In 2010 an array of 834 single-component geophones was deployed across the Bighorn Mountain Range... more In 2010 an array of 834 single-component geophones was deployed across the Bighorn Mountain Range in northern Wyoming as part of the Bighorn Arch Seismic Experiment (BASE). The goal of this deployment was to test the capabilities of these instruments as recorders of passive-source observations in addition to active-source observations for which they are typically used. The results are quite promising, having recorded 47 regional and teleseismic earthquakes over a two-week deployment. These events ranged from magnitude 4.1 to 7.0 (m b) and occurred at distances up to 10 •. Because these instruments were deployed at ca. 1000 m spacing we were able to resolve the geometries of two major basins from the residuals of several well-recorded teleseisms. The residuals of these arrivals, converted to basinal thickness, show a distinct westward thickening in the Bighorn Basin that agrees with industry-derived basement depth information. Our estimates of thickness in the Powder River Basin do not match industry estimates in certain areas, likely due to localized high-velocity features that are not included in our models. Thus, with a few cautions, it is clear that high-density single-component passive arrays can provide valuable constraints on basinal geometries, and could be especially useful where basinal geometry is poorly known.
ABSTRACT The core of the northern Teton Range is composed of an Archean gneiss complex consisting... more ABSTRACT The core of the northern Teton Range is composed of an Archean gneiss complex consisting of heterogeneous layered gneiss, interleaved homogeneous granitic gneisses, and cross-cutting Mount Owen Quartz Monzomte. Diverse whole rock chemistry and rare earth element (REE) suggest the layered gneiss has a mixed supracrustal and igneous origin. The homogeneous granitic Webb Canyon gneiss and augen gneiss are petrographically and chemically distinct units. REE patterns suggest derivation as partial melts of pre-existing felsic crust. Amphibolitic lenses contained within all gneissic units have flat REE patterns with low total values, suggesting derivation from the shallow mantle. The gneissic units are dominated by amphibolite facies assemblages with garnet-biotite temperatures averaging 600° C. Areas of relict granulite facies assemblages with two pyroxenes, garnet and rare kyanite indicate a previous granuhte facies event. The kyanite occurs as relict grains with cordierite reaction rims, suggesting a minimum pressure of 8 kilobars, which is anomalously high for Archean terranes. The northern Teton Range has undergone at least three major Precambrian deformations. Compositional layering in the layered gneiss is probably a combination of original bedding and subsequent metamorphic segregation, complicated by rootless isoclinal folding of the first deformation event (Di). This fabric was overprinted and folded during a second deformation, which resulted in isoclinal to open folding of various scales, transposition of (Di) foliations and formation of a secondary amphibolite facies foliation. (D2) mineral growth lineations parallel mineral stretching lineations and recumbent fold axes, suggesting that (D2) was characterized by sheath folding concurrent with retrograde metamorphism. The subsequent, post-orogenic intrusion of the Mount Owen Quartz Monzonite and satellite pegmatites was followed by the last major deformation, a greenschist facies shearing event, which formed mylonite zones as much as 45 meters wide.
