The Eastern California shear zone is a complex set of dextral faults that accommodates significant plate motion and has produced large earthquakes. The evolution of this system and why it consists of ...closely spaced, irregular faults that fail in multi‐fault ruptures are not well understood. Here we analyze the geometry, spatial distribution, and Quaternary slip activity of right‐lateral faults in the southern Mojave block. We find these faults are oriented favorably for accommodating regional dextral plate motion and do not show evidence of replacement following counterclockwise rotation to unfavorable positions, although activity may be migrating westward as previously proposed. We also confirm that the shear zone is transpressive, with widespread restraining bends, distributed convergent deformation, and significant impact on near‐fault topography. Observations also show that faults are geometrically complex, as represented by along‐strike variability in fault strike. We document a correlation between strike variability and fault activity (slip rate or net slip), which is evident within the shear zone as well as for a control group of other faults. We suggest that strike variability represents a form of geometric roughness, which may inhibit fault slip and result in complex ruptures, slip‐strengthening behavior, and a prevalence of off‐fault deformation. Other factors, including preexisting crustal fabric, edge effects, and changes in the stress field, may further complicate kinematics. These results suggest that faults of the shear zone are still juvenile and somewhat unique, yet offer an important window into how broadly distributed shear may evolve into a through‐going continental transform system.
Key Points
Dextral faults of the Eastern California shear zone are transpressive and unusually complex in orientation, connectivity, and geometry
Variability in strike of these faults may act as a form of geometric roughness, which may inhibit slip and result in secondary deformation
Shear zone kinematics may be slip‐strengthening and influenced by crustal fabric and lithospheric edge effects, but not major transrotation
Restraining bends influence topography, strike‐slip evolution, and earthquake rupture dynamics, however the specific factors governing their geometry and development in the crust are not well ...established. These relationships are challenging to investigate in field examples due to cannibalization and erosion of earlier structures with cumulative strain. To address this knowledge gap, we investigated the structure, morphology, and kinematics of 22 basement‐cored restraining bends on low net‐slip faults (<10 km) within the southern Eastern California shear zone (SECSZ) via mapping, topographic analyses, and 3D numerical modeling. The bends are self‐similar in form with most exhibiting focused relief between high‐angle bounding faults with an arrowhead shape in map view and a “whaleback” longitudinal profile. Slight changes in that form occur with increasing size indicating predictable growth that appears to be primarily controlled by local fault geometries (i.e., bifurcation angle), rather than the influence of fault obliquity relative to far‐field plate motion, due to inefficient slip‐transfer across interconnected irregularly trending closely spaced faults. Modeling results indicate that the self‐similar fault‐bound geometry of SECSZ restraining bends may arise from elevated shear strain at the outer corners of single transpressional fault bends with increasing cumulative slip. This, in turn, promotes growth of a new fault leading to efficient accommodation of local convergent strain via uplift between bounding faults. Finally, our results indicate that the kilometer‐scale restraining bends contribute minimally to regional contraction as they only penetrate the upper third of the seismogenic crust and are therefore also unlikely to impede large earthquake surface ruptures.
Key Points
Southern Eastern California shear zone restraining bends exhibit self‐similar forms that change predictably with growth
Uplift patterns within these bends are influenced more by local fault geometries than by obliquity of fault trend to plate motion
Positive flower structures may initially develop due to increased shear strain at bend corners
Large, resistant, quartz‐rich boulders deposited on hillslopes and in channels armour the landscape, trap sediment and influence hillslope angle and erodibility. In the Virginia Appalachians, such ...boulders are a significant component of hillslopes and channels. Establishing the timing of and processes responsible for bedrock fracture and boulder deposition is a critical piece of understanding the landscape as a system. In this study, we use cosmogenic 10Be exposure dating to resolve the timing of boulder deposition at three sites in the Virginia Valley and Ridge province: Gap Mountain, Brush Mountain and Little Stony Creek, and at one site in the Virginia Blue Ridge: Devil's Marbleyard. The correlation between measured boulder exposure ages (101.7 ± 6.9 ka to 10.8 ± 0.8 ka; n = 23) and the Wisconsin Glacial Stage and subsequent Laurentide Ice Sheet (LIS) deglaciation (~115–11.7 ka) suggests a periglacial origin for deposition of large hillslope and channel boulders in the Virginia Appalachians. The lack of boulder exposure ages corresponding to the Last Interglacial Stage or following Wisconsin LIS retreat suggests interglacial non‐deposition and stability. The absence of exposure ages from the penultimate Illinoian or older Quaternary Glacial Stages suggests that periglacial hillslope processes allow the landscape to be resurfaced with large boulders during each return to cold climate conditions. This cyclic resurfacing of hillslopes and channels is an example of how climatic oscillations insert disequilibrium into the landscape cycle and contributes to our appreciation of the timescales over which contemporary climate change may impact boulder dominated landscapes in rapidly warming alpine and arctic environments.
