The Traverse Ridge Paleoseismic Site and Ruptures Crossing the Boundary Between the Provo and Salt Lake City Segments of the Wasatch Fault Zone, Utah, United States

How structural segment boundaries modulate earthquake behavior is an important scientific and societal question, especially for the Wasatch fault zone (WFZ) where urban areas lie along multiple fault segments. The extent to which segment boundaries arrest ruptures, host moderate magnitude earthquakes, or transmit ruptures to adjacent fault segments is critical for understanding seismic hazard. To help address this outstanding issue, we conducted a paleoseismic investigation at the Traverse Ridge paleoseismic site (TR site) along the ∼7-km-long Fort Canyon segment boundary, which links the Provo (59 km) and Salt Lake City (40 km) segments of the WFZ. At the TR site, we logged two trenches which were cut across sub-parallel traces of the fault, separated by ∼175 m. Evidence from these exposures leads us to infer that at least 3 to 4 earthquakes have ruptured across the segment boundary in the Holocene. Radiocarbon dating of soil material developed below and above fault scarp colluvial packages and within a filled fissure constrains the age of the events. The most recent event ruptured the southern fault trace between 0.2 and 0.4 ka, the penultimate event ruptured the northern fault trace between 0.6 and 3.4 ka, and two prior events occurred between 1.4 and 6.2 ka (on the southern fault trace) and 7.2 and 8.1 ka (northern fault trace). Colluvial wedge heights of these events ranged from 0.7 to 1.2 m, indicating the segment boundary experiences surface ruptures with more than 1 m of vertical displacement. Given these estimates, we infer that these events were greater than Mw 6.7, with rupture extending across the entire segment boundary and portions of one or both adjacent fault segments. The Holocene recurrence of events at the TR site is lower than the closest paleoseismic sites at the adjacent fault segment endpoints. The contrasts in recurrence rates observed within 15 km of the Fort Canyon fault segment boundary may be explained conceptually by a leaky segment boundary model which permits spillover events, ruptures centered on the segment boundary, and segmented ruptures. The TR site demonstrates the utility of paleoseismology within segment boundaries which, through corroboration of displacement data, can demonstrate rupture connectivity between fault segments and test the validity of rupture models.

Role of rift-inheritance and segmentation for orogenic evolution: example from the Pyrenean-Cantabrian system

The Basque-Cantabrian junction corresponds to an inverted rift accommodation zone at the limit between the former hyperextended Pyrenean and Cantabrian rift segments. The recognition of an inherited rift segment boundary allows to investigate the reactivation associated with large-scale rift segmentation in an orogenic system. We use criteria from published field observations and seismic data to propose a new map of rift domains for the Basque-Cantabrian junction. We also provide balanced cross-sections that allow to define the along-strike architecture associated with segmentation during rifting and subsequent Alpine reactivation. Based on these results, this study aims to characterize and identify reactivated and newly formed structures during inversion of two rift segments and its intermitted segment boundary. It also aims to describe the timing of thin-skinned and thick-skinned deformation associated with the inversion of segmented rift systems. During convergence, two phases have been recognized within the rift segment (eastern Mauléon basin). The Late Cretaceous to Paleocene underthrusting/subduction phase was mostly governed by thin-skinned deformation that reactivated the former hyperextended domains and the supra-salt sedimentary cover. The Eocene to Miocene collisional phase, controlled by thick-skinned deformation that took place once necking domains collided and formed an orogenic wedge. At the rift segment boundary, the underthrusting/subduction phase was already controlled by thick-skinned deformation due to the formation of shortcutting thrust faults at the termination of overlapping V-shaped rift segments. This led to the formation of a proto-wedge composed of the Basque massifs. We suggest that this proto-wedge is responsible for the preservation of pre-Alpine structures in the Basque massifs and for the emplacement of subcontinental mantle rocks at a crustal level beneath the western Mauléon basin. These results argue for a first order cylindrical orogenic architecture from the Central Pyrenean segment to the Cantabrian segment (up to the Santander transfer zone) despite rift segmentation. They also highlight the control of 3D rift-inheritance for the initial phase of orogenic evolution and for the local architecture of mountain belts.

Late Pleistocene Record of Off‐Fault Deformation and Vertical Slip Rates from the Wasatch Fault Zone, Utah: Implications for Fault Segmentation from Lake Bonneville Shorelines

In an effort to better understand the Pleistocene history of the Wasatch fault zone, we evaluate the deformation and displacement of the Bonneville and Provo high‐stand shorelines of Lake Bonneville along the Wasatch Front. We apply an automated shoreline elevation measurement application developed as part of this study to measure Lake Bonneville shoreline elevations along the Weber and Brigham City segments of the fault, adding to a previously published dataset of shoreline elevations on the Salt Lake City segment. Tectonically deformed shorelines on the footwall of the fault demonstrate elevation patterns that are inconsistent with the idea that the Pleasant View salient, a bedrock salient marking the segment boundary between the Weber and Brigham City segments of the fault, is a persistent barrier to fault rupture since the late Pleistocene. Shoreline features are elevated ∼20 m across the segment boundary as compared to shoreline features on the northern part of the Brigham City segment. We suggest the possibility that fault rupture through the Pleasant View salient has been common since the late Pleistocene and speculate that a similar relationship could exist between the Provo and Salt Lake City segments, based on similarities between the shoreline elevation patterns on the Brigham City and Salt Lake City segments of the fault. Vertical slip rates measured from displaced shorelines at the Pleasant View salient (Brigham City–Weber segment boundary) are generally higher compared to those at the Honeyville spur (Collinston–Brigham City segment boundary). Statistically significant vertical slip rates calculated from the Provo shoreline at the Pleasant View salient (0.8±0.5 to 0.9±0.6 mm/yr and 0.7±0.5 to 0.9±0.6 mm/yr) suggest that late Pleistocene vertical slip rates are slightly lower than Holocene rates; however, large uncertainties in the shoreline elevation measurements exist.