ABSTRACT We present high-density, high-resolution receiver function (RF) images of the Bighorn Mo... more ABSTRACT We present high-density, high-resolution receiver function (RF) images of the Bighorn Mountains of north central Wyoming, to gain insight into the subsurface seismic structures of the range, as part of the Bighorn Arch Seismic Experiment (BASE). Our data set contains over 220 three component seismic stations in the Bighorns region, in some areas with spacing less than 5 km. BASE is a Flexible Array experiment integrated with Earthscope. In order to investigate the Bighorns, a large-scale deployment of seismic instrumentation was deployed in the summers of 2009 and 2010. This included 38 broadband and 172 short period seismic stations, as well as both passive and active source `Texan' deployments. Stations were placed to both densify the already present Transportable Array network as well as to create 5 linear transects. Station spacing along these transects range from four to ten kilometers, crossing the Bighorn Basin, through the Bighorn Arch, and into the Powder River Basin. The main objective of the BASE project is to better understand the tectonic processes involved in the formation of basement-cored arches. The formation of these structures remains a key unsolved tectonic problem. The Bighorn Mountains are an archetype of basement-involved foreland arches and therefore act as an excellent setting for the investigation of these types of structures. Four main formation models have been proposed for the Bighorns, each with unique crustal structures. Through a complete structural analysis of the range, relying heavily on seismic subsurface imaging, it will be possible to determine which of these models best fit observations. Moho topography is a crucial component in supporting these hypotheses, and should be well resolved with RF imaging. In this study P-S wave RFs are used to image the structures beneath the Bighorn Mountains. We present ideas for modeling and filtering approaches to dampen low velocity sedimentary layer reverberations in the Powder River and Bighorn basins, which can mask deeper structure (a problem increasingly affecting EarthScope seismic data as deployments move eastwards). Due to the large number of three component stations placed along transects, Common Conversion Point (CCP) stacking of the receiver function gives a high-resolution 2D slice of the crustal structure. Constraints from the active source `Texan' experiment provide independent P and S wave velocities and allow for a more accurate structural depth estimates in CCP images. Our results will be used to generate a Moho map of the Bighorn region which will be input into a 4D (3D geometry + time) model for foreland arch formation.
Page 1. Click to view Poster Panel in PDF format (1.96 MB) Click to view Map in PDF format (5.77 ... more Page 1. Click to view Poster Panel in PDF format (1.96 MB) Click to view Map in PDF format (5.77 MB) PS Two-Stage Mechanical Stratigraphy and Extensional Fracturing in the Wind River Basin, Wyoming* Ryan C. Thompson 1 and Eric Erslev 1 ...
Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Con... more Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages.
An x-ray fluorescence macroprobe was built for intermediate-scale compositional mapping to bridge... more An x-ray fluorescence macroprobe was built for intermediate-scale compositional mapping to bridge the gap in spatial resolution between bulk x-ray fluorescence and electron beam methods. The macroprobe was optimized for quantitative whole rock mapping on a millimeter scale to evaluate changes in bulk composition of fine-grained mineral aggregates.
In 2010 an array of 834 single-component geophones was deployed across the Bighorn Mountain Range... more In 2010 an array of 834 single-component geophones was deployed across the Bighorn Mountain Range in northern Wyoming as part of the Bighorn Arch Seismic Experiment (BASE). The goal of this deployment was to test the capabilities of these instruments as recorders of passive-source observations in addition to active-source observations for which they are typically used. The results are quite promising, having recorded 47 regional and teleseismic earthquakes over a two-week deployment. These events ranged from magnitude 4.1 to 7.0 (m b) and occurred at distances up to 10 •. Because these instruments were deployed at ca. 1000 m spacing we were able to resolve the geometries of two major basins from the residuals of several well-recorded teleseisms. The residuals of these arrivals, converted to basinal thickness, show a distinct westward thickening in the Bighorn Basin that agrees with industry-derived basement depth information. Our estimates of thickness in the Powder River Basin do not match industry estimates in certain areas, likely due to localized high-velocity features that are not included in our models. Thus, with a few cautions, it is clear that high-density single-component passive arrays can provide valuable constraints on basinal geometries, and could be especially useful where basinal geometry is poorly known.
