An official website of the United States government
The Black Hills of western South Dakota and eastern Wyoming were uplifted as a result of the Laramide orogeny occupying the suture between the Wyoming and Superior Cratons. Two exposures of Archean orthogneiss, the Bear Mountain Terrane and the Little Elk Granite (LEG), represent the oldest rocks exposed in the Precambrian core of the Black Hills and offer the opportunity to study tectonic processes involved in forming the Laurentian craton. This study presents new structural field data (orientation of foliation planes, stretching lineations, and cross cutting relations; n=270 measurements) along a ~5 km transect that record the deformation history of the LEG. Two dominant fabric types were found in outcrop: augen gneiss (type 1) and mylonitized granite (type 2). The type 1 fabric is characterized by 1-5 cm K-feldspar crystals aligned to give top-down or “normal” sense of shear, small-scale folding of the fabric, and is cross-cut by aplite dikes in multiple sites. The type 2 mylonitic fabric overprints the type 1 fabric and intensifies from east to west along the transect, resulting in a loss of the type 1 fabric. The stretching lineation in the type 2 fabric plunges down dip with shear sense indicators observable in outcrop. Both fabrics display a NW/SE striking and ~70°SW dipping foliation at every site. Yet, subtle folding of the type 1 fabric at some sites causes it to be crosscut by the type 2 fabric. Based on the high-temperature deformation features in the type 1 fabric and the cross-cutting relationship with aplite dikes, we interpret that the type 1 fabric formed during emplacement of the granite. Assuming the LEG has not experienced significant tilting since emplacement, the top-down shear sense recorded by alignment of K-feldspar may suggest emplacement of the LEG into an extensional setting. Our observations of the type 2 fabric, including down-plunge stretching lineations and opposing shear sense indicators support previous interpretations of transpressional deformation within the LEG and metasedimentary rocks sheared along its western margin. With the new data describing shear zone kinematics in the LEG, we interpret that the type 1 fabric formed prior to suturing of the Wyoming and Superior Cratons and the type 2 fabric formed during craton suturing.
more »
« less
Deformation kinematics and temperature conditions of the Dakota Tectonic Zone within the Little Elk Granite in the Black Hills near Nemo, South Dakota
Precambrian rocks in the Black Hills record multiple tectonic processes, including suturing of the Wyoming and Superior cratons from ca. 1.740-1.715 Ga. To date, studies focused on this suturing event have primarily focused on prograde metamorphism and structures that record shortening between the cratonic blocks. However, a strike-slip shear zone subparallel to the shortening structures, named the Dakota Tectonic Zone (DTZ), has also been documented but is poorly understood. We examined intracrystalline deformation and associated microstructures in oriented thin sections of the Little Elk Granite (2.560 Ga mylonitized augen gneiss) within the mapped domain of the DTZ to further document how the strike-slip deformation fits into the Precambrian structural evolution of the Black Hills. At the outcrop scale, the Little Elk Granite contains two types of fabrics. Fabric type 1 is an augen gneiss fabric characterized by alignment of ~1-5 cm K-feldspar crystals that is interpreted to have formed during emplacement of the Little Elk Granite. Fabric type 2 cross-cuts the augen gneiss fabric and is characterized by comminution of the large K-feldspar grains within mylonitic shear zones. Whereas the type 1 fabric is folded throughout the field area, the type 2 shear fabric is consistently oriented at ~150/70°SW and contains a down-dip stretching lineation. Oriented thin sections cut perpendicular to foliation and parallel to lineation contain broken feldspar crystals that in some cases also exhibit undulose extinction. Domains between paired fragments of broken feldspar crystals are filled in with equant polycrystalline quartz aggregates and are regularly oriented at a high angle (>45°) to the shear foliation. Quartz-rich domains in the type 2 fabric generally display undulose extinction and dynamic recrystallization textures. Kinematic indicators from asymmetric strain shadows associated with feldspar porphyroclasts and asymmetrically folded micas yield dominantly top-to-the-left sense of motion, but top-to-the-right shear sense is also common (46%). The sum of microstructural data from the Little Elk Granite suggests that the DTZ is an upper greenschist facies (~300-450°C) left-lateral pure shear dominated transpression zone that likely formed late in the suturing of the Wyoming and Superior cratons.
