Geology topics

Vera Schulte-Pelkum

Publications and source records attributed to Vera Schulte-Pelkum.

5 recordsLinked to original sources

USGS2024

Seismicity and anisotropic imaging reveal an active detachment beneath the northern Alaska Range foothills

North of the Denali Fault, the collision between the Yakutat block with North America is accommodated by a fold-thrust belt giving rise to the northern Alaska Range foothills. At the western end, the Kantishna Hills anticline hosts prominent microseismicity and surface deformation, interpreted as active folding of the Kantishna Hills anticline above a midcrustal detachment. We test for this detachment by using anisotropy-aware receiver functions to image fabric contrasts within the crust in context with seismicity. Seismic stations near the crest of the Kantishna Hills anticline and near its southern flank show a single strong contrast in dipping fabric at depths of 12–13 km near microseismicity clustering depths, consistent with a detachment plane beneath the fold. A minimum b -value at 10–13 km depth is consistent with seismicity on the detachment, compatible with the imaged anisotropic contrast, while off-fault seismicity is shallower and deeper with smaller magnitudes. South-dipping imbricate thrusts in schist characterize the northern Alaska Range foothills structure. This supports our interpretation of the observed anisotropy as reflecting SSW–SSE-dipping foliation above a detachment at ∼10–13 km depth that exploits existing crustal weaknesses along subtle fabric contrasts observed in the seismically quiescent region north of the actively deforming belt.

Alaska↗

USGS2024

Fault-dike-magma interactions inferred from transcrustal conical structures under Akutan volcano

Volcano monitoring and eruption forecasting requires accurate characterization of transcrustal magmatic structures in order to place volcanic unrest in context within the system where it occurs. Structural imaging using local seismicity is limited to seismogenic depths. Here, we exploit arrivals in teleseismic receiver functions that change polarity with backazimuth to image two surfaces beneath Akutan volcano in the Aleutian arc. The two surfaces delineate an upper to midcrustal inverted conical volume that deepens and thickens away from the volcanic center, with thicknesses of 3 to 13 km. The top of the volume is at depths of 2-3 km below sea level at distances of ~5-15 km from the caldera center. The bottom is at depths of 7-15 km at the same distances, and the cone’s thickness increases outward from ~5 to ~10 km depth. The signal is best fit by a volume with anisotropy with fast symmetry planes that dip outwards from the center and downward increases in shear velocity at both interfaces. The upper boundary coincides with the top of Akutan’s volcano-tectonic (VT) seismogenic zone, with the VT seismicity exhibiting outward dipping planar features matching the anisotropic fast plane orientation within the volume. The bottom of the anisotropic volume is below the termination depth of the majority of the VT seismicity and is therefore likely associated with the brittle-ductile transition. Long-period (LP) events associated previously with magma movement are concentrated below the anisotropic VT volume. Because of the strong spatial association with VT seismicity, we interpret the volume as consisting of concentric outward dipping faults/dikes that align the seismogenic response to stress changes from magmatic processes. Our observations map this volume independent of the present-day seismicity distribution and thus provide a spatially more complete image of the magmatic system.

Alaska↗

USGS2022

Don't judge an orogen by its cover: Kinematics of the Appalachian décollement from seismic anisotropy

As North America collided with Africa to form Pangea during the Alleghanian orogeny, crystalline and sedimentary rocks in the southeastern United States were thrust forelandward along the Appalachian décollement. We examined Ps receiver functions to better constrain the kinematics of this prominent subsurface structure. From Southeastern Suture of the Appalachian Margin Experiment (SESAME) and other EarthScope stations on the Blue Ridge–Piedmont crystalline megathrust, we find large arrivals from a 5–10-km-deep converter. We argue that a strong contrast in dipping anisotropic foliation occurs at the subhorizontal Appalachian décollement, and propose that such a geometry may be typical for décollement structures. Conversion polarity flips can be explained by an east-dipping foliation, but this orientation is at odds with the overlying northeast-trending surface tectonic grain. We suggest that prior to late Alleghanian northwest-directed head-on collision, the Appalachian décollement accommodated early Alleghanian west-vergence, independent of the overlying Blue Ridge–Piedmont structural inheritance. The geophysical expression of dipping anisotropic foliation provides a powerful tool for investigating subsurface kinematics, especially where they are obscured by overlying fabric, to disentangle the tectonic complexities that embody oblique collisional orogens.

