Furnace Creek Mapping Exercise
Field Methods in Active Tectonics
Purpose of the exercisesare as follows:
- Learn how to identify active ground offsets on young faults, including surface ruptures from historic or very recent earthquakes
- Learn to identify how many past earthquakes may be recorded at a particular site.
Furnace CreekThe Furnace Creek Fault zone is an excellent example of a single fault strand that gently changes its strike along a broadly linear rangefront. As such it offers an opportunity to map stranded remnants of stream channel and alluvial fan deposits offset by semi-continuous earthquake ruptures along its length. The goal is to measure and relate scarp heights along the map area to one another, in particular to estimate whether recent deformation is similar and consistent with changes in strike, or whether smaller rupture segments exist that might affect earthquake magnitude. Another key question is whether the most recent scarps were produced by only a single event.
Fault MapIn this exercise you will quickly make a strip map of the fault along its length. You should measure scarp height as often as possible along the rangefront to ask yourself the following questions (and you should continuously annotate the map with scarp height measurements – estimated at the scale of half a meter or less).
You will also need to make a histogram of the vertical displacements scaled to distance along the length of the scarp in order to visualize variations in offset. Students typically take 60-80 measurements along the length of the scarp. The data points on the histogram should be scaled to distance along the fault, whereas the vertical scale on the histogram should be such as to readily visualize differences in scarp height.
You should ask yourselves the following questions as you are writing up your report on your measurements.
- Does scarp height vary as a function of the strike of the fault, and can you develop a consistent kinematic model where slip toggles between greater or lesser amounts of lateral and dip slip, depending on the strike of the fault.
- Does scarp height vary as a function of sedimentation where areas with higher rates of deposition such as at fan heads bury sediments at the base of the scarp more rapidly (such as we saw along the Willow Creek scarps at Mormon Point).
- Does segmentation exist along the rangefront,, - in other words do recent ruptures appear to terminate at what might be segment boundaries.
Other important aspects of this map should include evidence you might find for the most recent earthquake along the fault (hint: the last event has been interpreted to occur about 2000 years ago). Are unaltered free faces present, do you see evidence for more than one event based on scarp morphology, such as a beveled crest or are there locations with stepped stream terrace deposits are present that might indicate multiple events?
When we reach the parking lot in the center of the strip map, be sure to look at the multiple indicators of fault displacement in the footwall of the fault (above your heads in the shaded area on the south side of the drainage). What features are present and what do they tell you about the sense of slip on the fault (assuming strain in the footwall is similar to displacement on the scarps at the rangefront).
Also, look for other indications of faulting, such as faceted spurs and map and note these on your strip map.
Products A strip map of the fault, with numerous measurements of scarp height along the rangefront (which should be annotated on the map) and whether you think they can be attributed to one or more of the bullets outlined above.
A short 1-2 page report outlining evidence you may have found for the most recent strain along the map area (i.e. can you estimate the amount of slip on the last one to ??? earthquakes, with an emphasis on the most recent one).