orogens in the world. Most of the current understanding of the Mesoproterozoic tectonic evolution of southern Laurentia comes from the southeastern portion of the Llano Uplift. To fully characterize the tectonic evolution Llano Uplift, detailed mapping is necessary in the less-studied western Llano Uplift. The Mesoproterozoic Llano Uplift exposes mid-crustal, poly-deformed and metamorphosed schists and gneisses and abundant pre- to post-tectonic granites through an erosional window of Phanerozoic sedimentary rocks. Three lithologic groups were mapped in the western Llano Uplift, from structural highest to lowest these are the Valley Spring Gneiss (VSG), Lost Creek Gneiss (LCG) and Packsaddle Schist (PS). The VSG consists of pelitic schists and pink quartzofeldspathic schists and gneisses. The LCG is a thick, homogeneous package of medium- to coarse-grained augen granite gneiss, interpreted to be a deformed, coarse-grained, porphyritic pluton. The PS consists of a heterogeneous package of interlayered quartzofeldspathic gneisses, amphibolites and minor marbles. These lithologies are consistent with the PS and VSG domains described in the southeastern Llano Uplift (Mosher, 1998; Reese et al., 2000). The exotic Coal Creek Domain (CCD) of the southeastern Llano Uplift is not observed in the western Llano Uplift. The western Llano Uplift, including the VSG, LCG and PS, records a deformational history that resulted in multiple fold generations (F1-F5) and is characterized by a penetrative axial planar foliation (S1-S5). F2s are isoclinal folds of S0 (primary layering) and S1 that locally fold F1 axial planes and have steeply plunging and generally easterly trending hinge lines. F3 folds are locally developed, nearly colinear and coplanar with F2s, tight to open, and fold all previous structures (F1/F2) and fabrics (S1/S2). F4s are open folds with northeast-trending axial traces that occur on a regional-scale. F5s are open to tight folds of all previous structures, with hinge lines that are primarily southeast trending and steeply plunging. S0 to S3 orientations vary from north to east dipping because of reorientation by younger folds. S4 foliations strike to the northeast and S5 foliations are northwest striking and nearly vertically dipping. Late left-lateral shear zones (D6) with generally an easterly trend and boudinage affects the VSG, LCG and VSG in this study area and is commonly associated with unfoliated granite material. Four generations of intrusive granitic sills and dikes are documented and provide relative and absolute age constraints on deformation. The oldest recognized deformation (D1-D3) is constrained between 1253 +5/-3 Ma and 1126 +5/-4 Ma (Roback, et al., 1999). D4 and D5 deformation are constrained between 1126 +5/-4 Ma and 1076 ± 5 Ma (Roback, et al., 1999). Although a change in metamorphic conditions is documented to have occurred between D2 and D3, metamorphic fabrics and assemblages indicate granulite facies conditions during D1, D2 and D3. Amphibolite facies metamorphism occurred during D4 and presumably D5. Deformation in the eastern Llano Uplift has a similar polyphase deformational history to that recorded here for the western Llano Uplift. Deformation in the eastern Llano Uplift is similarly constrained between ca. 1238 to 1091 Ma. In addition, the youngest fold generation (F5) can be directly correlated in orientation and timing from the western to the eastern Llano Uplift, and is constrained between ca. 1119 and 1091 Ma in the eastern uplift. Both the western and eastern Llano Uplift contain late shear zones and extensional structures. Structural differences between the western and eastern Llano Uplift include differences in style and orientation of all but the latest (D5 and D6) structures. In addition, dip of fabrics and, therefore, structural stacking of lithologic domains is opposite, and no mylonite zones were identified in the west. In conclusion, the lithologic domains appear to correlated across the Llano Uplift based upon gross lithologic similarities and the tectonic evolution is similar to the well-studied eastern Llano Uplift, though the kinematics and orientations differ. These conclusions may require that the kinematics of deformation in the southeastern uplift were controlled by the presence of the exotic island arc terrane (CCD) whereas the kinematics of deformation in the western uplift were controlled by continent-continent collision.Earth and Planetary Science
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From seafloor spreading to uplift: the structural and geochemical evolution of Macquarie Island on the Australian-Pacific plate boundary
No full texttextMacquarie Island (54º30’S, 158º54’E) is unique, consisting of a section of
uplifted oceanic crust and upper mantle that still lies within the ocean basin where
it formed. Earlier geophysical studies indicate that between ~40 and 6 Ma, this
plate boundary evolved from a spreading ridge to the modern transpressional
boundary. The rocks of Macquarie Island record both regimes. This study
combines structural, geochemical and geophysical data to describe the evolution
of Macquarie Island and the adjacent Australian-Pacific plate boundary from
spreading to transpression.
