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Lithosphere structure of Central Asia and Tien Shan Keith Priestley Bullard Laboratories, Department of Earth Sciences, University of Cambridge Earthquake Science and Hazard in Central Asia Almaty, Kazakhstan – 7 September 2016 Lithospheric structure of Central Asia and the Tien Shan Keith Priestley Almaty, Kazakhstan −− 7 September 2016 overview – main points of this talk I The LITHOSPHERE is the solid, rigid, brittle outer part of the Earth overlying the plastic, ductile, mechanically-weak ASTHENOSPHERE. overview – main points of this talk I The LITHOSPHERE is the solid, rigid, brittle outer part of the Earth overlying the plastic, ductile, mechanically-weak ASTHENOSPHERE. I There are large variations in the lithospheric thickness of Central Asia. overview – main points of this talk I The LITHOSPHERE is the solid, rigid, brittle outer part of the Earth overlying the plastic, ductile, mechanically-weak ASTHENOSPHERE. I There are large variations in the lithospheric thickness of Central Asia. I The nature of the lithosphere controls the size of felt ground motion area and the large-scale processes affecting the structure of the Earth’s crust. overview of talk I What is the lithosphere? overview of talk I What is the lithosphere? I Locating the lithosphere-asthenosphere boundary overview of talk I What is the lithosphere? I Locating the lithosphere-asthenosphere boundary I Variations in the lithospheric thickness of Central Asia overview of talk I What is the lithosphere? I Locating the lithosphere-asthenosphere boundary I Variations in the lithospheric thickness of Central Asia I Influence of the lithosphere on earthquakes What is the lithosphere? The strong, rigid, brittle outer layer of the Earth lithosphere asthenosphere The lithosphere includes the crust and uppermost mantle. What is the lithosphere? The strong, rigid, brittle outer layer of the Earth rigid plastic The lithosphere includes the crust and uppermost mantle. Locating the lithosphere-asthenosphere boundary Proxies for the lithosphere-asthenosphere boundary The lithosphere-asthenosphere boundary (LAB) coincides with the depth in the upper mantle where the response of materials to forces changes from brittle to plastic. This is primarily controlled by the increase of temperature with depth. wave speed anisotropy strain rate resistivity temperature conducting depth resistive various definitions change in anisotropy LAB rigid LAB LAB LAB deforming conducting convecting adapted from Eaton et al (2009) Locating the lithosphere-asthenosphere boundary seismic surface waves Locating the lithosphere-asthenosphere boundary seismic body waves Locating the lithosphere-asthenosphere boundary seismic receiver functions Locating the lithosphere-asthenosphere boundary seismic anisotropy Locating the lithosphere-asthenosphere boundary electrical resistivity Locating the lithosphere-asthenosphere boundary petrology Locating the lithosphere-asthenosphere boundary temperature vs. depth in the continents Steady−state continental geotherm surface heat flux 51.2 mW/m 2 flux 12.4 mW/m 2 Crust constant heat production granulite composition constant conductivity Mechanical boundary layer Mantle conductivity = f(T) no heat production Base of lithosphere 226 km Thermal boundary layer Isentropic interior Tp = 1315o C Locating the lithosphere-asthenosphere boundary seismic wave paths body waves P surface waves S SKS Fundamental mode higher modes 500 1000 1500 2000 seconds after origin time 2500 3000 surface waves seismograph earthquake P,S SKS mantle outer core core−mantle boundary surface Locating the lithosphere-asthenosphere boundary seismic structure of the Pacific Ocean A 40.000 N, 120.000 E A’ -33.336 S, 83.605 W Japan Hawaii Depth / km A A B’ B’ 4000 2000 0 EPR 0 100 200 300 400 C’ C’ 0 B 10 20 30 40 50 60 70 80 90 100 110 120 -25.000 S, 156.000 E 130 140 150 160 B’ 38.185 N, 123.868 W 4000 2000 0 Hawaii Vanuatu Depth / km 0 B B 100 200 300 C C 400 A’ A’ 0 C 10 20 30 40 50 60 70 -30.000 S, 160.000 E Tonga Depth / km 0 75 km 