Contractional Tectonics: Convergence and Collision Processes in Structural Geology & Tectonics Ben van der Pluijm WW Norton+Authors, unless noted otherwise 4/12/2017 5:21 PM
We Discuss Contractional Tectonics Convergent Plate Margins The Downgoing Slab The Trench The Accretionary Wedge The Forearc Basin And Volcanic Arc The Backarc Region Stages Of Collision Convergence Suturing Thickening Collapse Delamination Structures Of Collision Fold-thrust Belts Fold-thrust Belts Kinematics Related Processes Extrusion Tectonics Terrane Accretion Orogenic Architecture Synoptic Cross-sections and Orogenic Styles Curved Orogens (Oroclines) Contractional Tectonics PSG&T 2
Today s Major and Minor Plates Divergent Convergent Strike-slip Contractional Tectonics PSG&T 3
Generalized Cross-Section of a Convergent Margin and Main Structural Elements Crust Mantle Contractional Tectonics PSG&T 4
The Downgoing Slab and Peripheral Bulge (a) Peripheral bulge (east of Mariana Trench, W Pacific). Solid line is surface of downgoing slab; dashed line is calculated profile assuming plate is a rigid elastic sheet. (b) Table-top analogy of peripheral bulge. (c) Extension along outer swell produces horsts and grabens at surface of downgoing slab (also called bend-faults fault-bend folds). Contractional Tectonics PSG&T 5
Trench, Trench Fill and Tectonic Mélange Ancient (Paleozoic Dunnage) tectonic mélange near Gander, Newfoundland Contractional Tectonics PSG&T 6
Accretionary Wedge (or Accretionary Prism) (Univergent) wedge Bivergent wedge Contractional Tectonics PSG&T 7
Accretionary Wedge Structures Nankai Trench, Japan Contractional Tectonics PSG&T 8
FTB Kinematics: Imbricate Fan Type Relatively small displacements. Break-forward ( piggy-back ) thrusting. Successively younger thrusts cut into footwall, and older faults and folds become deformed by younger structures. Typical setting: Accretionary wedge. Contractional Tectonics PSG&T 9
Volcanic Arcs Continental Volcanic Arc (Cascades) Oceanic Volcanic Arc (Marianas) Contractional Tectonics PSG&T 10
The Backarc Region (a) Andean Type with contractional backarc region (b) Mariana Type with extensional backarc region Contractional Tectonics PSG&T 11
Continent-Continent Collision (map view) a) Zipper-like collision of continental crust b) Promontories and Recesses Contractional Tectonics PSG&T 12
Stages of Collision 1: Convergence W E Continent A has passive-margin basin on east coast, while Continent B has convergent margin (Andean-type) on west coast. Contractional Tectonics PSG&T 13
Stages of Collision 2: Abortive Subduction and Suturing During initial collision, passive margin is uplifted, and an unconformity develops. Turbidites from Continent B bury this unconformity (inset). Normal faults break up the passive-margin basin, due to stretching. Meanwhile, thrusts develop, transporting deeper parts of the basin over shallower parts. Contractional Tectonics PSG&T 14
Stages of Collision 3: Thickening, Erosion and Exhumation After collisional thickening, exhumation and erosion remove shallower tectonic elements and add new tectonic elements (foreland FTBs). The subducting slab brakes off, while suture (incl ophiolites) is preserved with basement rocks in the orogenic interior. Contractional Tectonics PSG&T 15
Elements of Continental Collision Fold-thrust belt (FTB) in an collisional orogen. FTB lies between foreland basin and metamorphic zone of hinterland (or internal zone) Thrusts eventually cut into foreland basin and incorporate basin material into FTB. Contractional Tectonics PSG&T 16
Extra: Interplay between Orogeny, Climate and Erosion Exhumation pathways under different climatic conditions: (a) Wind from east, so exhumed metamorphic rocks occur in a narrow belt on east side. (b) Wind from west, so broad metamorphic belt occurs on the west. Contractional Tectonics PSG&T 17
Stages of Collision 4: (Syn- to Late-)Orogenic Collapse South Tibetan Detachment at Everest. (Mike Searle) (a) During collision, crust thickens by thrusting. (b) Later, collapse occurs, when extensional faults develop in upper crust and plastic flow at depth. Contractional Tectonics PSG&T 18
Extra: Isostasy, Thickening, Surface Elevation and Keels Application of Archimedes Law of Buoyancy to Earth: the mass of water displaced by a block is equal to the mass of the block So, thickening of block (lithosphere) from collisions results in higher surface elevation and deeper keel. P A = P B is isostatic equilibrium Buoyancy estimates ~1km high continent has ~45km crust ~10km high mountain range has 80km crust matches observations. Contractional Tectonics PSG&T 19
Stages of Collision 5: Intrusion and Delamination (a) Thickening forms keel-shaped mass of cool lithosphere that protrudes into asthenosphere. (b) Keel drops off and sinks ( drips ), and is replaced by warm asthenosphere, causing partial melting and formation of post-orogenic plutons. Mantle drips Contractional Tectonics PSG&T 20
Foreland Fold-Thrust Belts (FTBs) Foreland Internal Zone Foreland Glarner Thrust (Swiss Alps) Canadian Rockies (Jasper, Alb.) Mt. Kidd (Alberta) Contractional Tectonics PSG&T 21
FTB Kinematics: Thrust Duplex Type Relatively large displacements. Duplex develops by progressive break-forward faulting. Roof thrust undergoes a sequence of folding and unfolding. Typical setting: Foreland FTBs. R. Allmendinger Contractional Tectonics PSG&T 22
Related Processes: Indentation and Tectonic Extrusion S China Sea Contractional Tectonics PSG&T 23
Related Processes: Continental Accretion ( Terranes ) and dismemberment Contractional Tectonics PSG&T 24
Future Terranes: Oceanic Plateaus Contractional Tectonics PSG&T 25
Extra: Terranes and Superterranes Terranes: Exotic, relatively buoyant blocks that are accreted to a large continent during subduction (many collisions) Superterranes: Amalgamation of exotic blocks into larger terranes prior to accretion to a larger continent (few collision pulses) Contractional Tectonics PSG&T 26
Synoptic Cross-Sections of Collisional Orogens: Foreland FTBs, Terranes and Sutures ( Appalachian Type ) Appalachians Sutures are fault zones at boundary between once separate plates, typically marked by the occurrence of ophiolite (remnants of oceanic lithosphere). Table Mountain peridotite, part of Bay of Islands ophiolite (Gros Morne, Newfoundland) Contractional Tectonics PSG&T 27
Synoptic Cross-Sections of Collisional Orogens: Nappes, Basement Uplifts and Deep Exhumation ( Alpine Type ) European Alps Morcles nappe of the Helvetic Alps (Switzerland) Coesite (Qz polytype) inclusions in near-colorless pyrope (garnet), exhumed from >100km depth (Dora Maira Massif, Italy) Contractional Tectonics PSG&T 28
Extra: Curved Orogens ( Oroclines ) Map-view curvature of orogens: Originally curved: primary orocline Later curvature: secondary orocline A combination of primary and secondary The Ibero-Armorican orocline of W Europe http://goo.gl/ebvi5j Picos de Europa, N Spain https://en.wikipedia.org/wiki/cantabrian_mountains Contractional Tectonics PSG&T 29
Timing of Tectonic Activity Contractional Tectonics PSG&T 30