The Finch-Langdon fault is the most significant spreading-related
structure on the island, juxtaposing upper crust and intrusive/mantle rocks. On
the basis of structural and petrologic data, I propose that this fault zone formed
near the inside corner of a ridge-transform intersection (RTI) and that structures
on the island are conformable with those in the surrounding seafloor.
Geochemical data for Macquarie Island basalts and peridotites suggest a
complex evolution during the last stages of seafloor spreading. The volcanic
section consists of enriched basalts formed by low degrees of partial melting.
Basalt geochemistry combined with stratigraphic relationships reveal early
intervals of variable enrichment followed by periods of more constant, decreasing
enrichment.
Peridotite and basalt geochemistries differ distinctly. Peridotites show
characteristics of a high degree of melting (heavy rare earth element, or REE, and
Al depletion), whereas low degrees of partial melting are inferred for the basalts.
The mantle rocks also have spoon-shaped REE patterns and anomalous Sr
enrichment. The depletion and trace element patterns are more typical of mantle
rocks in ophiolites than of abyssal mantle.
Ridge propagation proximal to an RTI exposing lower crust/uppermost
mantle would satisfy these structural and geochemical parameters.
Subsequently, transpression along the Australian-Pacific plate boundary
has resulted in transform motion along the plate boundary and vertical
deformation along the ~1500 km long Macquarie Ridge Complex. Uplift faults
on the island are dominantly high-angle, en echelon, normal faults. The
geometries and kinematics of the faults do not match predicted fault patterns for
transpression, but indicate domination by extensional relay zones between stepovers
of faults along the plate boundary.Earth and Planetary Science
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Deformation mechanisms in the Lewisian Gneiss and Cambro-Ordovician sediments at Heilam, Scotland
No full textHeilam, a 10 km2 area on the east coast of Loch Eriboll near the Moine Thrust in NW Scotland, is composed of three structural areas: 1) the Arnaboll Nappe, 2) the coastal schuppen zone and 3) the intermediate imbricate zone. Samples of each lithology taken from the zones show a progressive sequence of deformation. The least deformed rocks in the Arnaboll Nappe and in the intermediate imbricate zone have strain shadowed quartz grains and feldspars with deformation lamallae. Near the thrusts of the imbricate zone and in the schuppen zone, micas exhibit a strong preferred orientation, quartz grains show discontinuous undulatory extinction and recrystallization about the margins. Pressure shadows in the quartzites and styolites in the dolomites post-date earlier recrystallization in these areas. Mylonites, notably those along the Arnaboll Thrust, contain micas and quartz with a strong preferred orientation. All minerals show reduction in grain size by either recrystallization or brittle fracture. A sequence of deformation mechanisms affecting the lithologies is 1) initial recrystallization, 2) pressure solution, and 3) minor recrystallization. This sequence is best observed in the mylonites and schuppen zone.Earth and Planetary Science
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Microstructures and sense of shear in the Brevard Zone, southern Appalachians
No full textThe Brevard Zone, which separates the Blue Ridge and Inner Piedmont geologic provinces in the southern Appalachians, is a major structural feature with a multiple deformation history. Microstructures in oriented thin sections from rocks in the Brevard Zone in Tugaloo, Whetstone and Tamassee quadrangles, South Carolina, Rosman quadrangle, North Carolina, and in the sheared Ben Hill Granite in Atlanta, Georgia, indicate that there were at least two early ductile deformations and a later, locally developed, brittle deformation. The oldest recognizable microstructures arc a prominent foliation (S₁), quartz ribbons and garnets. The age of these features and the sense of shear during their formation is unknown. The remainder of the observed microstructures are categorized into groups A, B, C and D on the basis of orientation, overprinting relationships and direction of motion as indicated by sense of shear criteria present. Group A is the oldest of these microstructures and group D is the youngest. Group A features consists of northwest-verging, tight to isoclinal F₂ folds, a weakly developed, axial planar foliation (S₂), and scattered F₃ folds, coaxial with F₂. The F₁ folds of Roper