ABSTRACT The rifted Precambrian margin of western Laurentia is hypothesized to have consisted of a series of ~330°-oriented rift segments and ~060°-oriented transform segments. One difficulty with this idea is that the 87 Sr/ 86 Sr i = 0.706 isopleth, which is inferred to coincide with the trace of this rifted margin, is oriented approximately N-S along the western edge of the Idaho batholith and E-W in northern Idaho; the transition between the N-S– and E-W–oriented segments occurs near Orofino, Idaho. We present new paleomagnetic and geochronologic evidence that indicates that the area around Orofino, Idaho, has rotated ~30° clockwise since ca. 85 Ma. Consequently, we interpret the current N-S–oriented margin as originally oriented ~330°, consistent with a Precambrian rift segment, and the E-W margin as originally oriented ~060°, consistent with a transform segment. Independent geochemical and seismic evidence corroborates this interpretation of rotation of Blue Mountains terranes and adjacent Laurentian block. Left-lateral motion along the Lewis and Clark zone during Late Cretaceous–Paleogene time likely accommodated this rotation. The clockwise rotation partially explains the presence of the Columbia embayment, as Laurentian lithosphere was located further west. Restoration of the rotation results in a reconstructed Neoproterozoic margin with a distinct promontory and embayment, and it constrains the rifting direction as SW oriented. The rigid Precambrian rift-transform corner created a transpressional syntaxis during middle Cretaceous deformation associated with the western Idaho and Ahsahka shear zones. During the late Miocene to present, the Precambrian rift-transform corner has acted as a fulcrum, with the Blue Mountains terranes as the lever arm. This motion also explains the paired fan-shaped contractional deformation of the Yakima fold-and-thrust belt and fan-shaped extensional deformation in the Hells Canyon extensional province.

ABSTRACT The North American Cordillera experienced major contractional deformation during the Cretaceous–Paleogene, which is commonly attributed to normal subduction transitioning to shallow-slab subduction. We provide details of an alternative hit-and-run model, wherein the Insular superterrane obliquely collided with the North American margin from 100 to 85 Ma (the “hit”), followed by northward translation during continued oblique convergence with North America from 85 to 55 Ma (the “run”). This model assumes that the paleomagnetic evidence from the accreted terranes of the northern North American Cordillera, indicating up to thousands of kilometers of northward movement primarily between ca. 85 and 55 Ma, is correct. The hit-and-run model also incorporates new advances: (1) A worldwide plate reorganization occurred ca. 105–100 Ma; and (2) multiple subducted slabs have characterized subduction systems of the North American Cordillera since ca. 120 Ma. Finally, we explicitly address along-strike variations, such as the role of the preexisting rifted Precambrian margin and Permian–Triassic truncation of North America, in margin-parallel movement along western North America. The 100–85 Ma “hit” phase of the orogeny was characterized by dextral transpressional deformation that occurred simultaneously in the magmatic arcs of Idaho, northern Nevada, eastern California, and the Peninsular Ranges of southern California and northern Mexico. The hit phase also recorded incipient plateau formation, foreland block uplifts in the northern Rocky Mountains, and significant foreland sedimentation in adjacent North America. The transition from “hit” to “run” is hypothesized to have occurred because of the clockwise rotation of a Precambrian promontory in Washington State that was blocking northward translation: This rotation was accommodated by sinistral motion along the Lewis and Clark deformation zone. The 85–55 Ma “run” phase resulted in dextral strike-slip faulting of coastal blocks and significant contractional deformation in adjacent continental North America. The hit-and-run model is consistent with first-order geological and geophysical constraints from the North American Cordillera, and the proposed type of oblique orogeny requires a three-dimensional, time-dependent view of the deformation along an irregular and evolving continental margin.

