The Origins of Sulfate in Cenozoic Non-Marine Evaporites in the Basin and-Range Province, Southwestern North America

Cenozoic evaporites (gypsum and anhydrite) in southwestern North America have wide ranges of δ34S (−30 to +22‰; most +4 to +10‰) and δ18OSO4 (+3 to +19‰). New data are presented for five basins in southern Arizona. The evaporites were deposited in playas or perennial saline lakes in closed basins of Oligocene or younger age. Very large accumulations in Picacho, Safford and Tucson Basins have isotope compositions plotting close to a linear δ34S-δ18OSO4 relationship corresponding to mixing of two sources of sulfur: (1) sulfate recycled from Permian marine gypsum and (2) sulfate from weathering of Laramide-age igneous rocks that include porphyry copper deposits. In the large evaporites, sulfate with δ34S > +10‰ is dominantly of Permian or Early Cretaceous marine origin, but has locally evolved to higher values as a result of bacterial sulfate reduction (BSR). Sulfate with δ34S < −10‰ formed following exposure of sulfides, possibly formed during supergene enrichment of a porphyry copper deposit by BSR, and have values of δ18OSO4 higher than those of local acid rock drainage because of participation of evaporated water in BSR. Accumulations of 30 to 100 km3 of gypsum in Picacho and Safford Basins are too large to explain as products of contemporaneous erosion of Permian and Laramide source materials, but may represent recycling of Late Cretaceous to Miocene lacustrine sulfate.

From Terranes to Terrains: Geologic Field Guides on the Construction and Destruction of the Pacific Northwest

The eight field trips in this volume, associated with GSA Connects 2021 held in Portland, Oregon, USA, reflect the rich and varied geological legacy of the Pacific Northwest. The western margin of North America has had a complex subduction and transform history throughout the Phanerozoic, building a collage of terranes. The terrain has been modified by Cenozoic sedimentation, magmatism, and faulting related to Cascadia subduction, passage of the Yellowstone hot spot, and north and westward propagation of the Basin and Range province. The youngest flood basalt province on Earth also inundated the landscape, while the mighty Columbia watershed kept pace with arc construction and funneled epic ice-age floods from the craton to the coast. Additional erosive processes such as landslides continue to shape this dynamic geological wonderland.

Crustal Motion and Strain Rates in the Southern Basin and Range Province

New research teases out variations in strain rates and explores potential earthquake hazards across the southern Basin and Range and Colorado Plateau.

Quantifying Normal Fault Evolution from River Profile Analysis in the Northern Basin and Range Province, Southwest Montana, USA

Over the past few decades, tectonic geomorphology has been widely implemented to constrain spatial and temporal patterns of fault slip, especially where existing geologic or geodetic data are poor. We apply this practice along the eastern margin of Bull Mountain, Southwest Montana, where 15 transient channels are eroding into the flat, upstream relict landscape in response to an ongoing period of increased base level fall along the Western North Boulder fault. We aim to improve constraints on the spatial and temporal slip rates across the Western North Boulder fault zone by applying channel morphometrics, cosmogenic erosion rates, bedrock characteristics, and calibrated reproductions of the modern river profiles using a 1-dimensional stream power incision model that undergoes a change in the rate of base level fall. We perform over 104 base level fall simulations to explore a wide range of fault slip dynamics and stream power parameters. Our best fit simulations suggest that the Western North Boulder fault started as individual fault segments along the middle to southern regions of Bull Mountain that nucleated around 6.2 to 2.5 Ma, respectively. This was followed by the nucleation of fault segments in the northern region around 1.5 to 0.4 Ma. We recreate the evolution of the Western North Boulder fault to show that through time, these individual segments propagate at the fault tips and link together to span over 40 km, with a maximum slip of 462 m in the central portion of the fault. Fault slip rates range from 0.02 to 0.45 mm/yr along strike and are consistent with estimates for other active faults in the region. We find that the timing of fault initiation coincides well with the migration of the Yellowstone hotspot across the nearby Idaho-Montana border and thus attribute the initiation of extension to the crustal bulge from the migrating hotspot. Overall, we provide the first quantitative constraints on fault initiation and evolution of the Western North Boulder fault, perhaps the farthest north basin in the Northern Basin and Range province that such constraints exist. We show that river profiles are powerful tools for documenting the spatial and temporal patterns of normal fault evolution, especially where other geologic/geodetic methods are limited, proving to be a vital tool for accurate tectonic hazard assessments.