and Dunn (1973) are not observed due to later deformation. Group A microstructures are ductile features which formed during a west- to northwest-directed thrusting motion. Group B features include type II s-c mylonites, c-surfaces, scattered folds and garnet pressure shadows. The orientation of these features indicates that they formed during a period of dextral strike-slip shearing with a possible thrust component. Group C contains an extensional crenulation cleavage (ECC) which is relatively younger than features in groups A and B. The orientation of ECC is incompatible with dextral motion, thus they suggest a change in the direction of bulk motion in the Brevard Zone, the direction of which is unknown. Along strike a notable change in deformation conditions occurred during the ductile deformation(s) which formed features in groups A, B and C. This change is reflected in highly recrystallized quartz textures in Tugaloo, relative to partially recrystallized textures in Rosman quadrangle. Retrograde metamorphism postdates the formation of features in groups A, B and C. Group D contains the youngest microstructures which formed during a localized brittle deformation. Brecciation is visible in thin section and outcrop, however no sense of shear direction can be determined. Drag folds and faults are present in several outcrops but their geometry is highly variable. The bulk motion during brittle deformation is unknown. Sense of shear criteria in group A are compatible with tectonic models for both the Taconic and Alleghanian orogenies in the southern Appalachians. However, group A probably formed in the Taconic because the most intense ductile deformation has been reported for this time period. Microstructures in group B and C are found in the sheared, Permian Ben Hill Granite in Atlanta and thus are Alleghanian in age. Rocks containing group B and C in the northeastern study areas cannot be radiometrically dated with confidence, however, their orientations, deformation conditions and sense of shear is similar to group B and C in the Ben Hill Granite indicating that they are also Alleghanian in age. The dextral strike-slip motion indicated by groups B is compatible with the results of previous workers (Reed and Bryant, 1964; Bobyarchick, 1983) elsewhere along the Brevard Zone who have also demonstrated Alleghanian dextral motion. Thus the results of this study confirm an episode of ductile, dextral strike-slip motion in the Brevard Zone during the Alleghanian. Group C and D may also be Alleghanian or they may be the result of a separate and more recent deformation, possibly related to the Triassic opening of the present-day Atlantic Ocean.Earth and Planetary Science
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Kinematic and geometric evolution of the Buckskin-Rawhide metamorphic core complex, west-central Arizona
No full texttextReconstructing the structural evolution of metamorphic core complexes is critical to understanding how large-magnitude extension is accommodated in the middle to upper crust. This dissertation focuses on the Miocene geometric and kinematic evolution of the Buckskin-Rawhide metamorphic core complex in west-central Arizona, addressing controversial topics including the geometric development of mid-crustal shear zones, the formation of detachment fault corrugations, and the transition from detachment faulting to more distributed deformation. Detailed microstructural data from mylonites in the lower plate of the Buckskin-Rawhide detachment fault indicate that early Miocene mylonitization was characterized by consistent top-NE-directed shear and ~450-500°C deformation temperatures that varied by [less-than or equal to]50°C across a distance of ~35 km in the extension direction. The relatively uniform deformation conditions and strain recorded in mylonitized ~22-21 Ma granitoids are incompatible with models in which the lower plate shear zone represents the down-dip continuation of a detachment fault. Instead, lower plate mylonites initiated as a subhorizontal shear zone that was captured and rapidly exhumed by a moderately to gently dipping detachment fault system. Structural data and geologic mapping demonstrate that the prominent NE-trending Buckskin-Rawhide detachment fault corrugations are folds produced by extension-perpendicular (NW-SE) shortening during core complex extension. Dominant NE-directed slip on the detachment fault was progressively overprinted by NW- and SE-directed slip associated with corrugation folding. Orientation patterns of upper plate bedding across the corrugations are compatible with