Mountain peaks influence local climate, are proxies for topographic youth and exhumation, and are the most recognized features of mountain belts, yet are largely incidental to the modern conceptual framework of orogenic erosion that is focused on valley incision. Here it is shown that the three-dimensional distribution of prominent peaks is related to the confluences of major divides and thus to drainage basin structure. Divide-junction summits dominate mountain landscapes in both glacial and fluvial settings and likely result from the inherent stability of pyramidal peaks, in terms of both protection from valley erosion and greater mechanical stability relative to linear ridges. The potential stability of divide-junction peaks may make them anchor points for drainage divide networks, and thus the peaks work against the tendency of drainage divides to migrate. The influence of divide structure on peak heights also suggests that differences in headwall retreat rates may result in different frequency and relative relief of peaks in glacial versus fluvial settings. These results imply that interfluve topography and drainage divide structure offer relevant information for the understanding of landscape evolution.
The San Emigdio Mountains are an example of an archetypical, transpressional structural system, bounded to the south by the San Andreas strike-slip fault, and to the north by the active Wheeler Ridge thrust.Apatite (U-Th)/He and apatite and zircon fission track ages were obtained along transects across the range and from wells in and to the north of the range.Apatite (U-Th)/He ages are 4-6 Ma adjacent to the San Andreas fault, and both (U-Th)/He and fission track ages grow older with distance to the north from the San Andreas.The young ages north of the San Andreas fault contrast with early Miocene (U-Th)/He ages from Mount Pinos on the south side of the fault.Restoration of sample paleodepths in the San Emigdio Mountains using a regional unconformity at the base of the Eocene Tejon Formation indicates that the San Emigdio Mountains represent a crustal fragment that has been exhumed more than 5 km along the San Andreas fault since late Miocene time.Marked differences in the timing and rate of exhumation between the northern and southern sides of the San Andreas fault are difficult to reconcile with existing structural models of the western Transverse Ranges as a thin-skinned thrust system.Instead, these results suggest that rheologic heterogeneities may play a role in localizing deformation along the Big Bend of the San Andreas fault as the San Emigdio Mountains are compressed between the crystalline basement of Mount Pinos and oceanic crust that underlies the southern San Joaquin Valley.
Abstract 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 10 Be 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.
Low‐temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the San Andreas fault zone in southern California has experienced recent, rapid exhumation. Eight apatite (U‐Th)/He ages from a 1‐km‐relief section along Yucaipa Ridge in the San Bernardino Mountains range from 1.4 to 1.7 Ma. The minimal change in age with elevation implies exhumation of ∼5–7 mm yr −1 , sustained for at least several hundred thousand years. Three titanite helium ages from the ridge are much older, ranging from 57 to 82 Ma. These show a steep gradient with elevation, representing either an exhumed, partial retention zone or slow cooling through much of the Tertiary. These data imply that a total exhumation of ∼3 to 6 km has occurred since 1.8 Ma. It is uncertain whether this exhumation terminated as early as 1 Ma or has continued up to the present at a decelerated rate. We surmise that this exhumation represents rock uplift in the absence of major surface uplift, in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief. The record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion. Such rapid, large‐magnitude exhumation within the strands of the San Andreas fault zone is important for models of transpressional tectonics. It is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain. However, it is inconsistent with a stress‐partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike‐slip fault. In addition, a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust. These models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far‐field borderlands, rather than concentrated in the near field. Alternatively, the exhumation of Yucaipa Ridge may have been driven by the nearby restraining bend in the San Andreas fault at San Gorgonio Pass, in which case it represents local fault geometry rather than accommodation of far‐field plate motion.
