Lecture 8b: Introduction to California Geology

1. Lecture 8b: Introduction to California Geology • In the field one makes small-scale observations of many geological processes. It takes years to assemble such observation into large-scale regional processes, but this lecture sets the stage for understanding the importance of small-scale features retroactively by introducing the big picture. • Questions • How can we use the framework of plate tectonics to make a logical narrative of the geological history of a particular continental margin, California? • How do the features one sees in the field (mountains, valleys, faults, volcanoes, glacial deposits) relate to the stuff we have been discussing in lecture? • Tools • Your eyes • Maps 22

2. Owens Valley Long Valley Sierra Nevada Mojave Desert You are Here 23

3. Long Valley Owens Valley You are Here 24

4. Continental Margin settings • There are four recognized types of ocean-continent margin. California has been all four, at various times in the past billion years. • Atlantic-type: a passive margin, not a plate boundary • Andean-type: subduction close to shore, arc volcanoes built on continental basement • Japanese-type: subduction offshore, with a marginal sea between the arc and the mainland • Californian-type: transform fault, no subduction, no spreading 25

5. Synoptic history of Californian margin Hence the major events affecting the tectonic evolution of California are: • A rifting event in the latest Pre-Cambrian; • Two orogenies, the Antler (Devonian) and the Sonoma (Permian-Triassic): collisions of offshore arcs with North America; • Initiation of continental margin subduction with trench in today’s western Sierra foothills (Triassic-Jurassic); • Interruption of subduction by Late Jurassic Nevadan orogeny (accretion of another island arc terrane), initiation of mature Andean trench-gap-arc system at Franciscan-Great Valley-Sierra location; • Cenozoic subduction of Pacific-Farallon ridge leading to growing no-slab zone, Laramide orogeny in Rocky Mountains, then initiation of basin-and-range extension and San Andreas transform. 26

6. Before 700 Ma: Not a Margin • In the Proterozoic, the west coast of North America was attached to some other continent (East Antarctica?), and had not previously been a continental margin since at least 2 Ga. Beginning around 800 Ma, this other continent rifted away along an irregular margin that truncated the old age provinces and established a stepped western boundary of the continent. 27

7. 700-400 Ma: Atlantic-type Passive Margin • Through mid-Devonian time, this remained a stable passive continental margin (Atlantic type) and accumulated a passive margin sequence (“miogeoclinal belt”) of clastic and carbonate sediments up to several km thick. • The miogeocline is the part of this system on the continental crust, inboard of the continental slope. • It tends to be preserved when the margin is activated and rocks further out, on oceanic crust, are subducted and lost. • These Proterozoic-Paleozoic sediments still make up much of the exposed rock in the Western U.S. 28

8. 400-250 Ma: Japanese-type Offshore subduction • The ocean offshore widened and aged until it became unstable to subduction, which initiated sometime in the early Paleozoic under an offshore arc, with a steadily closing marginal sea attached to the North American plate. 29

9. 400-250 Ma: Japanese-type Offshore subduction • In the late Devonian, this offshore arc ran up against the North American margin in the Antler orogeny The arc terrane ended up accreted to the continent, overlying the miogeocline across the Roberts Mountain Thrust Fault The orogeny ends when the arc terrane is transferred to the North American plate and a new subduction boundary is initiated offshore. 30

10. 400-250 Ma: Japanese-type Offshore subduction • After a couple of arc-polarity reversals, the same thing happened again, more or less, in the Early Triassic Sonoma orogeny, bringing a new sequence of oceanic rocks on top of the miogeocline and the Antler rocks. The island arc so accreted forms the basement for the western Sierra Nevada batholith A modern example of arc-polarity reversal can be seen in the New Ireland-New Britain system in the Western Pacific, resulting from the arrival of the Ontong-Java plateau at the trench. 31

11. 250-50 Ma: Andean-type subduction • After Triassic time, the polarity of subduction remained “normal”; i.e., the North American continent was the upper plate. • Thus a recognizable Andean-type continental margin arc formed, built on basement of Pre-Cambrian North America, Paleozoic miogeocline, and the arc terranes accreted in the Antler and Sonoma events. • The dip angle of the slab was initially steep, with the arc rather close to the trench and minimal deformation deep in the continental interior 32

12. 250-50 Ma: Andean-type subduction • In the late Jurassic, another island arc terrane, riding on the subducting Farallon Plate, collided with the North American continent in the Nevadan orogeny. • This time the system responded by stepping subduction out beyond the new terrane to establish the subduction system that lasted through the Cretaceous and generated the still clearly recognizable sequences of Franciscan trench, Great Valley forearc, and Sierran arc • The Sierra Nevada batholith is the deep assemblage of plutonic rocks that formed under the arc volcanoes. 33