Structural models for Rocky Mountain foreland uplifts should be restorable to a continuous mosaic... more Structural models for Rocky Mountain foreland uplifts should be restorable to a continuous mosaic of basement blocks overlain by planar sedimentary beds. Line-length balancing of sedimentary strata shows that basement-cored anticlines are the result of shortening by underlying thrust or reverse faults. Once the effects of layer-parallel extension and relative tilt are subtracted, fault dips can be determined by calculating the vertical displacement and horizontal shortening indicated by the folds. Computer-generated downplunge projects show that basement motion can be closely approximated as translations and rotations on curvilinear fault surfaces without major penetrative deformation. Gradual changes in dip on the backside of many foreland uplifts suggest uniformly curved faults, not sharply kinked ramp-flat geometries. The relative tilt between individual basement blocks can be combined with fault-slip estimates to calculate the average curvature of intervening faults. These geometric constraints result in predictive models for individual foreland uplifts, which can be used to locate and evaluate potential structural traps for oil and gas. The extent of a basement overhang can be calculated from one fault attitude (calculated or observed) and the curvature on that fault. Basement balancing suggests that the extended zone in the crest of many foreland uplifts may correspond to themore » extent of the overhang. Regional block models predict a zone of smaller scale, potentially oil-trapping structures on the backside of large foreland uplifts.« less
ABSTRACT The Bighorns Arch Seismic Experiment (BASE) is a Flexible Array experiment integrated wi... more ABSTRACT The Bighorns Arch Seismic Experiment (BASE) is a Flexible Array experiment integrated with EarthScope. The goal of BASE is to develop a better understanding of how basement-involved foreland arches form and what their link is to plate tectonic processes. To achieve this goal, the crustal structure under the Bighorn Mountain range, Bighorn Basin, and Powder River Basin of northern Wyoming and southern Montana are investigated through the deployment of 35 broadband seismometers, 200 short period seismometers, 1600 ``Texan'' instruments using active sources and 800 ``Texan'' instruments monitoring passive sources, together with field structural analysis of brittle structures. The novel combination of these approaches and anticipated simultaneous data inversion will give a detailed structural crustal image of the Bighorn region at all levels of the crust. Four models have been proposed for the formation of the Bighorn foreland arch: subhorizontal detachment within the crust, lithospheric buckling, pure shear lithospheric thickening, and fault blocks defined by lithosphere-penetrating thrust faults. During the summer of 2009, we deployed 35 broadband instruments, which have already recorded several magnitude 7+ teleseismic events. Through P wave receiver function analysis of these 35 stations folded in with many EarthScope Transportable Array stations in the region, we present a preliminary map of the Mohorovicic discontinuity. This crustal map is our first test of how the unique Moho geometries predicted by the four hypothesized models of basement involved arches fit seismic observations for the Bighorn Mountains. In addition, shear-wave splitting analysis for our first few recorded teleseisms helps us determine if strong lithospheric deformation is preserved under the range. These analyses help lead us to our final goal, a complete 4D (3D spatial plus temporal) lithospheric-scale model of arch formation which will advance our understanding of the mechanisms accommodating and driving basement-involved arch formation as well as continental lithospheric rheology.
ABSTRACT Whereas basement-involved foreland arches, such as the Bighorn Arch in north-central Wyo... more ABSTRACT Whereas basement-involved foreland arches, such as the Bighorn Arch in north-central Wyoming, are typical of Laramide-style orogenesis, the mode of arch shortening at depth remains unresolved due to lack of geophysical imaging. Current hypotheses for lithospheric geometries and kinematics across the Bighorn Arch predict distinctly different lower crustal deformation and Moho topography. In order to determine the mode of arch shortening the 2010 Bighorn Arch Seismic Experiment (BASE) was designed to image the crust and mantle below the Bighorn Arch, measuring crustal velocity and thickness and identifying large-scale structures. Here, we present two-dimensional P-wave velocity models of the crust and upper mantle from an active-source wide-angle reflection and refraction survey conducted as part of BASE. Twenty one seismic shots recorded on ~1800 4.5 Hz vertical component geophones and 'Texan' dataloggers deployed in one east-west profile and one north-south profile resulted in ~15,000 total travel times available for inversion. The north-south profile lies on the western flank of the arch and crosses its southern extension. The velocity model from the profile shows little structural variation. Rather, crustal velocities on this profile are laterally continuous in the upper crust, with mantle velocities (>7.8 km/s) at ~50 km depth below surface elevation. The east-west profile is sub-parallel to the direction of contraction across the mountains. Low velocities (~2.8-4.2 km/s) in the foreland basins on either side of the arch within the upper ~5-10 km correlate with known basin geometries. Low-velocity zones (~5.2 km/s) within the upper 20 km of the crust locally coincide with known and predicted large-scale fault zones. These zones provide targets for kinematic modeling and reconstruction efforts. The vertical velocity gradient increases on both profiles at ~25 km depth, which we interpret as a mid-crustal transition associated with compositional changes within the crust. Above this mid-crustal transition, the east-west profile shows thickening of the upper crust under the arch culmination. Crustal thickness across the arch, constrained by PmP and Pn phase arrivals on the E-W profile, varies between ~45 km and ~50 km. The modeled Moho topography shows no major offsets and only minor deflections directly below the arch. The absence of large scale offsets of the Moho and the evidence for upper crustal thickening favors arch formation by crustal detachment, consistent with structural modeling suggesting listric thrusting on a west-dipping master fault that flattens at ~30 km depth. Integration with passive source results from BASE, including receiver function analysis (see Yeck et al.) and tomography (see O'Rourke et al.) will further constrain upper mantle structure and three-dimensional Moho topography in the region.