ABSTRACT The North American Cordillera experienced significant and varied tectonism during the Triassic to Paleogene time interval. Herein, we highlight selected questions and controversies that remain at this time. First, we describe two tectonic processes that have hindered interpretations of the evolution of the orogen: (1) strike-slip systems with poorly resolved displacement; and (2) the closing of ocean basins of uncertain size, origin, and mechanism of closure. Next, we divide the orogen into southern, central, and northern segments to discuss selected controversies relevant to each area. Controversies/questions from the southern segment include: What is the origin of cryptic transform faults (Mojave-Sonora megashear vs. California Coahuila transform fault)? Is the Nazas an arc or a continental rift province? What is the Arperos basin (Guerrero terrane), and did its closure produce the Mexican fold-and-thrust belt? How may inherited basement control patterns of deformation during subduction? Controversies/questions from the central segment include: Can steeply dipping mantle anomalies be reconciled with geology? What caused high-flux events in the Sierra Nevada batholith? What is the origin of the North American Cordilleran anatectic belt? How does the Idaho segment of the orogen connect to the north and south? Controversies/questions from the northern segment include: How do we solve the Baja–British Columbia problem? How big and what kind of basin was the Early Cretaceous lost ocean basin? What connections can be found between Arctic geology and Cordilleran geology in Alaska? How do the Cretaceous tectonic events in the Arctic and northern Alaska connect with the Cordilleran Cretaceous events? What caused the Eocene tectonic transitions seen throughout the northern Cordillera? By addressing these questions along the length of the Cordillera, we hope to highlight common problems and facilitate productive discussion on the development of these features.

ABSTRACT The Bighorn uplift, Wyoming, developed in the Rocky Mountain foreland during the 75–55 Ma Laramide orogeny. It is one of many crystalline-cored uplifts that resulted from low-amplitude, large-wavelength folding of Phanerozoic strata and the basement nonconformity (Great Unconformity) across Wyoming and eastward into the High Plains region, where arch-like structures exist in the subsurface. Results of broadband and passive-active seismic studies by the Bighorn EarthScope project illuminated the deeper crustal structure. The seismic data show that there is substantial Moho relief beneath the surface exposure of the basement arch, with a greater Moho depth west of the Bighorn uplift and shallower Moho depth east of the uplift. A comparable amount of Moho relief is observed for the Wind River uplift, west of the Bighorn range, from a Consortium for Continental Reflection Profiling (COCORP) profile and teleseismic receiver function analysis of EarthScope Transportable Array seismic data. The amplitude and spacing of crystalline-cored uplifts, together with geological and geophysical data, are here examined within the framework of a lithospheric folding model. Lithospheric folding is the concept of low-amplitude, large-wavelength (150–600 km) folds affecting the entire lithosphere; these folds develop in response to an end load that induces a buckling instability. The buckling instability focuses initial fold development, with faults developing subsequently as shortening progresses. Scaled physical models and numerical models that undergo layer-parallel shortening induced by end loads determine that the wavelength of major uplifts in the upper crust occurs at approximately one third the wavelength of folds in the upper mantle for strong lithospheres. This distinction arises because surface uplifts occur where there is distinct curvature upon the Moho, and the vergence of surface uplifts can be synthetic or antithetic to the Moho curvature. In the case of the Bighorn uplift, the surface uplift is antithetic to the Moho curvature, which is likely a consequence of structural inheritance and the influence of a preexisting Proterozoic suture upon the surface uplift. The lithospheric folding model accommodates most of the geological observations and geophysical data for the Bighorn uplift. An alternative model, involving a crustal detachment at the orogen scale, is inconsistent with the absence of subhorizontal seismic reflectors that would arise from a throughgoing, low-angle detachment fault and other regional constraints. We conclude that the Bighorn uplift—and possibly other Laramide arch-like structures—is best understood as a product of lithospheric folding associated with a horizontal end load imposed upon the continental margin to the west.

