GS Field Trip: End of Summer 2016 GS191/291 (Miller, no Klemperer (went to Tibet), Lund Snee, Gottlieb) Listed as Fall Quarter class under Miller Crossing the Cordillera: From the Colorado Plateau to the Sierra Nevada Field trip (1 credit), advanced ugrad to grad level, but all welcome Dates: September 15 to Sept. 21 (T.A. Eric Gottlieb) There is unprecedented controversy about the paleo-topographic evolution of the western United States. How thick and how high was the crust following Mesozoic crustal shortening? Did the region between the Sierra Nevada and the Colorado Plateau form a high plateau, the Nevadaplano, underlain by 60 km thick crust? When did this thick crust collapse to its present crustal thickness of only 30 km? Did extension happen as a result of, during and shortly after crustal thickening? Or did extension take place only in the Miocene, during formation of Basin and Range topography? How much stretching is represented by Basin and Range faulting? Can it account for thinning of crust by 30 km? This field trip will provide you with an exceptional opportunity to view a complete cross-section of the geology of the southern part of the North American Cordillera, from the undeformed Colorado Plateau on the east to Mount Whitney and the Mesozoic Sierra Nevada arc on the west. With these big questions in mind, our stops will focus on Mesozoic shortening, when the Cordillera is believed to have looked like the Andes, and on younger extensional structures (including a variety faults in Death Valley), with a critical view and discussion of age, geometry and offset along normal faults. Journal articles, field trip guidebooks and maps will be available before and during the trip. It will be quite hot but cooler in the evenings. Some of our camping is wilderness camping. Small tent, sleeping bag, etc., suggested. Aerial view of the Cretaceous Keystone thrust system, outside of Las Vegas: thrust fault places grey Cambrian limestone on top of red and white Jurassic Aztec sandstone 1 Introduction This trip will provide an exceptional opportunity to view a complete cross-section of the geology of the southern part of the North American Cordillera, from the Colorado Plateau to the Sierra Nevada. We will focus on the older Mesozoic history of shortening within the Cordillera, when it is believed to have looked like the Andes or southern British Columbia Rocky Mountain fold and thrust belt, as well as its younger history of Cenozoic extension and faulting, which has significantly chopped up and complicated our view of the older geology. Cenozoic normal faults are particularly controversial in terms of their style, geometry and amount of slip, and we will see faults that exhibit the observed range of geometries, dips, and slip amounts. Several of our stops in the western part of our transect will highlight metamorphic and igneous rocks of both Mesozoic and Cenozoic age to evaluate the roles and relationships between deformation, metamorphism and magmatism at deeper crustal levels of the Cordillera. The following section provides thumbnail sketches of what we will be visiting. Approximate routes and mileages follow. Original descriptions and references (if available) are in in the Appendix at the rear of the guidebook. These descriptions are resurrected from hard-to-find field trip guides that still form an important part of the “grey literature” of the geology of the west. It’s work in progress! Fig. 1. The “big picture”: Index geologic map of the western U.S. 2 Fig. 2. Combined California and Nevada state geologic maps. 3 Itinerary and Stops Day 1 (Sept. 15): Drive to campsite outside of Las Vegas in the Spring Mountains (Sites 1 and 2, Hilltop Campground, Spring Mountains National Recreation Area: 36.309345º, -115.607200º, Reservation numbers: 2- 36397156 & 2-36397155, Contact: 888-448-1474). Directions from I-15 northbound, starting south of Vegas. Sept. 15 Thursday Day 1: Drive to campsite outside of Las Vegas in the Spring Mountains The Spring Mountains constitute a large intact structural block that was little-affected by Cenozoic faulting, so it’s a great place to study Paleozoic stratigraphy and Mesozoic thrust fault history. The Spring Mountains expose a series of thrust faults that carry increasingly thick sections of Paleozoic stratigraphy in their hanging walls. Fig. 3. Tectonic sketch map of the Spring Mountains showing the various Mesozoic thrust plates, emplaced from NW to the SE (from Burchfiel et al., 1974). 