This three-year (2009-12) collaborative project integrates arch-scale upper crustal geometries from surface exposures and petroleum industry subsurface data (Eric Erslev, University of Wyoming; Christine Siddoway, Colorado College) with the results of a hybrid seismic experiment consisting of both passive and active components (BASE). The passive component of the seismic experiment consisted of a 1.25 year (2009-10) deployment of 38 broadband seismometers that densified the EarthScope transportable array (Megan Anderson, Colorado College), a 6.5 month deployment of 158 short period seismometers (Anne Sheehan, University of Colorado), and a 9 day deployment of 850 high frequency "Texan" seismometers (Kate Miller, Texas A&M University; Sheehan). Passive instruments were arrayed in a grid consisting of three SW-NE lines and two NW-SE lines. The active component consisted of 24 shots (summer 2010) recorded by the above broadband and short period instruments plus 1850 "Texan" seismometers deployed for 5 days (Steve Harder, University of Texas, El Paso; Miller). The 1850 Texans were deployed on the middle and west transect only. Joint inversion of active and passive results defines crustal and upper mantle velocities and interface structures within the Bighorn Arch. These new seismic results are integrated within a GIS-based, 3D geospatial framework including data from exposures, geologic maps and industry subsurface data for the study area. Kinematic information from fracture transects (Erslev, Siddoway) and gravity modeling (Miller) is used to guide 3D, lithosphere-scale structural restorations to test the compatibility of different components in our 3D geometric model.

left: Design of the BASE experiment on a shaded relief digital elevation model of the northern Rocky Mountains (BHM: Bighorn Mountains). Blue lines represent the 5 passive / active source transects, with active source shots marked by small red stars. Broadband seismic stations (green squares) infill the USArray grid (TA broadband stations, black triangles). Large red stars are source areas for mine blasts. Circles are epicenters for regional seismicity, 1973 to 2008.

• Receiver function images from passive source data show inline offsets in crustal layers (7.xx layer, Moho, and mantle layering) to the ESE of the Bighorn Arch.
• Passing source shear wave splitting measurements are affected by anisotropy in the mantle shear zone (if wide) ESE of the Bighorn arch.
• Active source data reveal offsets in layering of the crust, in the same relative position as seen in receiver functions, with possibility of a wide-angle reflection from the fault plane.
• 4D structural modeling balances upper crustal shortening and excess volume in the arch with offsets of 7.xx layer, Moho, and mantle layers.
• Gravity models with asymmetric long-wavelength negative anomalies, due to offset on Moho and crustal layers.

Case 2: Crustal detachment and buckling

• Receiver function and active source velocity models show evidence for mid-crustal detachment, Moho is fairly flat.
• Active source data contain near-vertical and wide-angle reflections that demarcate a mid- to lower-crustal detachment surface which correlates with receiver function anisotropy.
• Passive source - antistrophic receiver functions image a mid-crustal shear zone below arch.
• Shear-wave splitting detects no lithospheric mantle anisotropy.
• 4D modeling of arch geometry gives excess areas compatible with detachment folding and/or fault- propagation folding off sub-horizontal detachment in lower crust.
• Long wavelength gravity anomalies attributable to Moho topography are not observed because Moho is flat.

Case 3: Lithospheric buckling

• Receiver function and active source velocity models produce an upward deflection of continuous 7.xx and Moho beneath the major faults; OR major faults cut 7xx and Moho in areas where master fault is emergent.
• S-P receiver functions may help evaluate whether lithosphere is thin and could induce a buckling instability of small enough wavelength to produce the Bighorns arch.
• Shear wave splitting slightly affected, associated with minor strain predicted for buckling.
• 4D modeling gives excess areas requiring a sub- crustal detachment.
• Gravity models indicate that strong, positive long wavelength anomalies centered over the arch are due to shallow Moho (’antiroots’) directly beneath range.

Case 4: Pure shear thickening

• Receiver function and active source velocity models show downward deflection of continuous 7.xx and Moho beneath the midpoint of the arch due to lithospheric thickening.
• Active source data contain near-vertical and wide-angle reflections that define a mid- to lower-crustal detachment surface.
• Shear wave splitting affected by pure-shear mantle strain parallel to the arch axis.
• 4D modeling of arch geometry gives excess areas compatible with lower crustal thickening beneath arches that deflects the upper crust upward.
• Negative Bouger gravity is predicted, but positive Bouger is observed (PACES, 2007).