Search for: All records

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. 

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. 

The North American Cordillera formed by protracted subduction that led to the accretion of multiple exotic terranes during the Mesozoic. Subduction and terrane accretion are recorded throughout the Cordillera by fault-bounded mélange belts exposed between disparate terranes. South of the Denali fault in central Alaska, the Reindeer Hills Mélange (RHM) consists of pervasively sheared carbonate, ultramafic, and sandstone blocks in a shale and chert-breccia matrix. The presence of these oceanic rock types and correlation with nearby Cretaceous flysch has led to the interpretation that the RHM formed by subduction of oceanic lithosphere during the Cretaceous. However, the age of the RHM and its genetic relationship to surrounding terranes remain unclear. New structural and kinematic analysis along a ~5 km across-strike transect through the RHM reveals a steeply N-dipping penetrative cleavage, and asymmetric sandstone blocks in the shale matrix record distributed top-to-the-south shear. Detrital zircon U-Pb geochronology of grains taken from a sandstone block at the southern end of the transect present a dominant population of Silurian-Devonian grains that yield a youngest statistical population maximum depositional age of 416 +/- 6 Ma. Abundant Proterozoic grains ranging from 900-2000 Ma permit sediment input from peri-Laurentian sources, yet a distinctive population of 1450-1500 Ma grains may suggest input from Baltica basement or other Baltica-derived terranes recognized in the Cordillera (e.g., Alexander, Farewell). The new age data, along with Silurian-Devonian fossils from limestone blocks in the mélange and our recognition of Triassic diabase dikes that crosscut the mélange fabric, suggest that deposition and imbrication of Reindeer Hills clastic sediments took place in the Paleozoic. The new U-Pb data, Triassic mafic dikes, and published displacement estimates for the Denali fault suggest that the RHM correlates with the Mirror Creek Formation northeast of the Denali Fault in western Yukon, Canada, and may also have a link to Silurian-Devonian igneous rocks in the Alexander terrane of southeast Alaska. Altogether, the preliminary data presented here suggest that the RHM provides a record of early Devonian(?) subduction spatially associated with other Baltica-derived Cordilleran terranes. 

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.