13. 250-50 Ma: Andean-type subduction • A relatively low angle slab became more flat with time, causing a wide arc that propagated inland, farther from the trench (remember the volcanic front is always ~100 km above the Benioff zone, so a flatter slab causes more inland volcanism), and caused extensive compressional deformation in the continental interior. • Caltech geologists and geochemists (notably Silver and Taylor) have done much work to document the age progression of magmatic activity in the Sierra Nevada and Peninsular Ranges batholiths and the progressive incorporation of more continental source materials (higher 18O, 87Sr/86Sr, K2O, less mafic). 34

14. ~50 Ma: End of subduction • In the Cenozoic, the slab became completely horizontal, probably due to progressive decrease in age of the subducting Farallon Plate as the ridge approached the trench. Results include end of calc-alkaline volcanism, major compressive orogeny far inland (Laramide orogeny of the Rocky Mountains) and emplacement of Pelona and related schists under Southern California 35

15. ~50 Ma: End of subduction • The same process can be seen today in Chile, where a relatively flat region of the slab defined by depth to the seismic Benioff zone correlates with a gap between the Central and Southern Volcanic Zones of the Andes and extensive deformation and K-rich volcanism far inland in Argentina. 36

16. <50 Ma: Growth of San Andreas Transform Fault • The flat-slab event was probably related to decreasing age and increasing buoyancy of the slab as the Pacific-Farallon ridge approached the continent • This naturally leads to the next tectonic arrangement, as subduction of the Pacific-Farallon ridge leads to transform motion between Pacific and N. American plates between the Mendocino and Rivera triple junctions. 37

17. Modern Californian Margin • Subduction continues north of Cape Mendocino (the Cascade margin) and south of the Rivera triple junction (Mexican Volcanic Belt). • In between, some combination of drag from the Pacific plate, back-arc type tension from the cascades, and perhaps thermal doming of the North American itself have led to large scale extension of the Basin-and-Range province, forming the characteristic topography of the Great Basin, and perhaps also the Rio Grande Rift. 38

18. Topographic Expression of Extension in Western U.S. Basin and Range Owens Valley Rio Grande Rift 39

19. Modern California: Basin and Range Extension • Eastern California is the westernmost edge of the Basin and Range extensional province. (Owens Valley is a Basin; the Sierra Nevada and White-Inyo Mountains are Ranges). • Extension is accommodated by normal faults. When conjugate normal faults form, dipping in opposite directions with the same strike, the downdropped block between is called a graben. 40

20. Modern California: Strike-Slip tectonics • On the other hand, California is also a transform plate boundary zone, which is accommodated be a series of strike-slip faults. • There is evidence of strike-slip motion across the surface rupture of the 1872 Lone Pine earthquake. This air-photo of the San Andreas Fault shows a somewhat clearer offset drainage. 41

21. Modern California: Transtension and transpression • When you combine strike-slip motion with a component of extension or a component of compression, perhaps due to bends in the faults or motions oblique to the fault directions, you create a number of characteristic topographic features. • Death Valley, and parts of the Owens Valley, are pull-apart basins formed by combined extension and shear. • The Transverse Ranges, like the San Gabriel Mountains, are formed by compression due to a big bend in the San Andreas Fault. 42

22. Modern California: Post-arc magmatism • Large-scale extension of continental lithosphere leads to upwelling of asthenospheric mantle and basaltic volcanism. • This plot shows the thinning of mechanical and thermal lithospheres and the growth upwards of the partially molten region as a function of stretching factor b = (final area) / (initial area). There is substantial Miocene basaltic volcanism through the Basin and Range province. • If the basalt is too dense to erupt, it underplates and heats the crust. Basalt underplating due to finite extension leads eventually to crustal melting and Rhyolite volcanism as at the Rio Grande Rift (Valles Caldera) or Owens Valley (Long Valley Caldera). It takes time to conduct heat through the crust, so we expect a delay between onset of extension (Miocene) and rhyolite activity (Pleistocene). 43

23. Modern California: Post-arc magmatism • Long Valley is a Caldera, an elliptical hole formed by collapse of the roof of a magma chamber along a ring-shaped normal fault. It formed in a single catastrophic eruption 760,000 years ago, ejecting the Bishop Tuff. • The Bishop Tuff is a volcanic ash deposit consisting of airfall units (of individual ballistically emplaced particles), and ignimbrites or pyroclastic flow deposits (resulting from gravity-driven currents of air and suspended particles). The figure at right shows the stratigraphy of the Tuff units (Ig for ignimbrite, F for fall), as exposed at the Big Pumice Cut along Highway 395. 44

24. Pleistocene California: Valley Glaciers • There is considerable evidence of the action of Valley Glaciers that descended from the High Sierra during the Pleistocene ice ages. • Glaciers create a range of characteristic rock deposits and geomorphic features. We will have more time to discuss such things later, but here is the thumbnail version. • A deposit of unsorted, unlaminated debris from glacial outwash is called till. • Mounds of till pushed by glaciers or dropped at locations where the ablation front of the glacier stalled for a time are called moraines: • lateral moraine • medial moraine • terminal moraine • recessional moraine 45