more »
« less
- Award ID(s):
- 2050697
- PAR ID:
- 10508866
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Volume:
- 56
- Issue:
- 4
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The Black Hills, in western South Dakota and eastern Wyoming, were formed by Laramide orogeny deformation that was focused on a Precambrian suture along the eastern margin of the Wyoming Craton. Uplift of the Black Hills primarily took place by the development of monoclines on the eastern and western flanks of the Black Hills: west-vergent monoclines in the west and east-vergent monoclines in the east. Although the exhumed metamorphic core of the Black Hills contains abundant Precambrian structures that could have been reactivated during the Laramide orogeny, it remains unclear if the monoclines formed above reactivated basement structures. We present new balanced cross section modeling focused on the White Gates monocline along the eastern margin of the Black Hills to test whether it records reactivation of Precambrian basement structures. To better determine the geometry of the White Gates Monocline, we collected 22 bedding attitude measurements from the upper Deadwood and the Pahasapa formations along an 1,723-meter-long transect across the strike of the fold axis. We forward modeled monocline development related to slip on blind thrusts in three dip orientations: 30° (Andersonian thrust fault), 45° (maximum resolved shear stress), and 70° (orientation of nearby basement fabrics). Preliminary model results reveal that the 70° fault dip angle produces fold geometries most consistent with the geometry of the White Gate monocline. This result suggests that the reactivation of Precambrian fabrics during the Laramide orogeny influenced the formation of the White Gates monocline. Elsewhere in the Precambrian core of the Black Hills, conjugate thrust faults inferred to be Laramide age clearly cross-cut basement fabrics, which suggests that the role of structural reactivation in Laramide deformation varies spatially throughout the Black Hills.more » « less
-
none (Ed.)An obstacle to developing a general mechanical framework for magma mush is the emergence and complexity of a crystal fabric. To illuminate the conditions that produce a crystal fabric we performed time-dependent numerical simulations using a Computational-Fluid-Dynamics and Discrete-Element-Method (CFD-DEM) model in three dimensions. The specific focus was on the role of shear strain in the creation of a preferential orientation of crystals in mush. CFD-DEM method allows for the simultaneous coupling and frictional interactions of melt and crystals undergoing shear strain. The crystal shapes are represented using spheroids (either oblate or prolate). Simulations consist in imposing a compression stress (pressure) and a simple shear to a dense suspension of crystals in a viscous liquid, and monitoring the evolution of the orientation and strength of the shape fabrics. We ran a series of simulations by varying the size and aspect ratio of the particles. We considered samples in which all the solids have the same volume and shape, and cases including size and aspect ratio distributions. Results show that the strength of the shape fabric and the angle between the crystal preferential orientation and the compression plane both increase with the shear strain up to steady state values, which are primarily controlled by the aspect ratio of the particles. The stronger the aspect ratio, the greater the magnitude of the preferential orientation and the lower its angle relative to the compression plane. When introducing a distribution in the size of the crystals, we observed a decrease in the strength of the shape fabric and an increase in the angle between the preferential orientation of the crystals and the compression plane compared with samples composed of crystals having the same shape and size. Similarly, the distribution in the aspect ratio further decreases the strength of the shape fabric and increases the angle between the preferential orientation of the solids and the compression plane. Finally, we employed an alternative approach to quantify the amount of foliation and lineation and show that the samples always display a stronger foliation than lineation, although the shear strain increases both the foliation and the lineation.more » « less
-
Abstract Fluids are commonly invoked as the primary cause for weakening of detachment shear zones. However, fluid-related mechanisms such as pressure-solution, reaction-enhanced ductility, reaction softening and precipitation of phyllosilicates are not fully understood. Fluid-facilitated reaction and mass transport cause rheological weakening and strain localization, eventually leading to departure from failure laws derived in laboratory experiments. This study focuses on the Miocene Raft River detachment shear zone in northwestern Utah. The shear zone is localized in the Proterozoic Elba Quartzite, which unconformably overlies the Archaean basement, and consists of an alternating sequence of quartzite and muscovite-quartzite schist. In this study, we characterize fluid-related microstructures to constrain conditions that promoted brittle failure in a plastically deforming shear zone. Thin-section analyses reveal the presence of healed microcracks, transgranular fluid inclusion planes and grain boundary fluid inclusion clusters. Healed microcracks occur in three sets, one sub-perpendicular to the mylonitic foliation, and a set of two conjugate microcracks oriented at ∼40–60° to the mylonitic foliation. Healed microfractures are filled with quartz, which has a distinct fabric, suggesting that microcracks healed while the shear zone was still at conditions favourable for quartz crystal plasticity. Transgranular fluid inclusion planes also occur in three sets, similar in orientation to the healed microfractures. Fluid inclusions commonly decorate grain and subgrain boundaries as inter- and intragranular clusters. Our results document ductile overprint of brittle microstructures, suggesting that, during exhumation, the Raft River detachment shear zone crossed the brittle–ductile transition repeatedly, providing pathways for fluids to permeate through this shear zone.more » « less
-
Abstract Seismic anisotropy is controlled by aligned rock‐forming minerals, which most studies attribute to solid‐state shear with less consideration for magmatic fabric in plutonic rocks (rigid‐body rotation of crystals in the presence of melt). Our study counters this traditional solid‐state bias by evaluating contributions from fossil magmatic fabric. We collected samples from various tectonic settings, identified mineral orientations with electron backscatter diffraction and neutron diffraction, and calculated their bulk rock elastic properties. Results indicate that magmatic fabric may lead to moderate to strong anisotropy (3%–9%), comparable to solid‐state deformation. Also, magmatically aligned feldspar may cause foliation‐perpendicular fast velocity, a unique orientation that contrasts with a fast foliation typical of solid‐state deformation. Therefore, magmatic fabric may be more relevant to seismic anisotropy than previously recognized. Accordingly, increased considerations of magmatic fabric in arcs, batholiths, and other tectonic settings can change and potentially improve the prediction, observation, and interpretation of crustal seismic anisotropy.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