folding about a NE-trending axis. Extension-perpendicular shortening in the lower plate is recorded by synmylonitic constriction and folding. Upright m-scale and km-scale lower plate folds parallel the detachment fault corrugations and developed primarily by postmylonitic flexural slip that was coeval with detachment faulting. The total amount of NW-SE shortening across the lower plate is ~10%, but the amount of NW-SE shortening recorded by the younger detachment fault is only ~1%. The relatively late-stage development of corrugations in the Buckskin-Rawhide metamorphic core complex suggests that extension-perpendicular shortening was primarily driven by a reduction of vertical stresses through crustal thinning and tectonic denudation. Brittle fault data document the transition from large-magnitude, NE-directed extension to distributed E-W extension and right-lateral faulting. Following exhumation to brittle conditions, lower plate mylonites were extended up to ~20-30% by NE-dipping, syndetachment normal faults. Towards the end of detachment faulting, the extension direction rotated clockwise, and some portions of the Buckskin detachment fault record a transition from dominant top-NE slip to ENE- and E-directed slip. After detachment faulting ceased, E-W extension was accommodated primarily by steeply NE-dipping, right-lateral and oblique right-lateral-normal faults. The cumulative amount of right-lateral shear across the core complex is probably 7-9 km, which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction. Postdetachment right-lateral/transtensional faulting across the Buckskin-Rawhide metamorphic core complex reflects the increasing influence of the Pacific-North American transform plate boundary towards the end of the middle Miocene.Earth and Planetary Science
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Structural evolution and geochronology of the southeastern Llano Uplift, Central Texas
No full textThe Llano Uplift, central Texas, exposes Middle Proterozoic crystalline rocks that were deformed and metamorphosed during Grenville orogenesis. New U-Pb zircon geochronology from the Valley Spring Gneiss and Packsaddle Schist in the southeastern Llano Uplift, combined with previous data, demonstrate that several map units contain disparately aged constituents. Three new units are differentiated: the Comanche Creek Gneiss, Coal Creek Formation, and Inks Lake Gneiss. Results show that younger Packsaddle Schist lies in structural contact above older Valley Spring Gneiss. Also, zircons in a Valley Spring Gneiss unit are the oldest yet found in the uplift (~1360 Ma). Their age is consistent with those of the Western Granite-Rhyolite Terrane. The map distribution of U-Pb data shows four age suites of rocks of distinct tectonic origin. A 1248-1244 Ma Packsaddle Schist is structurally wedged between 1270 Ma northerly Valley Spring Gneiss, and southerly ~1300 Ma Big Branch Gneiss and older Coal Creek units. Younger meta-intrusive rocks intrude the metamorphic pile. Grenville orogenesis is constrained to have occurred between 1215 Ma and 1098 Ma. Detailed structural mapping shows that these rocks record a polyphase deformational history progressing from 1) NE-directed, collision(?)-related ductile thrusting and shortening (D2), to 2) regional-scale polyphase folding (D3, D4, D5), indicating continued N- to NE-directed contraction, to 3) late-stage, post-collisional N-S extension. D2 deformation was partitioned, with ductile thrust zones accommodating NE-directed shearing, and intensely folded units representing shortened and flattened zones. Strong development in different areas of different post-D2 fold generations suggests that these areas are in regional-scale fold hinges. Thus, the southeastern Llano Uplift consists of multiple generations of regional-scale folds. N-S extension is recorded by quartz vein fibers oriented normal to a WNW-striking, variably dipping extensional crenulation cleavage and subparallel to the stretching direction of boudins. The timing and orientation of these structures is consistent with late-stage N-S extension. The Llano Uplift represents the metamorphic core of a Grenville-age continental collision zone. Island-arc, accretionary complex, oceanic, and arc-flank/continental-slope assemblages were contracted between continental-scale blocks and emplaced onto North America's southern margin. Ductile thrusting and polyphase folding accommodated deformation of this material and its tectonic transport continentward. Increasing pressure-temperature conditions of dynamothermal metamorphism toward the orogenic core are attributed to greater crustal thickening related to continental margin underthrusting and tectonic stackingEarth and Planetary Science
