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Extensional Tectonics: Rifting and Divergence Processes in Structural Geology & Tectonics Ben van der Pluijm © WW Norton+Authors, unless noted otherwise 3/28/2016 7:45 PM We Discuss …Expressions of Extensional Tectonics • Today’s divergent plate boundaries • Ocean Ridges • Lithologies • Failed Rifts • Structural Styles • Pure shear systems • Simple shear systems • Stretching factor • Metamorphic core complexes • Isostasy • Rift evolution • Passive margins • Causes of rifting and extension Serra do Mar, along the east coast of Brazil. Mountains exposing Precambrian granite and gneiss rise directly out of the ocean along South America’s passive margin. Basalt flow on top of a rhyolitic ignimbrite in the Basin and Range Rift of the western United States. Extensional Tectonics ©PSG&T 2 Today’s Plates and Divergent Boundaries Extensional Tectonics ©PSG&T 3 Continental Rift Systems: East African Rift Gregory Rift Extensional Tectonics ©PSG&T 4 Gregory Rift Mt Kenya Extensional Tectonics ©PSG&T 5 Ancient Continental Rift Systems Stein et al., 2011 Midcontinent Gravity High Extensional Tectonics ©PSG&T 6 Rifts and Ocean Ridges Extensional Tectonics ©PSG&T 7 Morphology of Ocean Ridges and Spreading Rate Fast: East Pacific Rise (EPR); 10+cm/y Slow: MidAtlantic Ridge (MAR); few cm/y Extensional Tectonics ©PSG&T 8 Extra: Petrology of Ocean Ridges (“ophiolites”) Extensional Tectonics ©PSG&T 9 Structural Styles of Rift Systems Pure-shear model Simple-shear (or detachment) model Extensional Tectonics ©PSG&T 10 Examples of Rift Systems Extensional Tectonics ©PSG&T 11 Extension and Stretching Factor Stretch and extension: s= l/lo = 2 e = (l-lo)/lo = 1 (100% extension, so 50% thinning) (recall: s = 1 + e) e is typically 1-3 (s = 2-4) 100-400% extension, 50-75% thinning) Extensional Tectonics ©PSG&T 12 Continental Extension: Metamorphic Core Complexes (MCCs) MCC: idealized cross section Whipple Mnts, CA Extensional Tectonics ©PSG&T 13 Evolution of Metamorphic Core Complexes Whipple Mnts, CA Extensional Tectonics ©PSG&T 14 Extension and Isostasy 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, surface thinning of “block” (upper crust) results in upward flow of “water” (lower crust). In MCCs, thinning by normal faulting results in exhumation of deeper detachment and basement rock. PA = PB is isostatic equilibrium Extensional Tectonics ©PSG&T 15 Extra: Extension and Sedimentation Small basin with sediment fill (a) Thinning and basin formation (b) Cooling and further subsidence, forming broad basin fill (“steerhead basin”) Viking Graben (North Sea) Extensional Tectonics ©PSG&T 16 Rift Evolution and Lithologic Assemblages (a) Rift stage with nonmarine basins (b) rift–drift transition with evaporate deposition (c) drift stage, with seafloor spreading occurring and passive-margin basins evolving, and marine deposition in the basins. Extensional Tectonics ©PSG&T 17 Passive Margins (US Gulf Coast) Note: listric growth faults Extensional Tectonics ©PSG&T 18 Causes of Rifting a) b) c) Rifting above thermal plume. Outer-arc extension of bending slab at a subduction zone. Gravitationally driven extensional collapse of thickened crust in an orogen. Extensional Tectonics ©PSG&T 19 Causes of Rifting, cnt’d d) Backarc extension associated with convergence. e) As plates move apart, continent is stretched and broken apart. f) Pull-apart basin at a releasing bend along a lateral(=strike)-slip fault. Extensional Tectonics ©PSG&T 20