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Abstract Quartz deformation fabrics reflect stress and strain conditions in mylonites, and their interpretation has become a mainstay of kinematic and structural analysis. Quantification of grain size and shape and interpretation of textures reflecting deformation mechanisms can provide estimates of flow stress, strain rate, kinematic vorticity, and deformation temperatures. Empirical calibration and determination of quartz flow laws is based on laboratory experiments of pure samples; however, pure quartzite mylonites are relatively uncommon. In particular, phyllosilicates may localize and partition strain that can inhibit or enhance different deformation mechanisms. Experimental results demonstrate that even minor phyllosilicate content (<15 vol%) can dramatically alter the strain behavior of quartz; however, few field studies have demonstrated these effects in a natural setting. To investigate the role of phyllosilicates on quartz strain fabrics, we quantify phyllosilicate content and distribution in quartzite mylonites from the Miocene Raft River detachment shear zone (NW Utah, USA). We use microstructural analysis and electron backscatter diffraction to quantify quartz deformation fabrics and muscovite spatial distribution, and X-ray computed tomography to quantify muscovite content in samples with varying amounts of muscovite collected across the detachment shear zone. Phyllosilicate content has a direct control on quartz deformation mechanisms, and application of piezometers and flow laws based on quartz deformation fabrics yield strain rates and flow stresses that vary by up to two orders of magnitude across our samples. These findings have important implications for the application of flow laws in quartzite mylonites and strain localization mechanisms in mid-crustal shear zones.
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This content will become publicly available on January 21, 2026
Evidence for transpression in the Picuris orogen: The deformation record of the Marqueñas Formation metaconglomerate, Picuris Mountains, New Mexico, USA
Abstract The most recent models for the Mesoproterozoic (ca. 1.5–1.35 Ga) Picuris-Baraboo-Pinware orogeny call on transpression resulting from oblique, diachronous convergence at the southern margin of Laurentia to explain the patterns of deformation and magmatism along this transcontinental belt. The Marqueñas Formation metaconglomerate provides a rare opportunity to directly study the strain and kinematics of deformation within the intraplate Picuris segment of the orogen. Statistical analysis of deformed quartzite pebble and boulder dimensions shows flattening strain at the outcrop to map scale (kilometers to meters). Quartz crystallographic preferred orientation (CPO) records a combination of flattening and non-coaxial shear at the intraclast scale (millimeters to micrometers). Kinematic vorticity axes, determined by crystallographic vorticity analysis on quartzite clasts, are well preserved despite widespread static recrystallization and align with principal strain axes determined from quartz CPO. The compatibility of strain and kinematic vorticity data indicates that flattening was produced in progressive, 3-D general shear. Outcrop-scale and map-scale structural relations link Marqueñas Formation flattening strain to oblique slip, with components of north-directed thrusting and dextral shear, on the Plomo-Pecos shear zone. Quartz flattening CPO yields predominantly crossed-girdle c-axis figures with opening angles of 69°–92° and a mean of 80°. Quartz c-axis opening angle thermometry yields deformation temperatures of 601 ± 50 °C, suggesting that flattening was synchronous with prograde to peak metamorphism during the second phase of deformation (D2) in the Picuris Mountains. We conclude that flattening of the Marqueñas Formation records inclined transpression within the Picuris orogen, consistent with oblique convergence along the Mesoproterozoic Laurentian plate margin.