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Reactivation of fractures as discrete shear zones from fluid enhanced reaction softening, Harquahala metamorphic core complex, west-central Arizona
No full texttextDiscrete (mm- to m-scale) mylonitic shear zones in the northeastern Harquahala metamorphic core complex, Arizona, show evidence of fluid-mineral interactions catalyzing deformation and metamorphism. Many contain a deformed central epidote vein with adjacent bleached haloes and flanking paired shear zones that indicate significant fluid-rock interaction during deformation. An integration of structural and geochemical methods was employed to understand timing, metamorphic conditions, and physiochemical processes responsible for producing the discrete shear zones. Field and microstructural evidence suggest the zones initiated on antecedent fractures. Electron backscatter diffraction (EBSD) analyses show a significant coaxial contribution to the shear, and quartz deformation predominately by prism <a> slip, along with some rhomb <a> slip, suggesting amphibolite-facies conditions during shearing. Fourier Transform Infrared spectroscopy analyses of quartz reveal higher water contents within shear zones than within country rocks, indicating fluid infiltration synchronous with shearing. Stable isotope analyses of quartz and feldspar from mylonites are consistent with an igneous or metamorphic fluid origin. Microstructural observations suggest that the zone morphology with epidote veins, bleached haloes, and flanking discrete paired shear zones was developed predominantly from reaction softening mechanisms. The increase in deformation from bleached rock to flanking shear zones is marked by progressive modal increases in biotite and myrmekite, and modal decreases in K-feldspar, and locally epidote and titanite. Myrmekitic textures recrystallized readily and resulted in progressively greater grain size reduction of feldspar, which aided in the progressive alignment and linkage of the biotite grains, which together concentrated the deformation in bands. Volume reduction resulting from some of the metamorphic reactions may have led to a positive feedback cycle among fluid infiltration, metamorphism and deformation. U-Pb isotope analyses of syn-metamorphic titanite yield an age of ~70 Ma, suggesting the shear zones formed during cooling of the Late Cretaceous (75.5±1.3 Ma) Brown’s Canyon pluton, consistent with their top-to-the-southwest sense of shear, rather than during top-to-the-northeast directed Miocene metamorphic core complex exhumation. Petrography, EBSD analyses, and U-Pb dating of titanite from other (non-discrete) mylonites in the area imply most formed synchronously with the discrete shear zone mylonites. Only rare, scattered mylonites show features consistent with metamorphic core complex exhumation.Earth and Planetary Science
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Kinematics of bidirectional extension and coeval NW-directed contraction in orthogneisses of the biranup complex, Albany Fraser Orogen, Southwestern Australia
No full texttextGranulite-facies orthogneisses of the Mesoproterozoic Albany-Fraser Orogen from the locality of Bremer Bay, in southwestern Australia, record at least three phases of widespread, pervasive NW- and NE-trending bidirectional extension that alternate with shortening and/or shear related structures. Crustal extension occurred ca. 1180 Ma, based on SHRIMP U–Pb zircon geochronology of melts generated during deformation, which coincided with Stage II (1215-1140 Ma) of the Albany-Fraser Orogeny, a period of NW-directed contraction. Eight different deformation phases can be recognized in the Bremer Bay area: (1) formation of a pervasive migmatitic fabric, defined by alternating leucosomes and melanosomes, parallel to the main compositional layering, and axial planar to localized isoclinal folds of cm-wide melt bands; (2) first bidirectional extension phase, which formed cm-scale square boudins of mafic layers parallel to the main migmatitic fabric; (3) formation of open to isoclinal, upright to overturned, SW-plunging, NW-verging m-scale folds of early square and rectangular boudins and dominant migmatitic foliation; (4) renewed coeval NE- and NW-directed extension that produced intermediate (< 1 meter to a few meters) boudins of the migmatitic fabric and compositional layering; (5) formation of regional-scale, NW-verging, SW-plunging