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Back arc regions

Back arc regions. Extensional backarcs thick vs. thin skinned backarcs “ sevier ” vs. “ laramide ” structures. GEOS 425/525 Lecture 16,. Foreland forces and the backarcs. Rollback is most common force causing extension in a backarcs. Rules.

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Back arc regions

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  1. Back arc regions Extensional backarcs thick vs. thin skinned backarcs “sevier” vs. “laramide” structures GEOS 425/525 Lecture 16,

  2. Foreland forces and the backarcs

  3. Rollback is most common force causing extension in a backarcs

  4. Rules • Extensional backarcs appear to be more common in oceanic subduction systems; • They predominate in immature (not very old) subduction regions which tend to make more basaltic crust in the arc proper too; • Not common in Cordilleran sections in their late stages, but identified in earlier stages; • Generate ophiolitic pseudo-sutures in the geologic record; • Could be confused with intervening oceanic basins thus can be mislead for sites of terrane accretion.

  5. What is a compressional backarc • Requires that the upper plate of a subduction system is itself under compressional forces; • Same subduction system can be subject to both backarc compression and extension at different times; • Reasons: • A. strong coupling with the subducting plate (thought to operate in shallow subduction settings); E.g. the Laramide of western North America or parts of the modern central Andes; • B. Plate tectonics-driven, such as an acceleration of absolute plate motion of the upper plate toward the subduction system; E.g. the Sevier system of western North America and the subandean thrust belt in the Andes;

  6. Arc magmatism • Short, high flux events separated by lulls • Baseline fluxes coincide with steady state island arcs (10-30 km3/km Ma). Flare-ups generate 10 times more magma within short (5-15 My) periods. Most mass in continental arcs is made in flare-ups. • Don’t know what ignites the high flux events • We do know they are compressional events.

  7. shaded area: forearc contact between North America and subducting slabs diagonal area: regions of orogeny or extension 80 my: Kula oblique subduction 65 my: horizontal subduction 50 my: Kula/Farallon transform 40 my: horizontal subduction in SW USA and Mexico from: http://element.ess.ucla.edu/publications/1998_Laramide/1998_Laramide.htm

  8. oldest temporal + spatial overlap youngest from: http://www.sinc.sunysb.edu/Stu/ckramer/SevierOrogeny.htm

  9. Sevier schematic • foredeep basin in Utah > 10,000 feet thick • mountains were likely equivalent in elevation to modern Andes • crustal shortening > 100 km • similar deformation in Montana, British Columbia, Alberta from: http://www.ugs.state.ut.us/utahgeo/geo/

  10. Sevier Orogeny …Nevada, western Utah, Idaho… • thin-skinned • thrusting along pre-existing planes of weakness …shales, evaporites from: http://darkwing.uoregon.edu/~millerm/SunRiver1.html 7 thrust sheets thrusting above basal detachment (décollement)

  11. Sevier/Laramide structures in Utah from: http://www.ugs.state.ut.us/utahgeo/geo/

  12. DeCelles, Ducea, Zandt, Kapp, 2009

  13. Laramide Orogeny (late Cretaceous to early Tertiary) …structures in central and eastern Utah, western Colorado, and Wyoming …Uinta uplift, Waterpocket Fold, San Rafael Swell thick-skinned vs. thin-skinned …east-dipping reverse faults flat subduction? as low as 10° …sea-floor spreading rates much faster during this time Engebretson et al., GSA Spec. Paper

  14. Laramide uplifts

  15. Laramide thrusts and basins from: http://www.geolsoc.org.uk/template.cfm?name=fbasins

  16. Laramide cross-section from: http://www.geolsoc.org.uk/template.cfm?name=fbasins

  17. from: http://earth.leeds.ac.uk/postgrads/directory/tozer.htm

  18. from: http://www.sinc.sunysb.edu/Stu/ckramer/SevierOrogeny.htm

  19. Early Paleocene from: http://energy.usgs.gov/factsheets/tertiary/

  20. Middle Paleocene from: http://energy.usgs.gov/factsheets/tertiary/

  21. Late Paleocene from: http://energy.usgs.gov/factsheets/tertiary/

  22. flat subduction?

  23. subduction below South America

  24. flat subduction below Peru from: http://wwwrses.anu.edu.au/~uli/Teaching/PlateTec/SUB/SubAngle.html

  25. Sierra Pampeanas, Argentina: modern analogue for Laramide from: http://www.blackwell-synergy.com/links/doi/10.1046/j.1365-2117.2003.00214.x/full/

  26. cross-section from: http://www.blackwell-synergy.com/links/doi/10.1046/j.1365-2117.2003.00214.x/full/

  27. schematic history from: http://www.blackwell-synergy.com/links/doi/10.1046/j.1365-2117.2003.00214.x/full/

  28. Back-arc compressional styles • “Sevier” - thin skinned, large magnitude shortening; • “Laramide” thick skinned, minor shortening In both cases, a plateau forms.

  29. Both forearc and backarc mass balance require lateral crustal displacements directed into the root of arc regions. We know from regional examples that they end up (at least in some cases) underplated. This is a significant observation for understanding evolution of subduction systems. Barke and Lamb, 2006

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