The continental lithosphere

Transcription

1 Simplicity to complexity: The continental lithosphere Reading: Fowler p Sampling techniques Seismic refraction Bulk crustal properties, thickness velocity profiles Seismic reflection To image specific structures: faults, deformation patterns 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

2 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 Continental-Oceanic comparison Velocity structure Continental Oceanic crust mantle lithosphere low-velocity asthenosphere transition zone lower mantle? crust mantle lithosphere low-velocity asthenosphere transition zone lower mantle

3 Continental complexity Crustal thickness From surface wave observations North America: km Europe: km Global averages of Continental crust From Christensen & Mooney 1995 Average crustal velocity structure for different tectonic provinces Crustal thickness: Global average: 40 km

4 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: Ma more information about Earth history than Ma oceans Oldest parts of the continents typically at the centers implies continents are formed gradually by accretion at the margins North American velocity structure Thin lithosphere and asthenosphere of western US low velocity depth: 100km S-velocity model: NA95 van der Lee et al 1997 Thick, cold lithosphere of the Archaean craton high velocity North Atlantic lithosphere and asthenosphere

5 Growth of a continent What is the cause of the accretion to western North America? depth: 100km depth: 500km depth: 600km New continental lithosphere. All <250 Ma 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 Growth of a continent Volcanic emplacement Volcanic front corresponds to slab reaching ~100 km H 2 O 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)

6 Growth of a continent Makran subduction zone Arabian plate: subducting under Eurasian plate 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

7 Growth of a continent Subduction zone sediments Makran subduction zone: 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 new continental lithosphere is hot and weak horizontal motions: deformation Continental complexity Horizontal motions Stable, old craton Young, deforming margin Basin and range extension

8 Continental complexity Horizontal motions Basin and Range extension Total extension ~250 km across a 750 km transect Significant crustal thinning Growth of a continent India-Eurasia collision Himalaya Tibetan Plateau China India

9 Building Tibet Several crustal blocks have been sutured to the Eurasian continent 10 cm/yr 5 cm/yr Crustal thickening: Underthrusting of Indian plate Building the Himalaya by crustal underthrusting Having removed eroded material to present day levels: Himalaya Tibetan Plateau

10 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? Horizontal motions today Indian-Eurasian collision from earthquakes

11 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 Trusty timescale