Cosmogenic 10Be exposure dating indicates that the depositional timing of large boulders in the Virginia Appalachians is between 10.8 ± 0.8 ka and 101.7 ± 6.9 ka. The correlation between measured boulder exposure ages and the Wisconsin Glacial Stage (~115–11.7) suggests a periglacial origin for the deposition of boulders. The absence of pre‐Wisconsin ages suggests that periglacial hillslope processes allow the landscape to be resurfaced with large boulders during each return to cold climate conditions.
The effect of rapid erosion on kinematic partitioning along transpressional plate margins is not well understood, particularly in highly erosive climates. The Blue Mountains restraining bend (BMRB) ...of eastern Jamaica, bound to the south by the left-lateral Enriquillo-Plantain Garden fault (EPGF), offers an opportunity to test the effects of highly erosive climatic conditions on a 30-km-wide restraining bend system. No previous thermochronometric data exists in Jamaica to describe the spatial or temporal pattern of rock uplift and how oblique (>20°) plate motion is partitioned into vertical strain. To define the exhumation history, we measured apatite (n=10) and zircon (n=6) (U-Th)/He ages, 40Ar/39Ar (n=2; amphibole and K-spar) ages, and U/Pb zircon (n=2) crystallization ages. Late Cretaceous U/Pb and 40Ar/39Ar ages (74–68Ma) indicate rapid cooling following shallow emplacement of plutons during north-south subduction along the Great Caribbean Arc. Early to middle Miocene zircon helium ages (19–14Ma) along a vertical transect suggest exhumation and island emergence at ~0.2mm/yr. Older zircon ages 10–15km to the north (44–35Ma) imply less rock uplift. Apatite helium ages are young (6–1Ma) across the entire orogen, suggesting rapid exhumation of the BMRB since the late Miocene. These constraints are consistent with previous reports of restraining bend formation and early emergence of eastern Jamaica. An age-elevation relationship from a vertical transect implies an exhumation rate of 0.8mm/yr, while calculated closure depths and thermal modeling suggests exhumation as rapid as 2mm/yr. The rapid rock uplift rates in Jamaica are comparable to the most intense transpressive zones worldwide, despite the relatively slow (5–7mm/yr) strike-slip rate. We hypothesize highly erosive conditions in Jamaica enable a higher fraction of plate motion to be accommodated by vertical deformation. Thus, strike-slip restraining bends may evolve differently depending on erosivity and local climate.
•Jamaica offers a unique opportunity to study the effect of climate on tectonics.•The Blue Mountains restraining bend is a zone of rapid transpressive rock uplift.•Two phases of Miocene exhumation occurred from 20–5Ma and 5Ma-Present.•Rates of exhumation since 5Ma are ~1–2mm/yr across the entire BMRB.•Highly erosive tropical climates partition vertical deformation efficiently.
Bedrock erodibility exerts a fundamental control on fluvial incision, and therefore on the evolution of entire landscapes. However, the roles and relative significance of specific bedrock properties ...like rock strength and discontinuity spacing in setting erodibility remain poorly understood. As a result, erodibility is often overlooked or oversimplified in numerical and field‐based investigations of landscape evolution processes, leading to misinterpretation of channel profile convexities (i.e., knickpoints) in studies seeking to infer uplift or baselevel histories from longitudinal profile analyses. Here, we investigate the controls on fluvial bedrock erodibility and knickpoint expression by conducting detailed surveys of 21 lithologic knickpoints and non‐knickpoint reaches (representing end‐member bedrock erodibility cases) and corresponding bedrock properties from small channels in the south‐central Appalachian Mountains. We use binary logistic regression of field data to test the relative strength of rock strength and discontinuity spacing as predictors of knickpoint occurrence, and therefore their relative impact on fluvial bedrock erodibility. We find that discontinuity spacing more strongly influences bedrock erodibility in this setting, where both rock strength and discontinuity spacing vary widely, confirming for the first time quantitatively the hypothesis that discontinuities exert a dominant control on fluvial erodibility. We also find that knickpoint expression is unique to a given stratigraphic interval, implying that knickpoint morphology is intimately linked to local conditions and therefore effectively unpredictable without detailed field measurements. Finally, because all 21 of our study knickpoints occur within a single geologic unit, our results illustrate that intra‐unit heterogeneity must be accounted for when considering lithologic influence on channel profile convexities.