Abstract One clear distinction between thin-skinned thrust belts and the basement-cored structure... more Abstract One clear distinction between thin-skinned thrust belts and the basement-cored structures of the Rocky Mountain uplifts is the greater variability of thrust vergence in Rocky Mountain uplifts. In thin-skinned thrusting, nearly all the major thrusts verge toward the stable craton. This uniform polarity of thrusting is probably the result of a combination of preferential sliding on hinterland-dipping beds and topography-induced gravity spreading, factors of more limited importance in foreland uplifts. Important back-thrusting occurs in triangle zones at multiple scales in basement-cored foreland uplifts. Small-scale back thrusts occur in the sedimentary rocks in front of basement overhangs to facilitate the transition from basement block motion to flexural slip in the sedimentary cover. These can be seen on both sides of the west-vergent Rattlesnake Mountain and Forellen structures, which are actually larger-scale back-thrusts of opposite polarity. Increasing amounts of back-thrusting, commonly on several spaced structures (e.g., northwestern Beartooth and northeastern Front Range), reduces the slip on the master thrust, resulting in the termination of the master thrust in the basement. The southwestern vergence of the Wind River and northern Front Range suggests that their master thrusts are actually backthrusts off a mid-crustal detachment. Crustal wedging and basement involvement provide additional complexities to basement-cored structures that must be addressed in analytical and regional models of foreland uplifts.
In 2010 an array of 834 single-component geophones was deployed across the Bighorn Mountain Range... more In 2010 an array of 834 single-component geophones was deployed across the Bighorn Mountain Range in northern Wyoming as part of the Bighorn Arch Seismic Experiment (BASE). The goal of this deployment was to test the capabilities of these instruments as recorders of passive-source observations in addition to active-source observations for which they are typically used. The results are quite promising, having recorded 47 regional and teleseismic earthquakes over a two-week deployment. These events ranged from magnitude 4.1 to 7.0 (m b) and occurred at distances up to 10 •. Because these instruments were deployed at ca. 1000 m spacing we were able to resolve the geometries of two major basins from the residuals of several well-recorded teleseisms. The residuals of these arrivals, converted to basinal thickness, show a distinct westward thickening in the Bighorn Basin that agrees with industry-derived basement depth information. Our estimates of thickness in the Powder River Basin do not match industry estimates in certain areas, likely due to localized high-velocity features that are not included in our models. Thus, with a few cautions, it is clear that high-density single-component passive arrays can provide valuable constraints on basinal geometries, and could be especially useful where basinal geometry is poorly known.