Research Article| June 01, 2014 Volcanoes of the passive margin: The youngest magmatic event in eastern North America Sarah E. Mazza; Sarah E. Mazza 1Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, USA Search for other works by this author on: GSW Google Scholar Esteban Gazel; Esteban Gazel * 1Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, USA *E-mail: egazel@vt.edu. Search for other works by this author on: GSW Google Scholar Elizabeth A. Johnson; Elizabeth A. Johnson 2Department of Geology and Environmental Science, James Madison University, MSC 6903 Memorial Hall, Harrisonburg, Virginia 22807, USA Search for other works by this author on: GSW Google Scholar Michael J. Kunk; Michael J. Kunk 3U.S. Geologic Survey, 12201 Sunrise Valley Drive, MS 926A, Reston, Virginia 20192-0002, USA Search for other works by this author on: GSW Google Scholar Ryan McAleer; Ryan McAleer 3U.S. Geologic Survey, 12201 Sunrise Valley Drive, MS 926A, Reston, Virginia 20192-0002, USA Search for other works by this author on: GSW Google Scholar James A. Spotila; James A. Spotila 1Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, USA Search for other works by this author on: GSW Google Scholar Michael Bizimis; Michael Bizimis 4Department of Earth and Ocean Sciences, University of South Carolina, 701 Sumter Street, EWS 617, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar Drew S. Coleman Drew S. Coleman 5Department of Geological Sciences, University of North Carolina Chapel Hill, CB#3315, Chapel Hill, North Carolina 27599, USA Search for other works by this author on: GSW Google Scholar Geology (2014) 42 (6): 483–486. https://doi.org/10.1130/G35407.1 Article history received: 21 Dec 2013 rev-recd: 05 Mar 2014 accepted: 06 Mar 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Sarah E. Mazza, Esteban Gazel, Elizabeth A. Johnson, Michael J. Kunk, Ryan McAleer, James A. Spotila, Michael Bizimis, Drew S. Coleman; Volcanoes of the passive margin: The youngest magmatic event in eastern North America. Geology 2014;; 42 (6): 483–486. doi: https://doi.org/10.1130/G35407.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The rifted eastern North American margin (ENAM) provides important clues to the long-term evolution of continental margins. An Eocene volcanic swarm exposed in the Appalachian Valley and Ridge Province of Virginia and West Virginia (USA) contains the youngest known igneous rocks in the ENAM. These magmas provide the only window into the most recent deep processes contributing to the postrift evolution of this margin. Here we present new 40Ar/39Ar ages, geochemical data, and radiogenic isotopes that constrain the melting conditions and the timing of emplacement. Modeling of the melting conditions on primitive basalts yielded an average temperature and pressure of 1412 ± 25 °C and 2.32 ± 0.31 GPa, corresponding to a mantle potential temperature of ∼1410 °C, suggesting melting conditions slightly higher than average mantle temperatures beneath mid-ocean ridges. When compared with magmas from Atlantic hotspots, the Eocene ENAM samples share isotopic signatures with the Azores and Cape Verde. This similarity suggests the possibility of a large-scale dissemination of similar sources in the upper mantle left over from the opening of the Atlantic Ocean. Asthenosphere upwelling related to localized lithospheric delamination is a possible process that can explain the intraplate signature of these magmas that lack evidence of a thermal anomaly. This process can also explain the Cenozoic dynamic topography and evidence of rejuvenation of the central Appalachians. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract The nature of the connection between the Eastern California shear zone (ECSZ) and the San Andreas fault (SAF) in southern California (western United States) is not well understood. Northwest of San Gorgonio Pass, strands of the ECSZ may be migrating south and west into the convergent zone of the San Bernardino Mountains (SBM) as it is advected to the southeast via the SAF. Using high-resolution topography and field mapping, this study aims to test whether diffuse faults within the SBM represent a nascent connection between the ECSZ and the SAF. Topographic resolution of ≤1 m was achieved using both lidar and unmanned aerial vehicle surveys along two Quaternary strike-slip faults. The Lone Valley fault enters the SBM from the north and may form an along-strike continuation of the Helendale fault. We find that its geomorphic expression is obscured where it crosses Quaternary alluvium, however, suggesting that it may have a low rate of yet-undetermined activity. The Lake Peak fault is located farther south and cuts through the high topography of the San Gorgonio massif and may merge with strands of the SAF system. We find that this fault clearly cuts Quaternary glacial deposits, although the magnitude of offset is difficult to assess. Based on our interpretation of geomorphic features, we propose that the Lake Peak fault has predominantly dextral or oblique-dextral motion, possibly with a slip rate that is comparable to the low rates observed along other strands of the ECSZ (i.e., ≤1 mm/yr). Comparing the geomorphic expressions of these faults is difficult, however, given that the erosive nature of the mountainous landscape in the SBM may obscure evidence of active faulting. Based on these observations, as well as the occurrence of other diffuse faults in the region, we suggest that dextral strain is overprinting the actively convergent zone of the SBM, thereby creating a throughgoing connection between the ECSZ and the SAF west of San Gorgonio Pass.
