Oblique convergence along strike-slip faults can lead to both distributed and localized deformation. How focused transpressive deformation is both localized and maintained along sub-vertical wrench structures to create high topography and deep exhumation warrants further investigation. The high peak region of the Hayes Range, central Alaska, USA, is bound by two lithospheric scale vertical faults: the Denali fault to the south and Hines Creek fault to the north. The high topography area has peaks over 4000 m and locally has experienced more than 14 km of Neogene exhumation, yet the mountain range is located on the convex side of the Denali fault Mount Hayes restraining bend, where slip partitioning alone cannot account for this zone of extreme exhumation. Through the application of U-Pb zircon, 40Ar/39Ar (hornblende, muscovite, biotite, and K-feldspar), apatite fission-track, and (U-Th)/He geo-thermochronology, we test whether these two parallel, reactivated suture zone structures are working in tandem to vertically extrude the Between the Hines Creek and Denali faults block on the convex side of the Mount Hayes restraining bend. We document that since at least 45 Ma, the Denali fault has been bent and localized in a narrow fault zone (<160 m) with a significant dip-slip component, the Mount Hayes restraining bend has been fixed to the north side of the Denali fault, and that the Between the Hines Creek and Denali faults block has been undergoing vertical extrusion as a relatively coherent block along the displacement “free faces” of two lithospheric scale suture zone faults. A bent Denali fault by ca. 45 Ma supports the long-standing Alaska orocline hypothesis that has Alaska bent by ca. 44 Ma. Southern Alaska is currently converging at ~4 mm/yr to the north against the Denali fault and driving vertical extrusion of the Between the Hines Creek and Denali faults block and deformation north of the Hines Creek fault. We apply insights ascertained from the Between the Hines Creek and Denali faults block to another region in southern Alaska, the Fairweather Range, where extreme topography and persistent exhumation is also located between two sub-parallel faults, and propose that this region has likely undergone vertical extrusion along the free faces of those faults.
more »
« less
Why is Denali (6,194 m) so big? Caught inside the tectonic wake of a migrating restraining bend
Topography along strike‐slip fault restraining bends is theoretically self‐limited by erosion, block translation and the expected abandonment of fault bends. However, Denali (6,194 m) and Foraker (5,304 m) are located within a restraining bend of the dextral Denali Fault system. We reveal the role of bend evolution in mountain building with physical experiments scaled to simulate the Alaska Mount McKinley restraining bend (MMRB). Despite the natural complexity of the MMRB, first‐order patterns (of strike‐slip separation rates, uplift and lateral bend migration) from the geologic data align with patterns from scaled experiments. Thermochronology, seismicity, and slip rate data show that the persistence of a single Denali Fault strand through the ~6 Ma MMRB is facilitated by simultaneous advection of crust through the bend and migration of the eastern vertex of the bend.
more » « less- NSF-PAR ID:
- 10447344
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Terra Nova
- Volume:
- 34
- Issue:
- 2
- ISSN:
- 0954-4879
- Page Range / eLocation ID:
- p. 123-136
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Understanding mechanical conditions that lead to complexity in earthquakes is important to seismic hazard analysis. In this study, we simulate physics‐based multicycle dynamic models of the San Andreas fault (Carrizo through San Bernardino sections) and the San Jacinto fault (Claremont and Clark strands). We focus on a complex fault geometry based on the Southern California Earthquake Center Community Fault Model and its effect over multiple earthquake cycles. Using geodetically derived strain rates, we validate the models against geologic slip rates and recurrence intervals at various paleoseismic sites. We find that the interactions among fault geometry, dynamic rupture and interseismic stress accumulation produce stress heterogeneities, leading to rupture segmentation and variability in earthquake recurrence. Our models produce earthquakes with rupture extents similar to a recent comprehensive paleoseismic catalog. The “earthquake gates” of the Big Bend and the Cajon Pass occasionally impede dynamic ruptures. The angle of compression, which is the subtraction of the maximum shear strain rate direction from the local fault strike, can better determine the likelihood of the impedance of restraining bends to dynamic ruptures. Because the Big Bend has an angle of compression of ∼20°, ruptures that traverse the Big Bend, like the 1857 Fort Tejon earthquake, are more frequent than expected based on empirical relations which predict the ∼40° restraining bend to terminate most ruptures. Our models indicate that large ruptures tend to initiate north of the Big Bend and propagate southwards, similar to the 1857 earthquake, providing critical information for ground shaking assessment in the region.
