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Simplicity to complexity: The continental lithosphere Reading: Fowler p350-377 Physics and chemistry of the Earth’s interior – Continental lithosphere Sampling techniques Seismic refraction Seismic reflection To image specific structures: faults, deformation patterns Bulk crustal properties, thickness velocity profiles Surface wave analysis Velocity variations, crust/lithospheric thickness on a grand scale The Wind River Thrust extending to ~25 km depth Gravity observations To delineate tectonic blocks, isostasy Geodetic observations Present day horizontal motions; plates and crustal blocks OK, and a little bit of geochemistry and geology Physics and chemistry of the Earth’s interior – Continental lithosphere 1 Oceanic simplicity Crustal thickness: 7 km everywhere (almost) Horizontal lithospheric motions: rigid plates with motion at the boundaries Oceanic crust age distribution Oldest crust: ~160 Ma Oceanic crustal structure: Uniform and well defined Physics and chemistry of the Earth’s interior – Continental lithosphere Continental-Oceanic comparison Velocity structure Continental Oceanic crust crust mantle lithosphere mantle lithosphere ? low-velocity asthenosphere transition zone lower mantle low-velocity asthenosphere transition zone lower mantle Physics and chemistry of the Earth’s interior – Continental lithosphere 2 Continental complexity Crustal thickness From surface wave observations North America: 20-70 km Europe: 25-55 km Physics and chemistry of the Earth’s interior – Continental lithosphere Global averages of Continental crust From Christensen & Mooney 1995 Average crustal velocity structure for different tectonic provinces Crustal thickness: Global average: 40 km Physics and chemistry of the Earth’s interior – Continental lithosphere 3 Continental complexity Crustal age distribution Oldest: The continental core is an Archaean craton Newest: active plate margin accreting material today Grenville and Appalachian rocks: accreted to craton in continental collision Age range: 0-4000 Ma • more information about Earth history than 0-160 Ma oceans Oldest parts of the continents typically at the centers • implies continents are formed gradually by accretion at the margins Physics and chemistry of the Earth’s interior – Continental lithosphere North American velocity structure depth: 100km Thin lithosphere and asthenosphere of western US Î low velocity S-velocity model: NA95 van der Lee et al 1997 Thick, cold lithosphere of the Archaean craton Î high velocity North Atlantic lithosphere and asthenosphere Physics and chemistry of the Earth’s interior – Continental lithosphere 4 Growth of a continent What is the cause of the accretion to western North America? depth: 100km New continental lithosphere. All <250 Ma depth: 500km depth: 600km Farallon slab Subduction zones are responsible for the creation and growth of continents through processes of: • Melt production, intrusive and extrusive volcanism • Accretion of continental, oceanic and island-arc terrains Physics and chemistry of the Earth’s interior – Continental lithosphere Growth of a continent Volcanic emplacement Volcanic front corresponds to slab reaching ~100 km • H2O fluxed melting of asthenosphere • Basalt initiates melting in lower crust • MASH zones develop in lower crust (Melting + Assimilation + Storage + Homogenization) • Subequal proportions of mantle and melted continental crust comprise batholiths • Thicker, older crust promotes larger amounts of assimilation by ascending basaltic magmas • Difficult to detemine how crust gets into the magmas (Source contamination vs. crustal assimilation) Physics and chemistry of the Earth’s interior – Continental lithosphere 5 Growth of a continent Makran subduction zone Arabian plate: subducting under Eurasian plate Physics and chemistry of the Earth’s interior – Continental lithosphere Growth of a continent Subduction zone sediments Makran subduction zone • 6-7 km of sediments on the subducting oceanic Arabian plate • Sediments scraped of plate: frontal fold followed by thrusting • Coastline moving oceanward at ~1cm/yr Physics and chemistry of the Earth’s interior – Continental lithosphere 6 Makran subduction zone: Growth of a continent Subduction zone sediments Model of sedimentary accretion: Trench visible Thick accretionary sediments choke the trench an become part of over-riding plate Makran subduction zone currently at this stage Slab sinks: gradual extinction of volcanoes In the accretionary wedge: extension and development of volcanism – a new continental margin Î new continental lithosphere is hot and weak Î horizontal motions: deformation Physics and chemistry of the Earth’s interior – Continental lithosphere Continental complexity Horizontal motions Stable, old craton Young, deforming margin Basin and range extension Physics and chemistry of the Earth’s interior – Continental lithosphere 7 Continental complexity Horizontal motions Basin and Range extension • Total extension ~250 km across a 750 km transect • Significant crustal thinning Physics and chemistry of the Earth’s interior – Continental lithosphere Growth of a continent India-Eurasia collision Him Tibetan Plateau ala y a China India Physics and chemistry of the Earth’s interior – Continental lithosphere 8 Building Tibet Several crustal blocks have been sutured to the Eurasian continent 10 cm/yr 5 cm/yr Crustal thickening: Underthrusting of Indian plate Physics and chemistry of the Earth’s interior – Continental lithosphere Building the Himalaya …by crustal underthrusting Having removed eroded material to present day levels: Himalaya Tibetan Plateau Physics and chemistry of the Earth’s interior – Continental lithosphere 9 Himalaya-Tibet velocity structure • “Normal” crust and lithosphere beneath the Indian Shield • Very thick (~70 km) crust beneath Himalaya and Tibet, BUT little lithosphere? What happened to lower lithosphere? This question remains unresolved Maybe delamination? Physics and chemistry of the Earth’s interior – Continental lithosphere Indian-Eurasian collision Horizontal motions today …from earthquakes Physics and chemistry of the Earth’s interior – Continental lithosphere 10 Extrusion of China Of the 5cm/yr convergence between India and Eurasia only 1-2 cm/yr occurs across the Himalaya Thrusting within the Tibetan Plateau and extrusion of China accounts for the remainder Horizontal motions within continental lithosphere: More diffuse than in the oceans Physics and chemistry of the Earth’s interior – Continental lithosphere Trusty timescale Physics and chemistry of the Earth’s interior – Continental lithosphere 11