Orogenic Belts Flashcards
Fold Mountain Belts
- Curvilinear tracts of mostly high-standing, folded and faulted rocks
Active orogenic belts
- Ongoing deformation
- Rapid vertical motion
Orogenic belts require?
- Lateral compression or plate convergence
- Elevated regions w/o compression are not orogenic belts (eg. thermal doming near rift systems, Africa, or volcanic edifices, Hawaii)
Subduction orogen
- Andes style
- Ocean-continent subduction
- Continental crust built by thickening and possible underplating
- Minor role of magmatism in crustal addition
Collisional orogen
- Himalayas, Alps style
- Continent-Continent collision
- Little/no mantle-derived material added to crust
- Thickening through duplexing/shortening
Accretionary orogen
- Canadian Cordillera style
- Material added laterally and vertically (by stacking)
- Forms extensive new continental crust
Alpine-Himalayan fold belt
Continental collision following the closure of Tethyan oceans btwn Laurasia (North) and Gondwana (South)
Alpine-Himalayan fold belt
Continental collision following the closure of Tethyan oceans btwn Laurasia (North) and Gondwana (South)
- Affected an area at least 3000 x 4000 km
Tibetan plateau contains how much of the Earth’s surface?
- 82 percent of Earth’s surface >4km
Collision of India w/ Asia
- First contact 50Ma
- South edge of Eurasia has been displaced approx. 2000km north relative to Siberia (stable Eurasia)
- Convergence continues at 5cm/yr
Geology of Himalayan collision
- Main collision preceded by collision of several microcontinents and island arcs
Suture zones of Himalayan collision
- Preserve ophiolites, high-P metamorphic rocks
- Some ultra-high-P minerals from 60-140km depth
- Jinsha suture, Bangong Nujiang suture, Indus-Zangpo suture
Himalaya Tectonic History
- Permian-Triassic: Rifting of Tibet from Gondwana
- Late-Triassic-Early Jurassic, 200Ma: N.Tibet-Asia collision, possible back-arc extension separating N and S Tibet
- Late Jurassic-Early Cretaceous 100Ma: S.Tibet-N.Tibet collision, Collision and distributed shortening, closing of Paleo-Tethys ocean, subduction of ocean under Tibet, India moves closer
- Late Cretaceous 80-60Ma: Andean-type subduction margin, shortening w/ some uplift, 10cm/yr Neo-tethys closing
- Early Cainozoic 50Ma: 1st contact of India w/ Tibet and 1st terrestrial sedimentation
- Late Cenozoic 40Ma: Underthrusting at Indus-Zangpo suture, Shortening began, 5cm/yr convergence
- 20Ma: thrusting on MCT
Present Himalaya tectonic history
- Thrusting on Main Boundary Thrust
- Convergence at 1.5cm/yr
- India only partly underthrusting Tibet
- India underthrust Eurasia by at least a few 100 km
Himalaya-Tibetan seismicity
- MBT, MCT, MHT (Main Himalaya Thrust, detachment, top of underthrusting Indian plate)
- Shallow seismicity, thrust mechanisms on all 3)
- Deeper EQ’s (200-300km) to west (contorted continental slab?)
Large-Picture Himalayan collision
- N-S shortening, E-W extension
- N,S,W boundaries of Tibetan plateau well defined
- E margin of Tibet more diffuse, alt deep valleys and high mnt ranges running N-S
- Tibet/China are extruding to E, ‘lateral escape’
What produces double-thickness crust beneath Tibetan plateau?