Plain Language Summary
River incision into bedrock is a major driver of landscape evolution across most of Earth's terrestrial surface. The rate and pattern of this incision is strongly influenced by the erodibility of the underlying bedrock (defined as the rate at which erosion occurs for a given river slope and water discharge), which can vary significantly within a landscape. Currently, we have an incomplete understanding of the factors which influence bedrock erodibility and how it relates to river form and behavior. Therefore, the impacts of erodibility are not well captured by numerical models of river incision or adequately accounted for in field studies of river morphology. In this paper, we assume knickpoints (steep segments along a river's profile) are indicators of more resistant bedrock, and compare bedrock properties (e.g., sedimentary bed thickness and rock strength) between knickpoints and non‐knickpoint river reaches to infer which is a better predictor of knickpoint occurrence and therefore more impactful on bedrock erodibility. We find that within this setting, bed thickness is more important than rock strength in setting bedrock erodibility. We also observe that knickpoint morphology is fine‐tuned to local bedrock properties. Finally, because all of our study knickpoints occur within one geologic unit, studies seeking to interpret knickpoint distributions within a landscape must consider how heterogeneity within mapped geologic units may impact knickpoints within their study areas.
Key Points
Knickpoint occurrence, and therefore bedrock erodibility, is more strongly a function of bed thickness than rock strength in this setting
Knickpoint morphology is dictated by complex and unique combinations of local (sub‐meter scale) bedrock characteristics
Lithologic heterogeneity below the scale of mapped geologic units must be considered when interpreting spatial distributions of knickpoints
We have investigated the geologic controls on hydraulic geometry of bedrock rivers using a single large catchment, the New River, from a stable tectonic setting with variable, resistant lithology but ...spatially stable climate. Our survey of channel width at 0.5km spacing along 572km of the river shows major variation that only roughly fits the expected scaling relationships between width, drainage area, and slope. Considerable variations in width, including steps in trends and large spikes, relate to physiogeologic boundaries that the river passes through. A large fraction (15%) of the river's length classifies as bedrock reach, showing that it behaves more like a bedrock river than an alluvial river. Unlike established trends, the channel is wider in bedrock than in alluvium. Field observations show that aspect ratio (width to depth) is not constant, but fluctuates systematically with width from wide, shallow reaches to narrower, deeper reaches. Our observations of bedrock properties suggest that susceptibility to fluvial plucking versus abrasion may control this anomalous channel morphology. One end member form with aspect ratio as high as 500, which we term the incision plain, is associated with very closely spaced discontinuities (~10cm) in otherwise hard rock. We propose that the closely spaced discontinuities enable efficient plucking that leads to widening by lateral erosion. This morphology locally occurs in other passive margin rivers and may be a fundamental fluvial form that is similar to, but the inverse of, slot canyons. The other end member, which we term channel neck, is narrower and deeper with complex flow paths through blocky bedrock. This form occurs where discontinuity spacing is longer (>0.5m) and erosion is abrasion dominated. These results imply that changes in channel width do not necessarily reflect variations in uplift rate, but instead may result from complex response to bedrock properties.
•Bedrock channel width can be extremely variable along large passive margin rivers.•High aspect ratio (width/depth) occurs in bedrock with closely spaced joints.•Narrower channels occur where erosion is dominated by abrasion.•Morphology reflects local properties, not necessarily variation in uplift rate.
Syntaxial bends in convergent plate boundaries, or indentor corners, display some of the most intriguing deformation patterns on Earth and are type localities for “aneurysms” of coupled erosion, ...thermal weakening, and strain. The St. Elias orogen in Alaska is a small, young convergent system that has been dominated by a glacial climate for much of its history and exhibits two prominent indentor corners that are not well understood. We have added 40 new apatite (U–Th)/He ages to the already extensive dataset for the low-temperature cooling history of this orogen to constrain the pattern of exhumation in these indentor corners. Ages from the western syntaxis show minor variation across the structural hinge, suggesting that the bend has little effect on the pattern of exhumation and that structures, including the Bagley fault, connect smoothly from the orogen core to the subduction zone to the southwest. Rock uplift on the north flank of the range appears to increase steadily towards the eastern syntaxis, which represents the apex in the right-angle bend between a transform fault in the south and the collision zone in the west. Based on age–elevation relationships, zones of relative rock uplift can be defined in which the Mt. Logan massif, or the area just north of the eastern syntaxis, experienced ∼
4.8
km greater rock uplift than background levels northwest of the western syntaxis. A bulge in relative rock uplift is symmetric about the hinge in the eastern indentor corner. However, rates of denudation in this bulge are not as rapid as the core of the fold and thrust belt and are lower than those implied by detrital cooling ages from beneath the Seward Glacier. This implies that a large bull's eye of ultra-rapid (∼
5
mm/yr) exhumation does not occur and that the subpopulation of young detrital ages may be sourced from a narrow transpressional zone along the Fairweather fault. Unlike the Himalayan syntaxes, it thus appears that an aneurysm of coupled erosion–strain has not developed in either indentor corner of the St. Elias orogen. This may indicate a limit to the degree to which glacial erosion can partition strain in convergent orogens. Similarly, the likely existence of a through-going dextral fault, the Totschunda fault, through the eastern syntaxis implies that tectonics, rather than surface processes, exerts the main control on strain partitioning in these corners.
