State of stress in North America : seismicity, tectonics, and unconventional energy development
Keywords: Geophysics.

The state of stress in the Earth's brittle crust dictates the style of active deformation. In this dissertation, I present over 600 new orientations of the maximum horizontal principal stress (SHmax) and the first quantitative map of the style of faulting (relative principal stress magnitudes) across North America, the latter of which is based on > 1200 new estimates of the A휙 parameter and its uncertainty. I apply this dataset to (1) understanding the forces driving plate motions, intraplate deformation, and topographic change; (2) determining the faults that are best oriented to produce earthquakes and how much pore pressure perturbation might be required for failure; and (3) ensuring efficient subsurface energy development. The stresses vary over multiple scales across the continent. The stress field is compressive (strike-slip and/or reverse faulting) in the eastern USA and Canada, where SHmax is dominantly ENE--WSW to NE--SW. The faulting regime becomes less compressive west- ward, with strike-slip faulting in much of the Midwest, and normal/strike-slip faulting in the western Great Plains. Moving west over the same region, SHmax rotates gradually to be ENE--WSW to E--W in Oklahoma and much of the Great Plains. The transition in the faulting regime implies a continent-scale reduction in horizontal stress, which is likely balanced by viscous drag along the bottom of the WSW-moving North American lithosphere. In much of the U.S. Great Plains, normal and strike-slip faulting are active simultaneously. This relatively extensional stress field spans as far north as southern North Dakota, and as far south as central or southwestern Mexico, and it also covers much of the U.S. Intermountain West. As I show in Chapter 2, published geodynamic models do not reproduce this generally extensional stress field throughout the Great Plains and much of the Rocky Mountains. Models that include only estimates of GPE or basal tractions from mantle flow are reasonably effective at matching regional-scale SHmax orientations in some places. However, no published models replicate the newly apparent shorter-wavelength variability. On the margins of the extensional Rio Grande Rift (RGR) and Basin and Range provinces, SHmax rotates up to 90° over 10s of km, suggesting shallow sources of stress. Notably, in the Delaware Basin of west Texas and southeast New Mexico (part of the Permian Basin at the eastern margin of the RGR), SHmax rotates ~150° clockwise southward. Further east, in the Central Basin Platform and Midland Basin, SHmax is consistently ~ENE--WSW. The western margin of North America is dominantly compressive (strike-slip and/or reverse faulting) due to transpression associated with the San Andreas and Queen Charlotte transform systems, and compression above the Cascadia and Aleutian subduction zones. Strike-slip faulting dominates in western Mexico, including Baja California, despite the transtensional kinematics of the plate boundary. Western Canada is compressive (strike- slip and/or reverse faulting), with SHmax oriented ~NE--SW. Data are limited in central Canada, but the stress field appears to be less compressive there, perhaps normal/strike- slip faulting. In Alaska, the faulting regime transitions from strike-slip and/or reverse faulting above the subduction zone to normal and/or strike-slip faulting in the northern half of the state. The stress data presented here enable a new approach to managing the hazards associated with induced seismicity because they can be used nearly anywhere on the continent to indicate which faults are most likely to slip in response to fluid pressure perturbations (or naturally). In Chapter 3, I apply the data to determine whether several prominent sequences of earthquakes in Texas that were potentially triggered by industry activities (Azle, Snyder/Cogdell, Timpson, and Karnes City/Fashing) are well oriented for failure in the current stress field. In all cases, with the possible exception of Timpson, the causative faults were very well aligned for slip in the stress field, requiring minor pressure perturbations to fail. This conclusion has since been corroborated in general for induced seismicity in other regions. The new data are especially detailed in the Permian Basin of west Texas and southeast New Mexico, where seismicity has increased sharply in recent years. In Chapter 4, I apply the stress map to understand the sensitivity of mapped faults to a modest pore pressure increase. Using probabilistic fault slip potential (FSP) analysis, I illustrate how the orientations of potentially active faults rotate across the basin in response to the mapped stress variability. Chapter 5 (originally published by Hennings, Lund Snee, Zoback, et al. 2019) presents the results of similar analysis applied to a detailed new 3D fault model for the Fort Worth Basin, northeast Texas, mapping several faults below the Dallas-Fort Worth metroplex that we find to be sensitive to reactivation. Chapter 7, presents detailed views of SHmax and the first ever representation of the style of faulting (relative stress magnitudes) in each of the primary areas of unconventional oil, gas, and geothermal development in North America. In that chapter, I demonstrate graphically and mathematically how bounds on the principal stress magnitudes can be estimated anywhere using the mapped relative principal stress magnitudes. In appendices, I discuss several applications of the new data for subsurface energy development, including indicating the ideal orientations to drill horizontal wells and identifying the natural fractures most likely to form a permeable network during reservoir stimulation. The latter application builds upon the work presented in Chapter 6 (originally published by Zoback and Lund Snee, 2018), which illustrates why subparallel planes (e.g., bedding planes) are unlikely to slip during stimulation except under extremely high ambient pore pressures and/or in strike-slip/reverse or reverse faulting environments. The final chapter (Chapter 8) presents a new view of the factors responsible for Late Cretaceous to present topographic changes and tectonism in the Great Basin, western USA. I show that this region was a broad, low-relief, moderate-elevation plateau (probably < 2.5 km) subject to little erosion during and after Mesozoic Sevier-era shortening. Elevations increased markedly beginning in middle Eocene time as intense magmatism swept south over the region. I propose that migrating elevation caused a reorganization of the regional drainage network and progressively shifted the topographic divide into central Nevada from its prior position further west along the Cretaceous axis of the Sierra Nevada batholith. Basin and Range extension initiated later, in middle Miocene time, triggered by changing tectonic boundary conditions and driven by extension of crust that had high GPE and low strength as a result of high subsurface temperatures that persisted long after middle Cenozoic volcanism