Parts of the Sevier/ Laramide Orogeny AA. Accretionary Prism BB. Forearc Basin Sediment scraped off of subducting plate Sediment derived from the volcanic arc CC. Volcanic Arc Magmatic intrusion into the crust Subducting ocean plate The California Coast Ranges are largely composed of the Mesozoic accretionary prism The Great Valley of California is largely composed of Sevier forearc sediments The Sierra Nevada Mtns are the old magma chambers, of the arc, largely of Sevier age DD. Fold and Thrust Belt E E. Basement-cored uplifts Thin-sheet thrusting within the sedimentary layers Thrusting through upper crust (sediment layer is thin here) The Rocky Mtns are Laramide uplifts These ridges in western Wyoming are thrust sheets of Sevier age Basement-cored Laramide uplift. E D C 1
Evidence for major east-west crustal shortening during the Sevier orogeny Evidence for a central Nevada topographic high, the flow of ~45 m.y. old rivers. This was thinned by east-west extension (45-0 m.y. ago) related to core complex and recent Basin & Range extension. Map of old rivers flowing away from a high-standing central Nevada drainage divide. Nevada stood very high for these rivers to flow over the top of the Sierra Nevada. A Estimated crustal thickness after Sevier orogeny. This would have created a north-trending topographic high running through Nevada. B 2 A. Old east-flowing river valley in NE Nevada, preserved in a young, north-trending range of the Basin & Range. B. Old river sediments in the old valley.
Laramide orogeny >Beginning about 80 Ma >Unusual mountain building far inland; magmatism stops. >Caused by flat-slab subduction. >Ends ~ 40-50 Ma (depending on latitude) with a profound transition to extensional tectonics and vigorous magmatism. As the Laramide started, magmatism first swept inland, then shut off (all te while subduction continued). Laramide blocky uplifts (mostly basement cored) 3
Flat-slab subduction. They thought it couldn t happen. But then they found it beneath South America. And it does just what was observed for the Laramide: (1) basement cored uplifts far inland from the subduction zone, and (2) a cessation of magmatism ( volcanic gap ) between the uplifts and the subduction zone. Now the Laramide orogeny commonly is attributed 0 100 to the subduction of the twin of the Shatsky Rise. 200 Where this ocean plateau went beneath southern 300 California,it tore much of the crust apart. 400 500 600 700 km A 0 100 200 300 400 500 600 700 km B Laramide-style uplifts volcanic arc volcanic gap volcanic arc volcanic gap 4 volcanic arc
Flat-slab subduction. They thought it couldn t happen. But then they found it beneath South America. And it does just what was observed for the Laramide: (1) basement cored uplifts far inland from the subduction zone, and (2) a cessation of magmatism ( volcanic gap ) between the uplifts and the subduction zone. A B 0 100 200 300 400 500 600 700 km 0 100 200 300 400 500 600 700 km Now the Laramide orogeny commonly is attributed to the subduction of the twin of the Shatsky Rise. Where this ocean plateau went beneath southern California,it tore much of the crust apart. Shatsky Rise Twin Laramide-style uplifts 5
Cenozoic Major transition from non-volcanic and compressional to vigorously volcanic and extensional starting at about 55 m.y. ago and continuing to the present Ch a Volcanic flareup Cascade Arc lli (ages in m.y.) s (black with red outline) -t re vo nd lc 45 an - 5 ism 5 M a Siletzia accretion and the Ignimbrite flareup Siletzia ocean lithosphere accreted about 55 m.y. ago. It was quickly followed by the strong Challis-trend volcanic event and associated Pacific Northwest core-complex extension. This ended Laramide contraction in the Pacific Northwest and started the north-to-south propagation of volcanism and core-complex extension across the area now occupied by the northern Basin and Range (fine red outline). Meanwhile, another sweep of volcanism and core complex extension swept NW across the area now occupied by the central Basin and range (fine green outline). The Laramide was over by about 40 m.y. ago.
a 60 Ma 50 Ma 40 Ma b 1 10 100 count deg 2 m.y. 195 km -3 0 3 δv P (%) c 60 Ma 50 Ma 40 Ma Farallon plate C.E. Farallon plate Siletzia Challis Farallon plate Cascades JdF plate Farallon plate Siletzia Kr pluton Volcanic area Core complex Strike-slip fault Subduction zone d old Idaho batholith Challis Cascades 100 - Cascades Farallon Pc N.A. Siletzia 200-300 - 400-500 - 60 Ma 50 Ma 40 Ma 600 - km Present 7
Cenozoic Magmatism The two migrating fronts of volcanism. Where this magmatic event occurred it made weak lithosphere; it now is the Basin and Range province, and before that, it is where the core complexes occurred. Challis - trend volcanism Volcanic flareup (ages in m.y.) Cascade Arc 45-55 Ma From A. Glazner Magmatism in the western U.S. since 40 m.y. ago. The age of magma emplacement is shown below for the yellow dots. Mendocino triple junction 8
Hotspots in the Pacific Northwest Columbia River Flood basalts at Hells Canyon Yellowstone Columbia River Flood basalts Steens Flood basalts Columbia River/Steens Flood basalts 14-16.5 m.y. ago
KULA PLATE? KULA PLATE? FARALLON PLATE FARALLON PLATE? FARALLON PLATE? 10
NORTH AMERICAN PLATE NORTH AMERICAN PLATE NORTH AMERICAN PLATE NORTH AMERICAN PLATE Making the San Andreas Fault With sea floor spreading, the Pacific plate grew westward as the Farallon plate subducts. Since the Pacific plate moves nearly parallel to the margin of the North American plate, once it grows against North America it is separated from North America by a transform fault. When this shear zone steps into the North American plate about 5 m.y. ago, it (1) is called the San Andreas fault, and (2) opens the Gulf of California.
Deformation Rate Current-Day Deformation of the western U.S. Deformation Rate (% / m.y.) Yak f&th W Good Plate cn Vs b OR block LB No rth er ic a ecasz n Pl at e Bo rd er la nd ic Pl A fc o lf c if Gu Pa Makes geology (e.g., like here) Provides clues on > tectonic forces > rheology F SA Am Bad Plate at e Goals: Understand > the origin of forces and > the control of strength 12