ABSTRACT The core of the northern Teton Range is composed of an Archean gneiss complex consisting... more ABSTRACT The core of the northern Teton Range is composed of an Archean gneiss complex consisting of heterogeneous layered gneiss, interleaved homogeneous granitic gneisses, and cross-cutting Mount Owen Quartz Monzomte. Diverse whole rock chemistry and rare earth element (REE) suggest the layered gneiss has a mixed supracrustal and igneous origin. The homogeneous granitic Webb Canyon gneiss and augen gneiss are petrographically and chemically distinct units. REE patterns suggest derivation as partial melts of pre-existing felsic crust. Amphibolitic lenses contained within all gneissic units have flat REE patterns with low total values, suggesting derivation from the shallow mantle. The gneissic units are dominated by amphibolite facies assemblages with garnet-biotite temperatures averaging 600° C. Areas of relict granulite facies assemblages with two pyroxenes, garnet and rare kyanite indicate a previous granuhte facies event. The kyanite occurs as relict grains with cordierite reaction rims, suggesting a minimum pressure of 8 kilobars, which is anomalously high for Archean terranes. The northern Teton Range has undergone at least three major Precambrian deformations. Compositional layering in the layered gneiss is probably a combination of original bedding and subsequent metamorphic segregation, complicated by rootless isoclinal folding of the first deformation event (Di). This fabric was overprinted and folded during a second deformation, which resulted in isoclinal to open folding of various scales, transposition of (Di) foliations and formation of a secondary amphibolite facies foliation. (D2) mineral growth lineations parallel mineral stretching lineations and recumbent fold axes, suggesting that (D2) was characterized by sheath folding concurrent with retrograde metamorphism. The subsequent, post-orogenic intrusion of the Mount Owen Quartz Monzonite and satellite pegmatites was followed by the last major deformation, a greenschist facies shearing event, which formed mylonite zones as much as 45 meters wide.
ABSTRACT We present high-density, high-resolution receiver function (RF) images of the Bighorn Mo... more ABSTRACT We present high-density, high-resolution receiver function (RF) images of the Bighorn Mountains of north central Wyoming, to gain insight into the subsurface seismic structures of the range, as part of the Bighorn Arch Seismic Experiment (BASE). Our data set contains over 220 three component seismic stations in the Bighorns region, in some areas with spacing less than 5 km. BASE is a Flexible Array experiment integrated with Earthscope. In order to investigate the Bighorns, a large-scale deployment of seismic instrumentation was deployed in the summers of 2009 and 2010. This included 38 broadband and 172 short period seismic stations, as well as both passive and active source `Texan' deployments. Stations were placed to both densify the already present Transportable Array network as well as to create 5 linear transects. Station spacing along these transects range from four to ten kilometers, crossing the Bighorn Basin, through the Bighorn Arch, and into the Powder River Basin. The main objective of the BASE project is to better understand the tectonic processes involved in the formation of basement-cored arches. The formation of these structures remains a key unsolved tectonic problem. The Bighorn Mountains are an archetype of basement-involved foreland arches and therefore act as an excellent setting for the investigation of these types of structures. Four main formation models have been proposed for the Bighorns, each with unique crustal structures. Through a complete structural analysis of the range, relying heavily on seismic subsurface imaging, it will be possible to determine which of these models best fit observations. Moho topography is a crucial component in supporting these hypotheses, and should be well resolved with RF imaging. In this study P-S wave RFs are used to image the structures beneath the Bighorn Mountains. We present ideas for modeling and filtering approaches to dampen low velocity sedimentary layer reverberations in the Powder River and Bighorn basins, which can mask deeper structure (a problem increasingly affecting EarthScope seismic data as deployments move eastwards). Due to the large number of three component stations placed along transects, Common Conversion Point (CCP) stacking of the receiver function gives a high-resolution 2D slice of the crustal structure. Constraints from the active source `Texan' experiment provide independent P and S wave velocities and allow for a more accurate structural depth estimates in CCP images. Our results will be used to generate a Moho map of the Bighorn region which will be input into a 4D (3D geometry + time) model for foreland arch formation.
Page 1. Click to view Poster Panel in PDF format (1.96 MB) Click to view Map in PDF format (5.77 ... more Page 1. Click to view Poster Panel in PDF format (1.96 MB) Click to view Map in PDF format (5.77 MB) PS Two-Stage Mechanical Stratigraphy and Extensional Fracturing in the Wind River Basin, Wyoming* Ryan C. Thompson 1 and Eric Erslev 1 ...
Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Con... more Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages.
An x-ray fluorescence macroprobe was built for intermediate-scale compositional mapping to bridge... more An x-ray fluorescence macroprobe was built for intermediate-scale compositional mapping to bridge the gap in spatial resolution between bulk x-ray fluorescence and electron beam methods. The macroprobe was optimized for quantitative whole rock mapping on a millimeter scale to evaluate changes in bulk composition of fine-grained mineral aggregates.