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- Award ID(s):
- 2241801
- PAR ID:
- 10587104
- Publisher / Repository:
- Geosphere
- Date Published:
- Journal Name:
- Geosphere
- Volume:
- 21
- Issue:
- 2
- ISSN:
- 1553-040X
- Page Range / eLocation ID:
- 206 to 226
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The Paleoarchean Mt Edgar dome in the East Pilbara Terrane has long been studied as an archetypal dome within Archean dome‐and‐keel terranes, but the history of its formation is debated. Kinematic data presented in this study provide new insights into the late‐stage development of the Mt Edgar dome and East Pilbara Terrane. Quartz crystallographic preferred orientation (CPO), optical microstructures, and field structures all indicate that the granite‐greenstone contact of the Mt Edgar dome experienced reverse (greenstone‐up, dome‐down) sense of shear after the formation of the dominant schistosity. This reverse sense of shear is observed at localities along the entire extent of the sheared margin that rings most of the Mt Edgar dome, but is best documented along the southwest margin in the Warrawoona Greenstone Belt. Additionally, quartz CPO data from a dome triple junction outside of the sheared margin dominantly indicate a constrictional strain geometry, consistent with the previous interpretation that this area represents a zone of vertical foundering in a buoyancy‐instability driven system. However, buoyancy‐instability models do not necessarily predict the occurrence of greenstone‐up sense of shear preserved in solid‐state fabrics along the dome margin. Several geologic explanations are considered, including dome expansion or post‐doming deformation. The data are most consistent with explanations that directly relate to dome formation, especially when considered in tandem with recently published structural data from within the Mt Edgar dome. These kinematic data suggest that late dome development occurred in a near‐static crustal environment rather than an extensional or contractional setting.more » « less
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Abstract Orientated carbonate (calcite twinning strains; n = 78 with 2414 twin measurements) and quartzites (finite strains; n = 15) were collected around Gondwana to study the deformational history associated with the amalgamation of the supercontinent. The Buzios orogen (545–500 Ma), within interior Gondwana, records the high-grade collisional orogen between the São Francisco Craton (Brazil) and the Congo–Angola Craton (Angola and Namibia), and twinning strains in calc-silicates record a SE–NW shortening fabric parallel to the thrust transport. Along Gondwana's southern margin, the Saldanian–Ross–Delamerian orogen (590–480 Ma) is marked by a regional unconformity that cuts into deformed Neoproterozoic–Ordovician sedimentary rocks and associated intrusions. Cambrian carbonate is preserved in the central part of the southern Gondwana margin, namely in the Kango Inlier of the Cape Fold Belt and the Ellsworth, Pensacola and Transantarctic mountains. Paleozoic carbonate is not preserved in the Ventana Mountains in Argentina, in the Falkland Islands/Islas Malvinas or in Tasmania. Twinning strains in these Cambrian carbonate strata and synorogenic veins record a complex, overprinted deformation history with no stable foreland strain reference. The Kurgiakh orogen (490 Ma) along Gondwana's northern margin is also defined by a regional Ordovician unconformity throughout the Himalaya; these rocks record a mix of layer-parallel and layer-normal twinning strains with a likely Himalayan (40 Ma) strain overprint and no autochthonous foreland strain site. Conversely, the Gondwanide orogen (250 Ma) along Gondwana's southern margin has three foreland (autochthonous) sites for comparison with 59 allochthonous thrust-belt strain analyses. From west to east, these include: finite strains from Devonian quartzite preserve a layer-parallel shortening (LPS) strain rotated clockwise in the Ventana Mountains of Argentina; frontal (calcite twins) and internal (quartzite strains) samples in the Cape Fold Belt preserve a LPS fabric that is rotated clockwise from the autochthonous north–south horizontal shortening in the foreland strain site; Falkland Devonian quartzite shows the same clockwise rotation of the LPS fabric; and Permian limestone and veins in Tasmania record a thrust transport-parallel LPS fabric. Early amalgamation of Gondwana (Ordovician) is preserved by local layer-parallel and layer-normal strain without evidence of far-field deformation, whereas the Gondwanide orogen (Permian) is dominated by layer-parallel shortening, locally rotated by dextral shear along the margin, that propagated across the supercontinent.more » « less
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Abstract Documenting the magnitude of finite strain within ductile shear zones is critical for understanding lithospheric deformation. However, pervasive recrystallization within shear zones often destroys the deformed markers from which strain can be measured. Intensity parameters calculated from quartz crystallographic preferred orientation (CPO) distributions have been interpreted as proxies for the relative strain magnitude within shear zones, but thus far have not been calibrated to absolute strain magnitude. Here, we present equations that quantify the relationship between CPO intensity parameters (cylindricity and density norm) and finite strain magnitude, which we calculate by integrating quartz CPO analyses (n = 87) with strain ellipsoids from stretched detrital quartz clasts (n = 49) and macro‐scale ductile thinning measurements (n = 7) from the footwall of the Northern Snake Range décollement (NSRD) in Nevada. The NSRD footwall exhibits a strain gradient, with Rs(XZ)values increasing from 5.4 ± 1.4 to 282 ± 122 eastward across the range. Cylindricity increases from 0.52 to 0.83 as Rs increases from 5.4 to 23.5, and increases gradually to 0.92 at Rs values between 160 and 404. Density norm increases from 1.68 to 2.97 as Rs increases from 5.4 to 23.5, but stays approximately constant until Rs values between 160 and 404. We present equations that express average finite strain as a function of average cylindricity and density norm, which provide a broadly applicable tool for estimating the first‐order finite strain magnitude within any shear zone from which quartz CPO intensity can be measured. To demonstrate their utility, we apply our equations to published data from Himalayan shear zones and a Cordilleran core complex.more » « less
