Alluvial Fans – a primer

1. Alluvial Fans – a primer • Typically formed in arid regions where rainfall occurs infrequently but in large amounts (e.g. deserts) • Fan size and steepness depends on several things: 1) Sediment flux, itself a product of both catchment area and precipitation and 2) Basin shape or accommodation space. Rapidly subsiding edges of extensional basins typically stack or aggrade fans, gently tilted basin margins produce broader fans that prograde more readily. East side of Death Valley vs west side is a good example. • Fans are located at the mouths of stream and river channels where they exit uplifted fault blocks – deposition occurs due to the loss of confinement in the channel • Resulting deposits as sheet floods (flatter surfaces) and debris flows (steeper surfaces) and as small bars in confined channels.

2. Badwater fans. Note size of fans, catchment areas, aggradation

3. West side of Death Valley - broader, gently dipping and prograding fans. See fan size vary on west side as a function of catchments.

4. Alluvial Fans – a primer • Fans in Baja and Death Valley and environs have surfaces that range from the present to 100,000’s of years. • Fans typically undergo avulsion and shift depocenters like deltas • Preservation depends on burial, or abandonment of fan surfaces • Preservation is also related to local tectonics - coal train loader for strike slip faults, deflection of fans by scarps facing towards rangefronts, rapid progradation across scarps that face towards the basin (due to a local lowering of baselevel). “Uplift” in Death Valley is relative, actual movement is almost always subsidence.

5. Weathering of fan surfaces results in: smoothing of upper surface, development of desert pavement and armoring of silicic clasts by desert varnish, limestone will not preserve varnish (bacterial mediated deposits of aluminum, iron and manganese oxides).

6. Weathering of fan surfaces results in: smoothing of upper surface, development of desert pavement and armoring of silicic clasts by desert varnish, limestone will not preserve varnish (bacterial mediated deposits of aluminum, iron and manganese oxides).

7. Scarp Morphology and Local Tectonics • Baja, Death Valley and Panamint Valley are currently oblique systems (normal + dextral slip). All had an earlier history of high magnitude extension, associated with low angle normal faults and metamorphic complexes (footwall rocks exhumed from great depth) • Releasing bends result in high magnitude extension defined by antiformal culminations (turtlebacks), very low elevations (Badwater), and purely normal faults. Releasing bends at Canon Rojo, Mormon Point and elsewhere. • Scarp height for a given amount of strain increases as a function of the strike of the fault relative to the local sense of slip. • Scarp morphology depends on the history of earthquakes in a particular region. Large bedrock scarps erode to form faceted spurs, and exposed planar fault surfaces with slickenlines, etc.

8. Releasing bends result in high magnitude extension defined by antiformal culminations (turtlebacks), very low elevations (Badwater), and purely normal faults. Releasing bends at Canon Rojo, Mormon Point and elsewhere.

9. Scarp Morphology in Alluvial deposits • Most of the scarps you will map are located in alluvial fan deposits, - that is they offset young sediments. • Scarp height depends on the size and number of earthquakes that produce them. Scarps are a record of strain preserved since the fan surface was abandoned, or in young cases have not yet been buried. • Scarps start off as nearly vertical surfaces called free faces that erode quickly with time (in most arid environments, they don’t last more than a couple of hundred years). • Young scarps have a debris slope (from alluvium falling off the free face) and a wash slope, formed by erosion during rainfall. • Scarps “diffuse” with time and become more gently shaped as erosion of their crest and deposition at their base occurs. • Refreshed scarps are typified by a beveled crest where a diffused (older) scarp was offset again by a younger earthquake.

14. Purpose of the map exercises • Recognize faults and the recency of slip along them • Develop alluvial chronologies that help date faults and the rate they are slipping • Interpret the local tectonics of the region by integrating faults, the deposits they offset and regional structure (basins vs ranges).

17. Why are some fault scarps dark and others light? What is a shaded relief map? How are they made?