In 2010 an array of 834 single-component geophones was deployed across the Bighorn Mountain Range... more In 2010 an array of 834 single-component geophones was deployed across the Bighorn Mountain Range in northern Wyoming as part of the Bighorn Arch Seismic Experiment (BASE). The goal of this deployment was to test the capabilities of these instruments as recorders of passive-source observations in addition to active-source observations for which they are typically used. The results are quite promising, having recorded 47 regional and teleseismic earthquakes over a two-week deployment. These events ranged from magnitude 4.1 to 7.0 (m b) and occurred at distances up to 10 •. Because these instruments were deployed at ca. 1000 m spacing we were able to resolve the geometries of two major basins from the residuals of several well-recorded teleseisms. The residuals of these arrivals, converted to basinal thickness, show a distinct westward thickening in the Bighorn Basin that agrees with industry-derived basement depth information. Our estimates of thickness in the Powder River Basin do not match industry estimates in certain areas, likely due to localized high-velocity features that are not included in our models. Thus, with a few cautions, it is clear that high-density single-component passive arrays can provide valuable constraints on basinal geometries, and could be especially useful where basinal geometry is poorly known.
Structural models for Rocky Mountain foreland uplifts should be restorable to a continuous mosaic... more Structural models for Rocky Mountain foreland uplifts should be restorable to a continuous mosaic of basement blocks overlain by planar sedimentary beds. Line-length balancing of sedimentary strata shows that basement-cored anticlines are the result of shortening by underlying thrust or reverse faults. Once the effects of layer-parallel extension and relative tilt are subtracted, fault dips can be determined by calculating the vertical displacement and horizontal shortening indicated by the folds. Computer-generated downplunge projects show that basement motion can be closely approximated as translations and rotations on curvilinear fault surfaces without major penetrative deformation. Gradual changes in dip on the backside of many foreland uplifts suggest uniformly curved faults, not sharply kinked ramp-flat geometries. The relative tilt between individual basement blocks can be combined with fault-slip estimates to calculate the average curvature of intervening faults. These geometric constraints result in predictive models for individual foreland uplifts, which can be used to locate and evaluate potential structural traps for oil and gas. The extent of a basement overhang can be calculated from one fault attitude (calculated or observed) and the curvature on that fault. Basement balancing suggests that the extended zone in the crest of many foreland uplifts may correspond to themore » extent of the overhang. Regional block models predict a zone of smaller scale, potentially oil-trapping structures on the backside of large foreland uplifts.« less
ABSTRACT The Bighorns Arch Seismic Experiment (BASE) is a Flexible Array experiment integrated wi... more ABSTRACT The Bighorns Arch Seismic Experiment (BASE) is a Flexible Array experiment integrated with EarthScope. The goal of BASE is to develop a better understanding of how basement-involved foreland arches form and what their link is to plate tectonic processes. To achieve this goal, the crustal structure under the Bighorn Mountain range, Bighorn Basin, and Powder River Basin of northern Wyoming and southern Montana are investigated through the deployment of 35 broadband seismometers, 200 short period seismometers, 1600 ``Texan'' instruments using active sources and 800 ``Texan'' instruments monitoring passive sources, together with field structural analysis of brittle structures. The novel combination of these approaches and anticipated simultaneous data inversion will give a detailed structural crustal image of the Bighorn region at all levels of the crust. Four models have been proposed for the formation of the Bighorn foreland arch: subhorizontal detachment within the crust, lithospheric buckling, pure shear lithospheric thickening, and fault blocks defined by lithosphere-penetrating thrust faults. During the summer of 2009, we deployed 35 broadband instruments, which have already recorded several magnitude 7+ teleseismic events. Through P wave receiver function analysis of these 35 stations folded in with many EarthScope Transportable Array stations in the region, we present a preliminary map of the Mohorovicic discontinuity. This crustal map is our first test of how the unique Moho geometries predicted by the four hypothesized models of basement involved arches fit seismic observations for the Bighorn Mountains. In addition, shear-wave splitting analysis for our first few recorded teleseisms helps us determine if strong lithospheric deformation is preserved under the range. These analyses help lead us to our final goal, a complete 4D (3D spatial plus temporal) lithospheric-scale model of arch formation which will advance our understanding of the mechanisms accommodating and driving basement-involved arch formation as well as continental lithospheric rheology.