Implications of thermal disequilibrium during channelized melt-transport for the evolution of the lithosphere-asthenosphere boundary in intraplate settings

&lt;p&gt;This study explores how thermal disequilibrium during channelized melt-infiltration modifies the continental lithosphere from beneath. For this purpose, a 1D model of thermal disequilibrium between melt-rich channels and surrounding melt-poor material was developed, allowing us to estimate heat exchange across channel walls during melt transport at the lithosphere-asthenosphere boundary (LAB). &amp;#160;For geologically-reasonable values of volume fraction of channels (&lt;em&gt;&amp;#966;&lt;/em&gt;), relative velocity across channel walls (&lt;em&gt;v&lt;/em&gt;), channel spacing (&lt;em&gt;d&lt;/em&gt;), and timescale of episodic melt-infiltration (&lt;em&gt;&amp;#964;&lt;/em&gt;), disequilibrium heating may contribute &gt;10&lt;sup&gt;-3&lt;/sup&gt; W/m&lt;sup&gt;3&lt;/sup&gt; to the LAB heat budget.&amp;#160;During episodic melt-infiltration, a thermal reworking zone (TRZ) associated with spatio-temporally varying disequilibrium heat exchange forms at the LAB.&amp;#160;The TRZ grows by the transient migration of a disequilibrium-heating front at material-dependent velocity, reaching a maximum steady-state width &amp;#948;&amp;#8764;[&lt;em&gt;&amp;#966;vd&lt;sup&gt;-&lt;/sup&gt;&lt;/em&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;em&gt;&amp;#964;&lt;/em&gt;&lt;sup&gt;2&lt;/sup&gt;]. The model results have implications for the Cenozoic evolution of the western US, specifically during the time period following the middle-Cenozoic ignimbrite flareup, and can be used to interpret a disparate set of previously published geophysical and geologic observations from the western US. The spatio-temporal scales associated with establishment of the TRZ in the models are found to be comparable with those inferred for the migration of the LAB based on geologic and petrologic observations within the Basin and Range province. More generally, the geochemistry of Cenozoic basalts across the region indicate a process in which melt-infiltration may have hastened the thinning and weakening of the lithosphere during and following the mid-Cenozoic ignimbrite flare-up, prior to Neogene extension.&lt;/p&gt;

Diffuse Tectonic Deformation in the Drum Mountains Fault Zone, Utah, USA: Testing the Utility of Legacy Aerial Photograph-Derived Topography

The Basin and Range province in the western United States hosts numerous low-slip-rate normal faults with diffuse and subtle surface expressions. Legacy aerial photographs, widely available across the region, can be used to generate high-resolution digital elevation models of these previously uncharacterized fault systems. Here, we test the limits and utility of aerial photograph-derived elevation products on the Drum Mountains fault zone—a virtually unstudied and enigmatic fault system in the eastern Basin and Range province of central Utah. We evaluate a new 2-m digital surface model produced from aerial photographs against other remotely sensed and field survey data and assess the various factors that contribute to noise, artifacts, and distortions. Despite some challenges, the new elevation model captures the complex array of cross-cutting fault scarps well. We demonstrate that the fault zone has variable net east- or west-down sense of displacement across a c. 8-km-wide zone of antithetic and synthetic traces. Optically stimulated luminescence ages and scarp profiles are used to constrain net extension rates across two transects and reveal that the Drum Mountains fault zone has average extension rates of c. 0.1–0.4 mm yr−1 over the last c. 35 ka. These rates are both faster than previously estimated and faster than most other faults in the region, and could be an order of magnitude higher if steep faults at the surface sole into a detachment at depth. Several models have been proposed for local and regional faulting at depth, but our data show that the offsets, rates, and geometries of faulting can be generated by the reactivation of pre-existing, cross-cutting faults in a structurally complex zone between other fault systems. This study highlights how legacy aerial-photograph-derived elevation products, in lieu of other high-resolution topographic datasets, can be used to study active faults, especially in remote regions where diffuse deformation would otherwise remain undetected.