-
more » « less
Abstract With an obliquity of ∼30° relative to plate motion direction, the ∼300‐km‐long Big Bend of the San Andreas Fault is one of the world's largest restraining bends. The 5–6 Ma (∼160 km of total displacement) longevity of this mechanically inefficient structure and the lack of evidence for associated widespread uplift both challenge existing models of transpression and bend evolution. We focus on the structurally simplest section of the Big Bend (the Mojave section of the San Andreas: MSAF) to characterize the pattern of near‐field (<5 km from the San Andreas trace) uplift over two different timescales. The topography of vertically deformed alluvial surfaces is used to demonstrate that near‐field uplift along the least oblique segment of the MSAF has been significant over the last ∼40 ka (∼1–2 mm/yr), and driven by slip on two oppositely dipping blind reverse faults. Topographic and structural analyses of the MSAF near‐field are conducted at the scale of the entire fault to show that, at least on the NE side of the MSAF, these blind structures coincide with the front of a fault‐parallel bedrock ridge with clear characteristics of a young transpressive ridge. Structural, sedimentary, and geomorphic arguments converge to suggest that these blind structures were activated ∼315 ka ago and record a Mid‐Pleistocene kinematic reorganization of the MSAF fault‐zone. This reorganization is tentatively interpreted as a shift in the mode of accommodation of the transpressive component of plate motion, in turn driven by the strike‐slip advection of crustal strength gradients along the Big Bend.
-
Scaled physical experiments allow us to directly observe deformational processes that take place on time and length scales that are impossible to observe in the Earth’s crust. Successful evaluation of advection and uplift of material within a restraining bend along a strike-slip fault zone depends on capturing the evolution of strain in three dimensions. Consequently, we require deformation within the horizontal plane as well as vertical motions. While 3D digital image correlation systems can provide this information, their high costs have prompted us to develop techniques that require only two DSLR cameras and a few Matlab® toolboxes, which are available to researchers at many institutions. Matlab® plug-ins can perform particle image velocimetry (PIV), a technique used in many analog modeling studies to map the incremental displacements fields. For tracking material advection throughout experiments more suitable Matlab® plug-ins perform particle tracking velocimetry (PTV), which tracks the complete two-dimensional displacement path of individual particles. To capture uplift the Matlab® Computer Vision ToolboxTM, uses pairs of photos to capture the evolving topography of the experiment. The stereovision approach eliminates the need to stop the experiment to perform 3D laser scans, which can be problematic when working with materials that have time dependent rheology. We demonstrate how the combination of PIV, PTV, and stereovision analysis of experiments that simulate the Mount McKinley restraining bend reveal the evolution of the fault system and three-dimensional advection of material through the bend.more » « less
-
Recent field studies provide evidence of fault slip-rate variability over time periods of 10–100 k.y., yet researchers do not know how processes internal to the fault system (e.g., fault reorganization) impact records of fault slip rates. In this study, we directly observed fault-system evolution and measured slip-rate histories within a scaled physical experiment of a dextral strike-slip 15° restraining bend representative of a gentle crustal restraining bend. To assess the degree of slip-rate variability at particular sites along the experimental faults, such as would be revealed in a field study, we tracked fault slip rates at specific locations that advected throughout the experiment with accrued fault slip. Slip rates increased or decreased (5%–25% of the applied velocity) both during fault reorganization (e.g., fault growth and abandonment) and as sites migrated to new structural positions. Sites that advected into the restraining bend showed decreased slip rate. While we expect new fault growth to reduce slip rates along nearby fault segments, we document that the growth of new oblique-slip faults can increase strike-slip rates on nearby fault segments. New oblique-slip thrust faults within the experiment accommodated off-fault convergence and unclamped nearby strike-slip segments. The experimental results show that even under a constant loading rate, slip rates at sites located on stable fault segments can vary due to either reorganization elsewhere in the fault system or site advection.more » « less