ABSTRACT Whereas basement-involved foreland arches, such as the Bighorn Arch in north-central Wyo... more ABSTRACT Whereas basement-involved foreland arches, such as the Bighorn Arch in north-central Wyoming, are typical of Laramide-style orogenesis, the mode of arch shortening at depth remains unresolved due to lack of geophysical imaging. Current hypotheses for lithospheric geometries and kinematics across the Bighorn Arch predict distinctly different lower crustal deformation and Moho topography. In order to determine the mode of arch shortening the 2010 Bighorn Arch Seismic Experiment (BASE) was designed to image the crust and mantle below the Bighorn Arch, measuring crustal velocity and thickness and identifying large-scale structures. Here, we present two-dimensional P-wave velocity models of the crust and upper mantle from an active-source wide-angle reflection and refraction survey conducted as part of BASE. Twenty one seismic shots recorded on ~1800 4.5 Hz vertical component geophones and 'Texan' dataloggers deployed in one east-west profile and one north-south profile resulted in ~15,000 total travel times available for inversion. The north-south profile lies on the western flank of the arch and crosses its southern extension. The velocity model from the profile shows little structural variation. Rather, crustal velocities on this profile are laterally continuous in the upper crust, with mantle velocities (>7.8 km/s) at ~50 km depth below surface elevation. The east-west profile is sub-parallel to the direction of contraction across the mountains. Low velocities (~2.8-4.2 km/s) in the foreland basins on either side of the arch within the upper ~5-10 km correlate with known basin geometries. Low-velocity zones (~5.2 km/s) within the upper 20 km of the crust locally coincide with known and predicted large-scale fault zones. These zones provide targets for kinematic modeling and reconstruction efforts. The vertical velocity gradient increases on both profiles at ~25 km depth, which we interpret as a mid-crustal transition associated with compositional changes within the crust. Above this mid-crustal transition, the east-west profile shows thickening of the upper crust under the arch culmination. Crustal thickness across the arch, constrained by PmP and Pn phase arrivals on the E-W profile, varies between ~45 km and ~50 km. The modeled Moho topography shows no major offsets and only minor deflections directly below the arch. The absence of large scale offsets of the Moho and the evidence for upper crustal thickening favors arch formation by crustal detachment, consistent with structural modeling suggesting listric thrusting on a west-dipping master fault that flattens at ~30 km depth. Integration with passive source results from BASE, including receiver function analysis (see Yeck et al.) and tomography (see O'Rourke et al.) will further constrain upper mantle structure and three-dimensional Moho topography in the region.
Abstract One clear distinction between thin-skinned thrust belts and the basement-cored structure... more Abstract One clear distinction between thin-skinned thrust belts and the basement-cored structures of the Rocky Mountain uplifts is the greater variability of thrust vergence in Rocky Mountain uplifts. In thin-skinned thrusting, nearly all the major thrusts verge toward the stable craton. This uniform polarity of thrusting is probably the result of a combination of preferential sliding on hinterland-dipping beds and topography-induced gravity spreading, factors of more limited importance in foreland uplifts. Important back-thrusting occurs in triangle zones at multiple scales in basement-cored foreland uplifts. Small-scale back thrusts occur in the sedimentary rocks in front of basement overhangs to facilitate the transition from basement block motion to flexural slip in the sedimentary cover. These can be seen on both sides of the west-vergent Rattlesnake Mountain and Forellen structures, which are actually larger-scale back-thrusts of opposite polarity. Increasing amounts of back-thrusting, commonly on several spaced structures (e.g., northwestern Beartooth and northeastern Front Range), reduces the slip on the master thrust, resulting in the termination of the master thrust in the basement. The southwestern vergence of the Wind River and northern Front Range suggests that their master thrusts are actually backthrusts off a mid-crustal detachment. Crustal wedging and basement involvement provide additional complexities to basement-cored structures that must be addressed in analytical and regional models of foreland uplifts.
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