overturned folds of all previous structures; (6) third phase of bidirectional extension that formed large, decameter-scale boudins of the migmatitic fabric; (7) late folding phase that resulted in the formation of m-scale open to tight, SW-plunging, upright to moderately overturned, NW-verging folds; and (8) fracturing related to the intrusion of dominantly N-NW- and N-NE-trending intermediate and felsic few cm- to few dm-wide pegmatite veins. Melt generation was concurrent with all stages of deformation. The Albany-Fraser Orogen is reinterpreted as a diachronous orogen, resulting from the closure of the asymmetrically shaped ocean basin between the West Australian and Mawson cratons, which widens considerably from NE to SW along the length of the orogenic front. Subduction on the western side of the orogen was the driving force for NW-directed collision during Stage II of the orogeny. Slab breakoff and orogenic collapse following closure of an intracratonic ocean basin could account for the multiple phases of bidirectional extension, granulite facies metamorphism and pervasive partial melting throughout deformation.Earth and Planetary Science
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Structural analysis and detrital zircon provenance in the western Llano Uplift : implications for a southern collider
No full textStructural and metamorphic analysis, mapping and detrital zircon geochronology was conducted within the Mesoproterozoic Valley Spring Gneiss, in the western Llano Uplift, along an 8 km section of the Llano River west of Castell, TX. Comprising granitic gneisses and pelitic schists, with volcanic, plutonic and supracrustal protoliths, the protolith to the Valley Spring Gneiss was deposited on Laurentia and records an uppermost amphibolite facies, polyphase deformation history consistent with continent-continent collision during the Grenville orogeny. The Valley Spring Gneiss is characterized by both ortho- and paragneisses. To determine the provenance, three paragneiss samples were analyzed using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to establish ages for 80 zircons from each sample. All three samples show a major peak at 1200-1350 Ma and a smaller Paleoproterozoic peak. One sample had a small Archean peak. All data are consistent with a Laurentian origin for the sediment, with Mesoproterozoic sediment derived locally. Kalahari and Amazonia, which have been proposed as colliders with Laurentia, are unlikely sediment sources for rocks of the Llano Uplift. Six phases of synmetamorphic deformation have been documented. The mineral assemblage, including K-feldspar and sillimanite, indicates conditions above the second sillimanite isograd. Evidence for supersolidus conditions throughout deformation are widespread. Three types of leucosomes, indicative of partial melting, are parallel to early foliations. Pegmatites and granitic dikes are associated with late-stage folds and ductile shear zones; they increase in abundance eastward. The earliest deformation is characterized by two generations of isoclinal folds and penetrative metamorphic foliations, S₁ and S₂, which together form the dominant, northwest-striking, northeast-dipping metamorphic layering. The third phase, F₃, is characterized by tight folds that fold the S₁/S₂ foliation with an associated axial planar foliation. The F₃ fold axes plunge moderately to the southeast; axial planes are dominantly northwest-striking. Open, late generation folds (F₄ and F₅) refold earlier structures on both outcrop and map scales. F₄ and F₅ folds are northeast-plunging with northeast- to east-striking axial planes and southeast-plunging with northwest-striking axial planes, respectively. Late boudinage and shear zones containing melt are associated with an extensional D₆ phase of deformation. Granite and pegmatite intrusions are both syn- and post-tectonic as indicated by the presence or absence of the S₁/S₂ foliation, folding and boudinage. The sequence, style and orientation of the structures in this area correlate with those within the Packsaddle Schist and the Lost Creek Gneiss of the western Uplift. Orientations, structural stacking and definition of domain boundaries differ between the eastern and western portions of the Uplift, supporting the likelihood that deformation in the eastern Uplift was controlled by collision of the exotic arc terrane present there, whereas the western Uplift, where the island arc is absent, directly records deep-seated effects of a continent-continent collision.Earth and Planetary Science
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