_05B_Basin_RangeTwo

_05B_Basin_RangeTwo - GEOLOGY 20 - LECTURE 5B - BASIN &...

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1 GEOLOGY 20 - LECTURE 5B - BASIN & RANGE: Part 2 The Birth of Oceans and a Model for the Origin of the Paleozoic Passive Margin and the Modern Basin & Range Rift (Ch. 7, 1 & p.483-490 in Harden - read selectively!) You were introduced to plate tectonics back in lecture 2 when we discussed lithosphere & asthenosphere, convection currents, and convergent plate boundaries as they relate to the Cascadia margin. In this section, you’ll learn how ocean basins form, how continents split apart and the tectonic origins of the Paleozoic passive margin preserved in sedimentary rocks of the B&R. These notes expand upon the introduction to plate tectonics back in lecture 2. How ocean basins form and the tectonic origin of Paleozoic California: Divergent plate boundaries & seafloor spreading Linear patterns of earthquakes and volcanism extend directly through the ocean basins. This geologic activity reflects plate boundaries where plates are pulling apart (diverging), in the process creating new seafloor. Plate boundaries where two plates spread apart to create new lithosphere are called divergent boundaries - the primary example of divergent boundaries on Earth is the 60,000 km long underwater mountain chain known as the mid-oceanic ridge system that circles the globe like the seam on a baseball - running up the middle of the Atlantic is the Mid-Atlantic Ridge , whereas the mid- ocean ridge in the Pacific – the East Pacific Rise – extends up through the East Pacific & into the Gulf of California - in cross-section, mid-ocean ridges form an elongate, low mountain chain whose crest is about 2-2.5 km beneath the sea. The axis of the ridge ( axial ridge ) is marked by a narrow, steep-walled valley called a rift valley where the actual process of underwater volcanism occurs.
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_05B_Basin_RangeTwo - GEOLOGY 20 - LECTURE 5B - BASIN &...

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