ABSTRACT This field trip traverses the Sahwave and Nightingale Ranges in central Nevada, USA, and northward to Gerlach, Nevada, to the Granite, northern Fox, and Selenite Ranges. Plutonic bodies in this area include the ca. 93–89 Ma Sahwave nested intrusive suite of the Sahwave and Nightingale Ranges, the ca. 106 Ma Power Line intrusive complex of the Nightingale Range, the ca. 96 Ma plutons in the Selenite Range, and the ca. 105–102 Ma plutons of the Granite and Fox Ranges. Collectively these plutons occupy nearly 1000 km 2 of bedrock exposure. Plutons of the Sahwave, Nightingale, and Selenite Ranges intrude autochthonous rocks east of the western Nevada shear zone, while plutons of the Granite and Fox Ranges intrude displaced terranes west of the western Nevada shear zone. Integrated structural, geochemical, and geochronological studies are used to better understand magmatic and deformation processes during the Early Cretaceous, correlations with Cretaceous plutons in adjacent areas of Idaho and California, and regional implications. Field-trip stops in the Sahwave and Nightingale Ranges will focus on: (1) microstructure and orientation of magmatic and solid-state fabrics of the incrementally emplaced granodiorites-granites of the Sahwave intrusive suite; and (2) newly identified dextral shear zones hosted within intrusions of both the Sahwave and Nightingale Ranges. The Sahwave intrusive suite exhibits moderate to weak magnetic fabrics determined using anisotropy of magnetic susceptibility, with magnetic foliations that strike NW-NE and magnetic lineations that plunge moderately to steeply. Microstructural analysis indicates that these fabrics formed during magmatic flow. The older Power Line intrusive complex in the Nightingale Range is cross-cut by the Sahwave suite and contains a N-S–trending solid-state foliation that reflects ductile dextral shearing. Field-trip stops in the plutons of the Gerlach region will focus on composition, texture, and emplacement ages, and key differences with the younger Sahwave suite, including lack of evidence for zoning and solid-state fabrics. The field trip will utilize StraboSpot, a digital data system for field-based geology that allows participants to investigate the relevant data projects in the study areas.

Abstract The Dun Mountain ophiolite, South Island, New Zealand, records complex overprinting of mantle fabrics. Using structural observations, microstructural analysis, geothermometry, geobarometry, geochronology and rheological constraints from the Red Hills and Dun Mountain massifs, we propose that three deformation events occurred during the early stages of subduction initiation along the Permian margin of Gondwana. During the first deformation event, the lineated Two Tarns Harzburgite from the Red Hills formed in a transtensional setting associated with subduction initiation. Deformation was pervasive, homogeneous and simultaneous with boninitic melt migration through the unit; it also occurred at very fast strain rates (10 −9 –10 −8 s −1 ). During the second deformation event, progressive exhumation to c. 5 kbar and cooling to 1000°C led to the localization of melt and deformation into distinct zones (Dun Mountain, and the Plateau Complex, Plagioclase Zone and Ellis Stream Complex of the Red Hills). The third deformation event resulted in continued cooling and exhumation along serpentinized faults. This history provides a rare glimpse of the coupled fabric development and melt migration that sequentially develop in the early mantle wedge during the initiation of a subduction zone.

ABSTRACT The Salmon River suture zone is the boundary between the accreted (Blue Mountain) terranes and cratonic North America in western Idaho. This region was the focus of study by the EarthScope IDOR (IDaho-ORegon) project that integrated structural geology, geochemistry, geochronology, and seismology. This field trip traverses from western Idaho to eastern Oregon, covering the Atlanta lobe of the Idaho batholith, Blue Mountains terranes, and the middle Cretaceous western Idaho shear zone that separates these two domains. The main component of the Atlanta lobe is the Atlanta peraluminous suite, and it intruded from 83 to 65 Ma, was derived from crustal melting, and lacks a regionally consistent fabric. The crust below the Idaho batholith is relatively thick and seismic velocities are consistent with the entire crust being relatively felsic. The western Idaho shear zone overprints the Salmon River suture zone and obscures most evidence for the suturing. It is the present boundary between Blue Mountains terranes and cratonic North America. From studies along this transect, we have determined that the western Idaho shear zone exhibits dextral transpressional deformation, was active from ca. 103 to 90 Ma, and magmatism occurred during deformation; presently exposed levels on this transect record deformation conditions of 730 °C and 4.3 kbars. There is an ~7 km vertical step in the Moho at or slightly (<20 km) east of the current exposure of the western Idaho shear zone, separating thicker crust to the east from thinner crust to the west. Blue Mountains terranes immediately outboard of the western Idaho shear zone likely were located farther south during the middle Cretaceous and underwent strike-slip displacement during western Idaho shear zone deformation. The Olds Ferry terrane—the accreted terrane located immediately west of the western Idaho shear zone—was underplated by mafic magmatism, likely in the Miocene during eruption of the Columbia River basalt group. The field trip will utilize StraboSpot, a recently developed digital data system for structural geology and tectonics, so participants can investigate the relevant data associated with the IDOR EarthScope project.