Orogenic Belts Flashcards
(47 cards)
1
Q
Fold Mountain Belts
A
- Curvilinear tracts of mostly high-standing, folded and faulted rocks
2
Q
Active orogenic belts
A
- Ongoing deformation
- Rapid vertical motion
3
Q
Orogenic belts require?
A
- 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)
4
Q
Subduction orogen
A
- Andes style
- Ocean-continent subduction
- Continental crust built by thickening and possible underplating
- Minor role of magmatism in crustal addition
5
Q
Collisional orogen
A
- Himalayas, Alps style
- Continent-Continent collision
- Little/no mantle-derived material added to crust
- Thickening through duplexing/shortening
6
Q
Accretionary orogen
A
- Canadian Cordillera style
- Material added laterally and vertically (by stacking)
- Forms extensive new continental crust
7
Q
Alpine-Himalayan fold belt
A
Continental collision following the closure of Tethyan oceans btwn Laurasia (North) and Gondwana (South)
8
Q
Alpine-Himalayan fold belt
A
Continental collision following the closure of Tethyan oceans btwn Laurasia (North) and Gondwana (South)
- Affected an area at least 3000 x 4000 km
9
Q
Tibetan plateau contains how much of the Earth’s surface?
A
- 82 percent of Earth’s surface >4km
10
Q
Collision of India w/ Asia
A
- First contact 50Ma
- South edge of Eurasia has been displaced approx. 2000km north relative to Siberia (stable Eurasia)
- Convergence continues at 5cm/yr
11
Q
Geology of Himalayan collision
A
- Main collision preceded by collision of several microcontinents and island arcs
12
Q
Suture zones of Himalayan collision
A
- Preserve ophiolites, high-P metamorphic rocks
- Some ultra-high-P minerals from 60-140km depth
- Jinsha suture, Bangong Nujiang suture, Indus-Zangpo suture
13
Q
Himalaya Tectonic History
A
- 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
14
Q
Present Himalaya tectonic history
A
- 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
15
Q
Himalaya-Tibetan seismicity
A
- 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?)
16
Q
Large-Picture Himalayan collision
A
- 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’
17
Q
What produces double-thickness crust beneath Tibetan plateau?
A
- Underthrusting by India crust for 1000km
- And/or
- India is ‘rigid indentor’ and thickens Tibetan crust by lateral compression
18
Q
Lithospheric Delamination
A
- 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
19
Q
Himalayan seismic tomography
A
- Delaminated upper mantle, cold mantle root
- Unstable, detaches, sinks
- Replaced by upwelling hot asthenosphere
- Further surface uplift
20
Q
Rigid Indenter, 2 possible effects
A
- Confined at both sides
- Open on one side
21
Q
Rigid Indenter effects, Himalaya
A
- 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
22
Q
Lake Baikal
A
- Extension in Russia, North of Himalaya collision
23
Q
Extrusion phases of Himalaya collison
A
- 50-20Ma
- 20-0Ma
- Recent and current, mostly in Himalayas and Lake Baikal, while previous was in N and E of Himalayas, China, Indochina
24
Q
GPS motions relative to stable Eurasia
A
- 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)
25
Tectonic Assemblage
- Distinctive association of rock types, structure, metamorphism linked to a unique tectonic setting (4 settings)
26
4 plate tectonic settings
1. Divergent, ocean plate rocks
2. Convergent, accretionary prism, forearc, volcanic arc, back-arc
3. Transform, SS/pull-apart basins
4. Intraplate
27
Intraplate tectonic setting
- Passive margin (cont-ocean) extensional basin
- Foreland basin (Alberta)
- Mantle plume volcanics
28
Accreted terrane
- Mappable unit w/ different geological history than adjacent units
- Separated from other units by major faults/complex zones/intrusions
- Allochthonous, originates elsewhere
29
Distinguishing features of Accretionary Orogens
- Stratigraphy/sedimentary history
- Magmatic History
- Deformation
- Paleontology
- Paleomagmatism
- Rock geochemistry/isotopes
30
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
31
Large Igneous Provinces
- Thick crust > 20km
| - Ontong-Java, Rockall, Kerguelen plateaus
32
Modern accretion of island arc can be seen where?
Timor
33
Canadian Cordillera Accretionary orogen
- Western N. America
- Approx. 500km wide zone
- 30 percent of continent
- Paleomag shows major northward displacements
34
N.Am. Craton
- Rifted margin formed approx. 750Ma (breakup of Rodinia)
| - E. Antarctica, Australia bordered W. NA
35
West from Cordillera Craton
1. Passive margin sequences 700-160Ma w/ thick (12km) section deposited in Rockies (mainly carbonate and shale)
2. 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
3. Accreted terranes of mostly volcanic arc rocks, some ocean floor material, mostly late Devonian to mid-cretaceous (360-100Ma)
36
Superterranes of Cordillera
- Intermontane
| - Insular
37
Intermontane
- Cache creek, Quesnellia, Stikinia
- Terranes together by end of Triassic
- Accreted in Mid-Jurassic (180-170Ma)
38
Insular
- Wrangellia, Alexander
- Approx. 300Ma stitching pluton
- Accreted in mid-Cretaceous (100Ma)
39
Collision zones
Structural, metamorphic, plutonic tectonic welts
40
Omineca belt
- Overlap btwn cratonic margin and intermontane belt
41
Coast belt
- Collision btwn intermontane and insular belts
42
Cross-sections of Rockies show?
- Most terranes: crustal flakes (<10km, Stikinia is exception)
- Moho relatively flat
- Major strike-slip faults
43
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
44
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)
45
Cache Creek Terrane Stages of accretion
1. Initial impingement by plateau (modern analogue is Emperor Seamounts, indents faults)
2. Late triassic: Indentation (modern analogue, Carolina Rise interacting w/ Mariana Trench)
3. Early Jurassic: Enclosure, rotations around indentation (Modern analogue, Banda Sea )
4. Mid-Jurassic: Collision w/ N. Am. (modern analogue, Molucca Sea arc-arc collision)
46
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
47
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