- Underthrusting by India crust for 1000km
- And/or
- India is ‘rigid indentor’ and thickens Tibetan crust by lateral compression
Lithospheric Delamination
- Underthrusting by India
- Brittle-Ductile transition of crust (quartz, 340-400C), Mantle (olivine, 800C)
- Weak ductile channel in lower crust: detachment zone where strong cooler India crust can be inserted, process of crustal delamination or mantle wedging
Himalayan seismic tomography
- Delaminated upper mantle, cold mantle root
- Unstable, detaches, sinks
- Replaced by upwelling hot asthenosphere
- Further surface uplift
Rigid Indenter, 2 possible effects
- Confined at both sides
- Open on one side
Rigid Indenter effects, Himalaya
- Crustal thickening N of inventor, e.g. Tibetan Plateau
- Strike-slip faults N and E of India (late-tertiary, some still active)
- Lateral escape/extrusion of material
Lake Baikal
- Extension in Russia, North of Himalaya collision
Extrusion phases of Himalaya collison
- 50-20Ma
- 20-0Ma
- Recent and current, mostly in Himalayas and Lake Baikal, while previous was in N and E of Himalayas, China, Indochina
GPS motions relative to stable Eurasia
- Crustal shortening across Himalayas and Tibetan plateau
- Lateral escape (E-W extension and SS faulting) across Tibetan plateau and to E (greatest in S. Tibet)
Tectonic Assemblage
- Distinctive association of rock types, structure, metamorphism linked to a unique tectonic setting (4 settings)
4 plate tectonic settings
- Divergent, ocean plate rocks
- Convergent, accretionary prism, forearc, volcanic arc, back-arc
- Transform, SS/pull-apart basins
- Intraplate
Intraplate tectonic setting
- Passive margin (cont-ocean) extensional basin
- Foreland basin (Alberta)
- Mantle plume volcanics
Accreted terrane
- Mappable unit w/ different geological history than adjacent units
- Separated from other units by major faults/complex zones/intrusions
- Allochthonous, originates elsewhere
Distinguishing features of Accretionary Orogens
- Stratigraphy/sedimentary history
- Magmatic History
- Deformation
- Paleontology
- Paleomagmatism
- Rock geochemistry/isotopes
Constraints on timing of accretion
- Deposition of sediments across terrane boundaries
- Presence of sediments originating from adjacent terranes
- Stitching plutons
- Cross-cutting relationships determined from faults
Large Igneous Provinces
- Thick crust > 20km
- Ontong-Java, Rockall, Kerguelen plateaus
Modern accretion of island arc can be seen where?
Timor
Canadian Cordillera Accretionary orogen
- Western N. America
- Approx. 500km wide zone
- 30 percent of continent
- Paleomag shows major northward displacements
N.Am. Craton
- Rifted margin formed approx. 750Ma (breakup of Rodinia)
- E. Antarctica, Australia bordered W. NA
West from Cordillera Craton
- Passive margin sequences 700-160Ma w/ thick (12km) section deposited in Rockies (mainly carbonate and shale)
- Marginal basin terranes, deep water strata deposited on thinned cont. crust and or oceanic crust, some craton affinity (old detrital zircons), displaced along margin by unknown amount
- Accreted terranes of mostly volcanic arc rocks, some ocean floor material, mostly late Devonian to mid-cretaceous (360-100Ma)
Superterranes of Cordillera
- Intermontane
- Insular
Intermontane
- Cache creek, Quesnellia, Stikinia
- Terranes together by end of Triassic
- Accreted in Mid-Jurassic (180-170Ma)
Insular
- Wrangellia, Alexander
- Approx. 300Ma stitching pluton
- Accreted in mid-Cretaceous (100Ma)
Collision zones
Structural, metamorphic, plutonic tectonic welts
Omineca belt
- Overlap btwn cratonic margin and intermontane belt
Coast belt
- Collision btwn intermontane and insular belts
Cross-sections of Rockies show?
- Most terranes: crustal flakes (<10km, Stikinia is exception)
- Moho relatively flat
- Major strike-slip faults
Mode of Terrane Accretion
- N. Am plate moves west (in absolute sense) since Jurassic opening of N. Central Atlantic
- N. Am collides w/ and accretes terranes to west
- Crustal delamination, only upper crust accreted as large thrust sheet, lower crust/mantle subduct or are underplated
- Terrane Duplication (lateral widening), Stikinia arc bending around Cache Creek ocean, Wrangellia-Alexander intraplate SS faulting along margin, Great Alaska terrane wreck
Cache Creek Terrane
- Oceanic
- Exotic Tethyan faunas
- Between 2 arc terranes (Stikinia, Quesnellia)
- Orocline? (Strontium isotopes, 2 arcs form limbs, Paleomag rotations of Stikinia CCW and Quesnellia CW)
Cache Creek Terrane Stages of accretion
- Initial impingement by plateau (modern analogue is Emperor Seamounts, indents faults)
- Late triassic: Indentation (modern analogue, Carolina Rise interacting w/ Mariana Trench)
- Early Jurassic: Enclosure, rotations around indentation (Modern analogue, Banda Sea )
- Mid-Jurassic: Collision w/ N. Am. (modern analogue, Molucca Sea arc-arc collision)
Molucca Sea
- Arc-arc collision
- W-dipping seismicity to 600km depth
- E-dipping to 200km depth
- Closure of 2 arc-trench systems
- Trapped accretionary wedge melange (now flooring Molucca sea), emplaced ophiolite
Seismic Tomography of N. Am.
- Old slabs in lower mantle beneath NA
1- Deepest, oldest slabs (vertical slab walls, stationary trench, NA was to east), Westward subduction, ocean-ocean
2- Westward motion of NA, Arc-continent collision, slab detached
3- Subduction polarity change, E subduction of Farallon, Slab rollback as NA continues W