Alabama, Georgia, North Carolina, South Carolina, ↗

USGS2022

From crystals to crustal-scale seismic anisotropy: Bridging the gap between rocks and seismic studies with digital geologic map data in Colorado

Deep continental crustal structures are enigmatic due to lack of direct exposures and limited tools to investigate them remotely. Seismic waves can sample these rocks, but most seismic methods focus on coarse crustal structures while laboratory measurements concentrate on crystal-scale rock properties, and little work has been conducted to bridge this interpretation gap. In some places, geologic maps of crystalline basement provide samples of the intermediate-scale fabrics and structures that may represent in situ deep crust. However, previous research has not considered natural geometric variations from map data, nor is this heterogeneity typically included in map-scale seismic property calculations. Here, we test how map-scale fabrics influence crustal seismic anisotropy in Colorado by analyzing structural data from geologic maps, combining those data with bulk rock elastic tensors to calculate map-scale seismic properties, and evaluating the resulting comparisons with observed receiver function A1 (360° periodic) arrivals. Crystalline fabrics, predicted seismic properties, and tectonic structures positively correlate with shallow and deep crustal A1 arrivals. Additionally, widespread correlations occur between mapped fault traces and regional foliations, implying that preexisting mechanical heterogeneity may have strongly influenced subsequent reactivation. We interpret that various mapped geologic contact types (e.g., lithologic and structural) generate A1 arrivals and that multiple parallel features (e.g., faults, foliations, and intrusions) contribute to a seismically visible tectonic grain. Therefore, Colorado's exhumed basement, as expressed in outcrops and maps, offers insight into modern deep crustal geological and geophysical structure.

Colorado↗

USGS2020

Imaging the tectonic grain of the Northern Cordillera orogen using Transportable Array receiver functions

Azimuthal variations in receiver function conversions can image lithospheric structural contrasts and anisotropic fabrics that together compose tectonic grain. We apply this method to data from EarthScope Transportable Array in Alaska and additional stations across the northern Cordillera. The best‐resolved quantities are the strike and depth of dipping fabric contrasts or interfaces. We find a strong geographic gradient in such anomalies, with large amplitudes extending inboard from the present‐day subduction margin, the Aleutian arc, and an influence of flat‐slab subduction of the Yakutat microplate north of the Denali fault. An east–west band across interior Alaska shows low‐amplitude crustal anomalies. Anomaly amplitudes correlate with structural intensity (density of aligned geological elements), but are the highest in areas of strong Cenozoic deformation, raising the question of an influence of current stress state. Imaged subsurface strikes show alignment with surface structures. We see concentric strikes around arc volcanoes implying dipping magmatic structures and fabric into the middle crust. Regions with present‐day weaker deformation show lower anomaly amplitudes but structurally aligned strikes, suggesting pre‐Cenozoic fabrics may have been overprinted or otherwise modified. We observe general coherence of the signal across the brittle‐plastic transition. Imaged crustal fabrics are aligned with major faults and shear zones, whereas intrafault blocks show imaged strikes both parallel to and at high angles to major block‐bounding faults. High‐angle strikes are subparallel to neotectonic deformation, seismicity, fault lineaments, and prominent metallogenic belts, possibly due to overprinting and/or co‐evolution with fault‐parallel fabrics. We suggest that the underlying tectonic grain in the northern Cordillera is broadly distributed rather than strongly localized. Receiver functions thus reveal key information about the nature and continuity of tectonic fabrics at depth and can provide unique insights into the deformation history and distribution of regional strain in complex orogenic belts.

Alaska↗