4 Day 2 (Sept. 16): Overview: We begin our tour with the stratigraphy of the “foreland” of the Cordillera, known as the “Grand Canyon sequence” a thin platformal sequence that covered the western portion of the North American continent after it rifted away from another continent > 600 Ma. This initial rifting led to the deposition of the Cordilleran passive margin sequence in the Paleozoic. The platformal sequence is exposed at Frenchman Mountain, east of Las Vegas, where we will see its basal unconformity above > 1.7 Ga metamorphic and igneous rocks of the North American craton. We will then examine some of the major Mesozoic thrust faults of the Sevier foreland fold and thrust belt. These emplace thicker stratigraphic sequences of the Cordilleran passive margin (subsiding continental shelf) eastward across the much thinner cratonic sequence. We will have ample opportunity to view and learn about the units of the Cordilleran passive margin and you will want of refer frequently to the stratigraphic columns included in this guide. We will discuss the timing of thrusting (Mesozoic) and proceed westwards through the thrust belt, looking at the Red Springs and Keystone thrust faults. Stop 1: The Great Unconformity at Frenchman Mountain. Driving: From Hilltop Campground (1.25 hr / 51 mi without traffic). East on Lake Mead Blvd to where the road passes over the northern part of Frenchman Mountain. Stop and hike to see basal unconformity and talk about the stratified units of the platform sequence and their thicknesses. Reference: Steve Rowland, Geology UNLV. http://geoscience.unlv.edu/pub/rowland/Virtual/virtualfm.html Fig. 4. Location map to Frenchman Mountain. 5 Fig. 5. Google image of Frenchman Mountain. Fig. 6. The basal unconformity is defined by the Tapeats Sandstone (~ 500 Ma) that lies above the Vishnu Schist (1.7 Ga and older here). Reference: Steve Rowland, Geology UNLV, http://geoscience.unlv.edu/pub/rowland/Virtual/virtualfm.html Looking south. Altogether the section above the unconformity is less than 5 km thick. Let’s compare that to the shelf sequences deposited to the west in the next stops and on the next figures! 6 The Great Unconformity is best known from dramatic exposures in the Inner Gorge of the Grand Canyon. The unconformity was named by Clarence Dutton in his 1882 book Tertiary History of the Grand Cañon District. People are sometimes surprised to learn that at Frenchman Mountain they can visit the Great Unconformity without taking a long hike down into a deep canyon. Thank you, Miocene Basin and Range faulting and tilting! Let’s estimate the tilt of units here and the probable geometry of the normal fault bounding Frenchman Mountain. Fig. 7. Cross Section of Frenchman Mountain by James E. Faulds. Reference: Steve Rowland, Geology UNLV, http://geoscience.unlv.edu/pub/rowland/Virtual/virtualfm.html Note that the horizontal and vertical scales are the same and that most of these faults (except for the range-bounding fault) have small displacements. Questions: What do you think of the angles between faults and bedding? Does the Frenchman Mountain normal fault (which bounds the western side of Frenchman Mt.) bottom into a detachment fault with 80 km of net slip? Or does it merge at depth with the DBTZ and is motion compensated by flow of the crust below? The cartoon below illustrates one of the end-member interpretations of what happens to normal faults at depth. Here they are shown as merging into a brittle shallow angle detachment fault with a significant magnitude of slip, increasing to the west! The alternative end-member interpretation is that normal faults merge down into a brittle-ductile transition zone in the crust where their horizontal component of slip is matched by ductile stretching and flow beneath (e.g. the McKenzie pure shear model below): 7 Fig. 8. Cartoon of the low angle brittle fault model of Wernicke (19XX) (top) from Steve Rowland, Geology UNLV, http://geoscience.unlv.edu/pub/rowland/Virtual/virtualfm.html, the classic uniform pure shear model of McKenzie (reference) (middle and a crustal scale cross section from the Sierra east by Surpless et al. (2002) illustrating the Stanford view of normal faults. Stops 2 and 3: Red Rock Canyon Scenic area Driving Directions from the Las Vegas Strip south of Russell Road: Get on to the I-15 south Take exit 36 for Russell Road/215 west Keep left at the fork, follow signs for Interstate 15 south Keep right at the fork, follow signs for 215 west and merge onto 215 west for 13.5 miles Take exit 26 for Charleston Boulevard and turn left Continue onto State Route 159/ West Charleston Boulevard for 5.5 miles. Turn right into the entrance of Red RockCanyon National Conservation Area Driving Directions from State Route 160:Get on to the I-15. Take exit 33 toward State Route160 west /Blue Diamond Road. Keep right at the fork and merge onto State Route 160 west /Blue Diamond Road for 10.5 miles. Turn right onto State Route 159 east/Charleston Boulevard (opposite the gas station) drive for 10.5 miles. Turn left into the entrance of Red Rock Canyon. Fig. 9. Driving to Red Rock Canyon 8 Fig. 10.