Low‐angle detachment fault systems are important elements of oblique‐divergent plate boundaries, yet the role detachment faulting plays in the development of such boundaries is poorly understood. The ...West Salton Detachment Fault (WSDF) is a major low‐angle normal fault that formed coeval with localization of the Pacific‐North America plate boundary in the northern Salton Trough, CA. Apatite U‐Th/He thermochronometry (AHe; n = 29 samples) and thermal history modeling of samples from the Santa Rosa Mountains (SRM) reveal that initial exhumation along the WSDF began at circa 8 Ma, exhuming footwall material from depths of >2 to 3 km. An uplifted fossil (Miocene) helium partial retention zone is present in the eastern SRM, while a deeper crustal section has been exhumed along the Pleistocene high‐angle Santa Rosa Fault (SFR) to much higher elevations in the southwest SRM. Detachment‐related vertical exhumation rates in the SRM were ~0.15–0.36 km/Myr, with maximum fault slip rates of ~1.2–3.0 km/Myr. Miocene AHe isochrons across the SRM are consistent with northeast crustal tilting of the SRM block and suggest that the post‐WSDF vertical exhumation rate along the SRF was ~1.3 km/Myr. The timing of extension initiation in the Salton Trough suggests that clockwise rotation of relative plate motions that began at 8 Ma is associated with initiation of the southern San Andreas system. Pleistocene regional tectonic reorganization was contemporaneous with an abrupt transition from low‐ to high‐angle faulting and indicates that local fault geometry may at times exert a fundamental control on rock uplift rates along strike‐slip fault systems.
Plain Language Summary
The southern San Andreas Fault system is an important component of the Pacific‐North America plate tectonic boundary, but the age of initiation of this significant geological feature in southern California is disputed. We investigated the uplift history of the Santa Rosa Mountains, west of the San Andreas Fault in Coachella Valley, California, by measuring helium in crystals called apatite. The technique known as thermochronometry lets us learn the cooling history and thus the uplift history of mountain ranges. Our work shows that the Southern San Andreas system began forming with extension of Earth's crust along a low‐angle normal fault 8 Ma ago. The timing of southern San Andreas formation was probably synchronous with the beginning of widespread strike‐slip faulting in the Baja Mexico area that would later become the Gulf of California. Much later, around 1 Ma ago, a new strike‐slip fault, called the San Jacinto Fault, deactivated and cut the low‐angle fault and uplifted the southwest Santa Rosa Mountains. This event was associated with punctuated, surprisingly high rates of uplift. These results help geologists to reconstruct the history of this plate boundary and to learn how mountains form along oblique‐extensional and strike‐slip fault systems.
Key Points
Exhumation of the Santa Rosa Mountains (SRM) initiated at circa 8 Ma via low‐angle extension along the West Salton Detachment Fault
Correction of Pleistocene to recent NE tilt of the SRM and thermal modeling improve precision of exhumation initiation timing
Local fault geometry, not plate motion obliquity, exerts primary control on rock uplift rates in the SRM since circa 1.2 Ma
The Blue Ridge escarpment of the southern Appalachian Mountains is a striking and rugged topographic feature of the ancient passive margin of eastern North America. The crest of the escarpment ...generally coincides with an asymmetric regional drainage divide, separating steep streams of the escarpment face from low-gradient streams of the Blue Ridge Upland. Recent exhumation and erosion rate studies suggest that the escarpment has evolved by inland erosional retreat, but the mechanism, timing, and magnitude of retreat remain poorly understood. Longitudinal stream profiles and slope–drainage area relationships of several upland basins draining the divide have led to the identification of 14 previously unknown fluvial terrace deposits preserved at the escarpment crest. These relict terraces and the associated beheaded drainages indicate the role of large stream capture events in producing ongoing escarpment retreat through landward divide migration and subsequent topographic adjustment. Terrace location and preservation suggest that rectilinear drainage patterns and divide asymmetry generate discrete high order captures and episodes of rapid localized retreat that collectively produce slower evolution of the escarpment at large. While overall retreat magnitude and rate remain unknown, roundness of terrace alluvium suggests that the most recent captures have locally produced tens of kilometers of retreat within the limited preservation lifetime of the deposits. In contrast with recent numerical modeling and cosmogenic studies, these data show the potential for stream capture and divide migration to sustain passive margin escarpment evolution long after the cessation of rifting. The fluvial record of divide retreat preserved atop the Blue Ridge escarpment suggests the potential for using field methods to better constrain the histories of younger, taller, and potentially more dynamic passive margin escarpments.
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