80 90 C’ 15.350 N, 79.263 W EPR Central America 100 200 300 δ VS (%) VS ref = 4.408 km/s 400 0 -9 -6 -3 0 3 6 9 10 20 30 40 50 60 70 Distance / degrees 80 90 100 110 120 4000 2000 0 Locating the lithosphere-asthenosphere boundary Relationship between shear wave speed and seafloor age Locating the lithosphere-asthenosphere boundary Relationship between shear wave speed and temperature (b) depth (km) shear wave speed vs seafloor age 100 200 300 400 25 50 75 100 125 plate temperature vs seafloor age 20 500C 60 1000C 100 25 150 50 age (mya) 4.0 4.2 4.4 4.6 4.8 5.0 shear wave speed (km/s) (c) 4.7 4.6 shear wavespeed (km/s) depth (km) (a) 4.5 4.4 4.3 50 km 4.2 75 km 4.1 600 800 1000 temperature (C) 1200 100 75 age (mya) 125 150 Locating the lithosphere-asthenosphere boundary seismic estimates of the continental geotherm temperature (oC) 400 800 1200 1600 s idu 100 gr di aph am it on e d temperature (oC) 400 800 1200 0 sol depth (km) 0 100 MBL TBL (a) 179 km 175 km isentropic 200 200 300 (b) Variations in the lithospheric thickness of Central Asia Eastern Asia – topography, seismicity, lithosphere topography and earthquakes lithospheric thickness and earthquakes 55˚ 55˚ Siberian Shield East European Craton 180 160 50˚ 200 160 240 50˚ 220 220 210 190 45˚ 180 180 45˚ Tien Shan 160 190 40˚ Tarim Basin 100 140 100 40˚ 100 160 100 North China Craton 210 35˚ 275 35˚ 160 100 10 0 30˚ Tibet 30˚ 140 275 25˚ 100 180 140 230 210 South China Craton 210 25˚ 18 0 190 20˚ 160 20˚ 15˚ 15˚ 140 140 140 100 10˚ 100 5˚ Lithospheric thickness (km) 100 120 140 160 180 200 220 240 260 110˚ 100˚ 90˚ 80˚ 10˚ 70˚ South Indian Shield Variations in the lithospheric thickness of Central Asia Tien Shan – topography, seismicity, lithosphere lithospheric thickness and earthquakes 160 180 topography and earthquakes 160 45˚ 0 19 45˚ 100 100 140 100 18 0 40˚ 230 40˚ 190 0 20 210 180 16 240 70˚ 275 80˚ 90˚ Lithospheric thickness (km) 100 120 140 160 180 200 220 240 260 0 100 275 35˚ Variations in the lithospheric thickness of Central Asia Baikal Rift – topography, seismicity, lithosphere topography and earthquakes lithospheric thickness and earthquakes 60˚ 60˚ 200 Siberian Craton Siberian Craton 200 0 21 220 160 55˚ 200 0 55˚ 160 16 180 140 100 50˚ 50˚ 180 160 45˚ 45˚ 10 0 100 180 200 140 40˚ 100 90˚ 100˚ 110˚ 120˚ Lithospheric thickness (km) 100 120 140 160 180 200 220 240 260 130˚ Influence of the lithosphere on earthquakes North America – comparison of felt areas topography and earthquakes lithospheric thickness and earthquakes 50˚ 50˚ 50˚ 230 50˚ 19 45˚ 40˚ 35˚ 35˚ New Madrid 1811 −− M 7.5 100 San Francisco 1906 −− M 7.8 Charleston 1886 −− M 7.3 180 30˚ 30˚ 30˚ 35˚ 0 35˚ 40˚ 100 100 20 40˚ 45˚ 20 0 40˚ 0 210 45˚ 45˚ 30˚ 160 140 160 25˚ 25˚ 25˚ 25˚ Modified Mercalli Intensity 140 V 20˚ 100 100 −100˚ −110˚ −120˚ −100˚ −120˚ 0 2000 4000 6000 8000 −80˚ topography (m) −6000 −4000 −2000 −90˚ −80˚ −90˚ −110˚ 100 Lithospheric thickness (km) 100 120 140 160 180 200 220 240 260 Comparison of felt areas (Modified Mercalli Intensity Scale V ) for similar magnitude earthqauakes. The felt area is strongly affected by the presence of the thick lithosphere. 20˚ Influence of the lithosphere on earthquakes different tectonic settings shortening rifting (Tien Shan) (Baikal) Summary – main points of this talk I The LITHOSPHERE is the solid, rigid, brittle outer part of the Earth overlying the plastic, ductile, mechanically-weak ASTHENOSPHERE. Summary – main points of this talk I The LITHOSPHERE is the solid, rigid, brittle outer part of the Earth overlying the plastic, ductile, mechanically-weak ASTHENOSPHERE. I There are large variations in the lithospheric thickness of Central Asia. Summary – main points of this talk I The LITHOSPHERE is the solid, rigid, brittle outer part of the Earth overlying the plastic, ductile, mechanically-weak ASTHENOSPHERE. I There are large variations in the lithospheric thickness of Central Asia. I The nature of the lithosphere controls the size of felt ground motion area and the large-scale processes affecting the structure of the Earth’s crust.