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Plate Tectonics and the Geologic Record What causes the Earth’s cycles of uplift, erosion, and deposition? Hutton’s angular unconformity at Siccar Point, Scotland Plate Tectonics rifting lift up subduction up lif t subduction Tectonic plates separate (diverge) and collide (converge) as they move across the globe. C Plate Margins D transform convergent C D divergent Divergent Plate Boundaries • Plates are moving away from each other. • Magma is upwelling to form new oceanic crust. • Mid-ocean ridges • Continental rift zones (east Africa) Rifting in east Africa Eroding uplands Rift lake Normal fault Rift valley deposition African rift volcanoes Basalt Rock formation in a growing rift basin Rift Lake and shoreline beige siltstone and dark shale Braided river / floodplain red sandstone and siltstone Alluvial fan conglomerate and arkose Basalt lava flows Bounding fault Geologic Evidence for Continental Rifting • Normal faulting - rift valley basins • River and lake sediments – Red beds, siltstones, black shales – Fresh water and terrestrial fossils • Basalt lava flows and subsurface intrusives • Evaporite beds Rifting in east Africa Lake Assal - seawater-fed salt lake at the northern end of the East African Rift. Passive margin Passive Margin •Tectonically inactive •Ocean crust and continent are connected on single plate Passive margin •Continental margin sinks (subsides) as ocean floor cools. Clastic sediments Ancient rift sediments Carbonate sediments Evaporite deposits Geologic Evidence for a Passive Continental Margin • • • • Thick accumulations of limestone. Thin sandstones and shales. Slow rate of deposition of detrital rocks. No volcanoes, no metamorphism, no folding of rock layers. Interbedded limestone, dolostone, and shale Niagara Gorge, NY Convergent Plate Boundaries • Plates are colliding with each other. • 1. Ocean crust - ocean crust • 2. Ocean crust - continental crust • 3. Continental crust - continental crust Ocean - Ocean Convergence Geologic Evidence for OceanOcean Collision • All ocean floor is eventually subducted back into the mantle - little record of the deep sea floor is preserved in the rock record. • Island arc my eventually be pushed against a continent in an island arc to continent collision Ocean - Continent Convergence Foreland Basin foreland basin Andes Mountains - folded and uplifted sedimentary rock Erupting Andean volcano, Ecuador Geologic Evidence for OceanContinent Collision • • • • Metamorphic rock Folded sedimentary rock Granite plutons (magma bodies) Foreland basin filled with thick sequence of eroded sediments (gravel, sand, mud) • Deep sea sediments, volcanic islands, and oceanic crust sutured onto the margin of the continent. Serpentinite - metamorphosed oceanic crust San Francisco, CA (Mesozoic) Staten Island, NY (Paleozoic) Serpentinite bodies are remnant slivers of metamorphosed oceanic crust caught up in ancient plate tectonic collisions. Continent - Continent Convergence Crust thickens greatly, produces high mountains Foreland basin Foreland basin Metamorphism Deposition of detrital sediment eroded from mountains. Geologic Evidence for ContinentContinent Collision • Metamorphic rock • Folded sedimentary rock • Foreland basin filled with eroded sediments – Thick accumulations of mud, silt, and sand – Deposited over geologically short interval of time (several millions of years). Carboniferous sandstone and shale, Pound Gap, Kentucky 37 Foreland basin deposits
