[1] The detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide. In particular, the propagation of slab detachment ("tearing") of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift. However, there is little understanding of the causes and dynamics associated with three-dimensional (3-D) slab tearing, especially in the case of ridge-trench collision. Here we show using fully dynamic 3-D numerical models that the process of detachment due to ridge-trench collision depends on the geometry of the ridge segments approaching the trench. For a finite laterally symmetric slab, the 3-D detachment process occurs nearly simultaneously along strike by way of boudinage-type necking and opening of holes central to the slab. For a case involving the approach of two offset ridge segments to the trench, slab tearing occurs in the form of (1) a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and (2) horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments. However, lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone. Whether detachment occurs simultaneously along strike or propagates laterally, the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull.
[1] The approach of a buoyant spreading ridge to a subduction zone may lead to detachment of a subducted slab. Previous work has called upon the detachment process as an explanation for observed ridge abandonment and slab window related magmatism (e.g., in Baja California), but such a scenario has not been tested using fully dynamic numerical models. We use dynamic two-dimensional models including a non-Newtonian rheology to study the approach of a trench-parallel spreading ridge to a subduction zone. We find that before the ridge approaches within 100 km of the trench, detachment of the subducted slab occurs due to the combined effects of increased buoyancy and reduced strength of the lithosphere with ridge proximity. In models exploring effects of subducted slab length, distance of the ridge from the trench, shear zone strength, and lithospheric yield strength, we find the following: (1) a decrease in subduction velocity as the ridge approaches the trench, (2) a shrinking surface plate that maintains a uniform plate-like subduction velocity, (3) ridge abandonment distances 100-275 km from the trench, and (4)
For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit http://www.usgs.gov/ or call 1-888-ASK-USGS (1-888-275-8747).For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod/.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.Suggested citation: Perry, S.C., Blanpied, M.L., Burkett, E.R., Campbell, N.M., Carlson, A., Cox, D.A., Driedger, C.L., Eisenman, D.P., FoxGlassman, K.T., Hoffman, S., Hoffman, S.M., Jaiswal, K.S., Jones, L.M., Luco, N., Marx, S.M., McGowan, S.M., Mileti, D.S., Moschetti, M.P., Ozman, D., Pastor, E., Petersen, M.D., Porter, K.A., Ramsey, D.W., Ritchie, L.A., Fitzpatrick, J.K., Rukstales, K.S., Sellnow, T.S., Vaughon, W.L., Wald, D.J., Wald, L.A., Wein, A., and Zarcadoolas, C., 2016, Get your science used-Six guidelines to improve your products: U.S. Geological Survey Circular 1419, 37 p., http://dx.doi. org/10.3133/cir1419. Executive SummaryNatural scientists, like many other experts, face challenges when communicating to people outside their fields of expertise. This is especially true when they try to communicate to those whose background, knowledge, and experience are far distant from that field of expertise. Why This Publication?At a recent workshop, experts in risk communication offered insights into the communication challenges of probabilistic hazard products, suggested tips, and shared their strategies for making products that a targeted audience can understand and use. Although the workshop was held to broaden the understanding and use of the U.S. Geological Survey (USGS) National Seismic Hazard Maps (NSHM), the workshop outcomes presented in this report can benefit anyone who develops products based on technical information. Why the Workshop?In the United States, earthquakes threaten people in 42 of the 50 States, with 16 States at high risk. The NSHM, which forecast earthquake ground shaking, are important products for earthquake loss reduction and thus are a flagship application of the earthquake hazards research done at the USGS. The seismic provisions of U.S. building codes use the NSHM to save lives, 1 U.S. Geological Survey.2 Natural Hazards Center, University of Colorado Boulder. Get Your Science Used-Six Guidelines to Improve Your Productsand to date, the main user group has been engineers. However, because the NSHM provide a broad view of earthquake ground-shaking hazard across the Nation, they have untapped value for planning, risk reduction, and education, and they have potential users as yet unreached.To expand the use and understanding of the NSHM, the USGS Science Application for Risk Reduction (SAFRR) ...
ShakeAlert-An Earthquake Early Warning System for the United States West Coast E arthquake early warning systems use earthquake science and the technology of monitoring systems to alert devices and people when shaking waves generated by an earthquake are expected to arrive at their location. The seconds to minutes of advance warning can allow people and systems to take actions to protect life and property from destructive shaking. The U.S. Geological Survey (USGS), in collaboration with several partners, has been working to develop an early warning system for the United States. ShakeAlert, a system currently under development, is designed to cover the West Coast States of California, Oregon, and Washington.
Recent seismic imaging of the mantle beneath western North America reveals complexities interpreted as structures ranging from plumes to lithospheric drips and slab fragments. A prominent high-velocity "curtain" beneath Idaho has been interpreted as a remnant of the subducted Farallon plate left dangling within the upper mantle since >40 Ma. Consequently, using numerical models, we explore the rheological, chemical, geometrical, and dynamic conditions under which a slab fragment might persist in the mantle for tens of millions of years. With thermal buoyancy alone, stalled slabs extending to 500 km depth tend to detach and sink vertically within ~17 m.y. for the slab age and rheologic conditions explored here, and shorter slabs <300 km deep have the greatest impact on delaying detachment up to 28 m.y. Otherwise, we fi nd that an unrealistic chemical density contrast of 90 kg/m 3 with respect to the mantle is required for the stalled slab to remain attached to the lithosphere >40 m.y. An increase in upper-to lower-mantle viscosity contrast (1.4× to 100×) can slow sinking velocities and extend slab dangling time by up to 5 m.y. Dynamic effects such as those arising from active nearby subduction only slightly delay or do not affect stalled slab detachment timing but do affect the geometry of the slabs as they respond to suction pressures within the wedge. Overall, a combination of buoyant, viscous, geometric, and dynamic factors may allow cases of extended slab stalling, and conditions we explore here within realistic ranges can so far account for a delay of up to 28 m.y.
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