Mormon Point Exercise (Day 4) Reading: Frankel et al, 2016

After viewing the Confidence Hills from a distance, we are now at Mormon Point, the outer corner of the large southernmost releasing bend in the Death Valley Fault Zone. As we drove north along the valley, we passed a number of obvious fault scarps (right lateral + normal slip components) that cut low hills of Late Quaternary sediment stranded above a detachment as well as basement rocks that underlie an extensional fault zone than dips and extends (no pun intended) under the basin. Mormon Point also contains a spectacular northwest plunging antiformal culmination in the detachment that exhumes basement rocks (a “Turtleback” which also defines the sense of slip, down to the NE on the detachment). Mormon Point is thus a remarkable opportunity to view exhumed mid crustal rocks brought up by the detachment and their plastic to brittle overprinting of strain. A range of arguments have been proposed for how turtlebacks form, including fault corrugations and rotation of previously steeper faults.

Shaded relief map of Mormon Point area. We will hike partway up the canyon in the lower part of the image.

What are the parent rocks in the most highly exhumed rocks (lower exposures)? Sedimentary, igneous or metamorphic?

Exposures here are dramatically altered from their original rock types. What do you see in the exposures that suggest deformation or alteration has occurred, including both mineralogical and deformational evidence?

Given this is an extensional fault zone, with exhumation of deeper rocks to progressively shallower levels, plastically deformed rocks (i.e. mylonites) are overprinted by brittle fault breccias and cataclasites as they move upward. How much total displacement do you think has occurred across the entire shear zone? Tens of meters, a thousand meters, several kilometers, or more than 10 kilometers?

As you climb up into the higher exposures, what direction are you going in the crust? Up or down? What are the youngest, shallowest rocks in the outcrops (located at the highest part of the exposures)?

Mormon Point fault scarps

As we have now rounded the corner of Mormon Point itself, look at how far the range front steps away to the east. Is this a right, or left step in the (oblique-right lateral) Death Valley fault system? What do you think is the sense of slip on the faults that extend across the fault step?

You should now have determined whether this is a releasing bend or a restraining bend. Which one is it? What type of faults do the scarps define? Normal or reverse?

As we walk along the rangefront, what type of unconsolidated sediments do the fault scarps offset? How old do you think these might be? Hundreds, thousands, hundreds of thousands or millions of years? See the map legend by Knott for details.

Draw a block diagram illustrating the morphology and height of the fault scarps where they cross alluvial fans linked to incised upstream channels. Are they old and degraded looking, or are they sharp and fresh, with a vertical face of alluvium (i.e. a free face, which indicates a recent rupture). Does the height of the scarp change in some regular way in relation to fan heads? Why is this? Because 1) slip varies systematically so that it always decreases towards fan heads? Or 2) because the scarps form at basically the same height in earthquakes along the short length of the range front, but then are partly buried by subsequent deposition on the fan (rates of which are higher at the fan head).

What is the vertical displacement rate across the Mormon Point scarps based on the work of Frankel et al., 2016?

How do they estimate the slip rate on the fault (i.e. parallel to its slipping surface). What other data are required to get a unique slip rate? What assumptions does Frankel use to get a true slip rate?

Mormon Point slot canyon hike. Five miles roundtrip and probably pretty warm with some shade where the canyon narrows. Bring plenty of water.

Cross section from Hayman et al. Our hike will take us along the surface of cross section A-A’ from left to right.

As we walk from the active fan surface into the narrow canyon, how old are these conglomerates? See Knott’s map legend for details.

Do these sediments get older or younger as we hike up the canyon? What is their age range?

Have these sediments been tilted in any direction? Which direction are they tilted in? Is the sense of rotation consistent with normal sense slip on the frontal normal faults? Is the rotation of the sediments and the sense of slip on the normal faults part of a consistent kinematic system? Can you think of a simple analogy for this type of closely spaced normal faulting above a detachment, or sliding surface?

Why do you think there is so much extension in the hangingwall of the detachment as expressed by the array of young, NW dipping faults? Hint: what is it about the geometry and relative strength of the hangingwall that might facilitate its breakup? Do you think the amount of normal faulting increases in intensity towards the NW, or decreases in intensity. Why is this?

As we hike through the narrows of the Mormon Point slot canyon, what geologic structures do you see?

Assuming we get this far, what does the detachment surface separate (i.e. what are the rocks above and below the detachment)? Is this the same fault surface we looked at first along-side the road south of Mormon Point?

Badwater Salt Pans Note the sign on the hillside opposite the parking lot that denotes sea level 282 feet above us. Why is this area, and central Death Valley at such low elevations? (Hint 1: What type of faults are present here and how does their orientation affect whether or not extension and crustal thinning occurs; Hint 2: How might sediment flux from the adjacent mountains elevations along the rangefront?. Look at the fault map in Frankel et al., 2016 to answer this question…

Lateral Spreading Faults The GE image shown below illustrates a series of grabens that are oriented parallel to the shoreline along the edge of the alluvial fan. These are interesting extensional features caused by lateral spreading of the alluvial sediments during a large Holocene earthquake when the fan was accelerated towards the playa. Spreading features can be common in earthquakes, and they pose problems when they are interpreted as primary ground ruptures (i.e. they form during the earthquake but are not actual faults per se).

Shaded relief map of grabens at Badwater from 1m lidar data.

What type of grabens are these? Full grabens or half grabens? Do they accommodate significant differential offset across them (measured across their tops)? If not, what is required for full grabens with no differential offset to form (i.e. what must exist, at what orientation beneath the grabens). Draw a simple cross section illustrating this geometric relationship.