This chapter contains sections titled: Introduction Previous Models for Axial Valleys Model Formulation Results for Stretching a Single Layer Crustal Accretion and Mantle Stretching Conclusions
The great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting. To investigate how these and other factors affect rifting and the development of passive continental margins, we develop a simplified model of lithospheric extension. We consider the evolution of extensional deformation for a three‐layer model lithosphere bounded laterally by much stronger lithosphere. The cold part of the crust and mantle are treated as thin brittle/plastic layers. The lower crust is approximated as a thin viscous channel. Each brittle/plastic layer can extend in only one location determined by the strength of the layer, shear of the lower crust, and buoyancy forces related to both crustal thickness variations and thermally induced density differences. The lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle/plastic layers are horizontally offset, a situation we term shear decoupling. As in previous studies, we see three distinct patterns, or modes, of extensional deformation that occur under different sets of model parameters: the core complex mode, the wide rift mode, and the narrow rift mode. Shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values. We are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions. We show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas. Predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins, including very wide and asymmetric margins.
Research Article| August 01, 1998 Styles of extensional decoupling John R. Hopper; John R. Hopper 1Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar W. Roger Buck W. Roger Buck 2Lamont-Doherty Earth Observatory and the Department of Earth and Environmental Sciences, Columbia University, Palisades, New York, 10964 Search for other works by this author on: GSW Google Scholar Author and Article Information John R. Hopper 1Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark W. Roger Buck 2Lamont-Doherty Earth Observatory and the Department of Earth and Environmental Sciences, Columbia University, Palisades, New York, 10964 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1998) 26 (8): 699–702. https://doi.org/10.1130/0091-7613(1998)026<0699:SOED>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation John R. Hopper, W. Roger Buck; Styles of extensional decoupling. Geology 1998;; 26 (8): 699–702. doi: https://doi.org/10.1130/0091-7613(1998)026<0699:SOED>2.3.CO;2 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 Simplified models of continental lithospheric extension demonstrate that the strength of the lower crust is an important factor in the evolution of rifting. When the lower crust is strong, both the crust and the mantle lithosphere should extend in the same place. When the lower crust is weak, however, the upper crust can mechanically decouple from the lithospheric mantle during extension. Two distinct styles of extensional decoupling can be recognized. Diffuse decoupling occurs when the lower crust flows laterally in response to topographically induced pressure gradients. Offset decoupling occurs when stretching of the upper crust is horizontally displaced from mantle lithospheric stretching. We show that diffuse decoupling is expected for a range of possible crustal mineralogies, but that offset decoupling only occurs for extremely weak mineralogies. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
We simulate jumps of ocean spreading centers with axial high topography using elastoplastic thin plate flexure models. Processes considered include ridge abandonment, the breaking of a stressed plate on the ridge flank, and renewed spreading at the site of this break. We compare model results to topography at the East Pacific Rise between 15°25′N and 16°N, where there is strong evidence of a recent ridge jump. At an apparently abandoned ridge, gravity data do not suggest buoyant support of topography. Model deflections during cooling and melt solidification stages of ridge abandonment are of small vertical amplitude because of plate strengthening, resulting in the preservation of a “frozen” fossil high. The present‐day high is bounded by slopes with up to a 40% grade, a scenario very difficult to achieve flexurally given generally accepted constraints on lithospheric strength. We model these slopes by assuming that the height at which magma is accreted increases rapidly after the ridge jumps. This increase is attributed to high overburden pressure on melt that resided in an initially deep magma chamber, followed by a rapid increase in temperature and melt supply to the region shortly after spreading began. The high is widest at the segment center, suggesting that magmatic activity began near the center of the segment, propagated south and then north. The mantle Bouguer anomaly exhibits a “bull's‐eye” pattern centered at the widest part of the high, but the depth of the axis is nearly constant along the length of the segment. We reconcile these observations by assigning different cross‐axis widths to a low‐density zone within the crust.
Significance The efficiency of erosion in leveling relief mainly depends on climate and strength of exposed rocks. However, whether erosion is sufficiently efficient to influence the architecture of a tectonic plate boundary remains a topic of debate. Here, we analyze continental rift landscapes reworked by river incision to assess a globally representative range of fluvial erosion efficiency. We then simulate crustal extension exposed to surface processes acting within this documented range. We find that more efficient erosion favors the growth of half-grabens over horsts, which can explain contrasting tectonic styles across the Basin and Range province and the East African Rift. This suggests that variability in Earth’s geological structures partly reflects variability in hydrological conditions and associated surface processes.
Abstract Crustal properties of young oceanic lithosphere have been examined extensively, but the nature of the mantle lithosphere underneath remains elusive. Using a novel wide-angle seismic imaging technique, here we show the presence of two sub-horizontal reflections at ∼11 and ∼14.5 km below the seafloor over the 0.51–2.67 Ma old Juan de Fuca Plate. We find that the observed reflectors originate from 300–600-m-thick layers, with an ∼7–8% drop in P-wave velocity. They could be explained either by the presence of partially molten sills or frozen gabbroic sills. If partially molten, the shallower sill would define the base of a thin lithosphere with the constant thickness (11 km), requiring the presence of a mantle thermal anomaly extending up to 2.67 Ma. In contrast, if these reflections were frozen melt sills, they would imply the presence of thick young oceanic lithosphere (20–25 km), and extremely heterogeneous upper mantle.
We present a novel explanation for absolute trench‐normal motions of slabs surrounding the Pacific. Rapid subduction‐zone retreat on the eastern side of the Pacific and slow advance in the west can result from the large‐scale asymmetric plate configuration. We use simple fluid dynamics to explain the mechanical background of this hypothesis, and we use the results of a simple finite difference scheme to estimate the effect on trench motion velocities. The hypothesis is based on two key assumptions. First, we follow the concept of plate‐scale horizontal counterflow in the asthenosphere driven by accretion of asthenosphere into lithosphere and by plate motion. Second, we assume that horizontally wide slabs without large slab windows drift passively in the mantle flow field and do not retreat as a result of flow around the slab. If the asthenosphere transfers flow‐related horizontal shear stress into deeper levels of the mantle, an asymmetry in the plate configuration leads to different net pressure forces on the two slabs and thus affects the retreat behavior. In an ocean with an asymmetric ridge position, the slab of the smaller plate should retreat faster than the slab of the large plate, which may even advance. Also, the domain of a slower moving plate should collapse faster than the domain of the faster plate. Our model explains the counterintuitive negative correlation between slab age and retreat velocity observed in the Pacific. It also accords with the topographic asymmetry of the ridge flanks along the Pacific rise.
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