Back arc regions

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Back arc regions

• Extensional backarcs
• Retroarc cycle,thick vs. thin skinned backarcs
• sevier vs. laramide structures

GEOS 425/525 Lecture 11, Sept 14 2009
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Foreland forces and the backarcs
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Rollback is most common force causing extension
in a backarcs
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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.

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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

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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.
• Dont know what ignites the high flux events
• We do know they are compressional events.

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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/199
8_Laramide/1998_Laramide.htm
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oldest
temporal spatial overlap
youngest
from http//www.sinc.sunysb.edu/Stu/ckramer/Sevie
rOrogeny.htm
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Sevier schematic
foredeep basin in Utah gt 10,000 feet thick
mountains were likely equivalent in elevation to
modern Andes crustal shortening gt 100 km
similar deformation in Montana, British Columbia,
Alberta
from http//www.ugs.state.ut.us/utahgeo/geo/
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Sevier Orogeny Nevada, western Utah, Idaho
thin-skinned thrusting along pre-existing
planes of weakness shales, evaporites
from http//darkwing.uoregon.edu/millerm/SunRive
r1.html
7 thrust sheets
thrusting above basal detachment (décollement)
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Sevier/Laramide structures in Utah
from http//www.ugs.state.ut.us/utahgeo/geo/
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The concept of a tectono-magmatic cycle in
major orogenic belts
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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
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Laramide uplifts
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Laramide thrusts and basins
from http//www.geolsoc.org.uk/template.cfm?name
fbasins
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Laramide cross-section
from http//www.geolsoc.org.uk/template.cfm?name
fbasins
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from http//earth.leeds.ac.uk/postgrads/directory
/tozer.htm
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from http//www.sinc.sunysb.edu/Stu/ckramer/Sevie
rOrogeny.htm
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Early Paleocene
from http//energy.usgs.gov/factsheets/tertiary/
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Middle Paleocene
from http//energy.usgs.gov/factsheets/tertiary/
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Late Paleocene
from http//energy.usgs.gov/factsheets/tertiary/
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flat subduction?
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subduction below South America
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flat subduction below Peru
from http//wwwrses.anu.edu.au/uli/Teaching/Plat
eTec/SUB/SubAngle.html
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Sierra Pampeanas, Argentina modern analogue for
Laramide
from http//www.blackwell-synergy.com/links/doi/1
0.1046/j.1365-2117.2003.00214.x/full/
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cross-section
from http//www.blackwell-synergy.com/links/doi/1
0.1046/j.1365-2117.2003.00214.x/full/
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schematic history
from http//www.blackwell-synergy.com/links/doi/1
0.1046/j.1365-2117.2003.00214.x/full/
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Back-arc compressional styles

• Sevier - thin skinned, large magnitude
  shortening
• Laramide thick skinned, minor shortening
• In both cases, a plateau forms.

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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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On Tuesday

• Trenches and forearcs