GP-5 Slip Rates...Andreas system) (Fig. 1). These two stress regimes overlap in the Sierra Nevada/Owens Valley region where both normal faults and strike slip faults criss-cross the

Geol  305   Lab  #5;  Slip  Rates     Due  May  13,  2011    

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Lab #5: Slip Rates on young faults

I N T R O D U C T IO N Southern California is a structurally complex region, with the eastern area undergoing continental

extension (the Basin and Range) and the western area accommodating strike-slip deformation (San Andreas system) (Fig. 1). These two stress regimes overlap in the Sierra Nevada/Owens Valley region where both normal faults and strike slip faults criss-cross the country side.

Sierra Nevada frontal fault zone (SNFFZ): is a normal fault system responsible for uplifting the Sierra Nevada Mtn. range above the adjacent Owens Valley. While it is clear that the SNFFZ is currently a normal fault system, it is unclear if it has always accommodated normal offset, or perhaps it accommodated strike slip motion in the past. To determine the offset history of the SNFFZ, Le et al (2007) conducted a detailed mapping, geomorphic, and geochronologic investigation along the SNFFZ. Their data provides quantitative constraints on fault-slip rates on the SNFFZ in Quaternary time, and provides insight into the structural evolution of the region.

Geol  305   Lab  #5;  Slip  Rates     Due  May  13,  2011    

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CWU A N D M E A S U R IN G F A U LT-S LIP R A T E S To understand how the crust accommodates the transition between Basin and Range extension

and San Andreas strike-slip motion, Le et al (2007) looked at the offset of Quaternary surfaces that are cut by the SNFFZ. By determining the age and offset of these Quaternary surfaces, they were able to determine the history of slip rates along the SNFFZ. The question they posed was; ”Did the style and/or rate of fault-offset (e.g., normal, strike slip, oblique slip) change through time? Or has the SNNFZ consistently maintained the same style and rate of offset through the Quaternary?”

T H E D A T A Le et al (2007) recognized six distinct Quaternary surfaces (Fig 2). .To determine slip rates of

the faults that offset these surfaces (Fig 3), they first determined the amount of offset (in meters) and constrain the timing of offset (in ky). Of the six Quaternary surfaces, they were able to determine the age of four (Table 1). All six of the surfaces have been offset by the SNNFZ, but they were able to directly measure the offset of only three of the Quaternary surfaces (Table 1).

Geol  305   Lab  #5;  Slip  Rates     Due  May  13,  2011    

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Surface Age (ka) Vertical offset of

dated surface (m) Maximum vertical

offset (m) Q1 123.7 ± 16.6 24 ±1 — Q2a Q1>Q2a>Q2b — 41 ± 2 Q2b 61 ±7 11.9 ± 0.6 — Q3a 26 ± 8 10.2 ± 0.5 10.2 ± 0.5 Q3b Q3a>Q3b>Q3c — 6.4 ± 0.3 Q3c 4 ±1 — 6.9 ± 0.3

Table 1: Summary of Surface Ages and Vertical offsets

D A T A A N A L Y S IS To properly determine fault slip rates based on the ages and fault offsets of Table 1, we must

clearly understand what these values represent. All of the surfaces studied in this project were alluvial fan surfaces. This means that sediments

were being transported across these surfaces as flash floods tore through the canyons and flowed across the alluvial fan. The ages of these surfaces represent the last time that the surface was “active”, when sediments were being transported across it. In the instances that the surface could not be directly dated, the law of superposition constrains the relative surface age (i.e., surfaces Q2a and Q3b). The reported offset of a surface must have occurred after the surface became inactive.

The vertical offset of a surface is the measured vertical displacement of a surface across the SNFFZ (i.e., fig 3). In those cases that the vertical offset could not be directly measured, cross-cutting relations were used to determine maximum vertical offset.

To determine the fault slip rate, you can divide the vertical offset by the surface age Fault slip rate = (vertical offset of surface) (Eq. 1) (age of surface) Eq. 1 calculates the minimum, average fault slip rate since the surface became inactive. Since there can be significant uncertainties in the measurements of both age and offset, it is

important to properly propagate the uncertainties of the slip rates.

Geol  305   Lab  #5;  Slip  Rates     Due  May  13,  2011    

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Assignment: You will analyze the provided data to determine the history of slip rates across the SNFFZ, and

answer the question: “have slip rates changed significantly over time?” Propagating uncertainties is critical to answer this question.

Suppl ies: Data from Table 1 .

Procedure: 1) Determine the vertical slip rates for surfaces Q1, Q2b, and Q3a, including uncertainties

Question 1: What does it mean that the slip rate from Eqn 1 is the minimum, average slip rate? Question 2: Has the rate of vertical silp varied significantly in the past 124 ka? What criteria

did you use to come to this conclusion? 2) In step 1 you determined the minimum, average slip rates for three time spans, since ~124 ka,

since ~61 ka, and since ~26 ka. To get a more detailed estimate of how slip rates vary through time (i.e, between 124 and 61 ka) you divide the difference in offset by the time span:

Slip rate from time 1 to time 2=(offset1-offset2)/(age1-age2)

Question 3: Has the rate of vertical silp varied significantly during the past 124 ka? What criteria did you use to come to this conclusion?

3) Plot the data (vertical offset on the y axis, surface age on the x axis) for surfaces Q1, Q2b, Q3a, with error bars. By hand, attempt to draw as few straight lines as possible that pass within the uncertainty of the three data points (be sure to include the origin in your lines, since there has been zero slip since time zero). Question 4:a) What is the slope of the line(s)?

b) What are the units of slope? c) Discuss the evolution of fault slip rate based on the slope of the line(s).

4) For surfaces Q2a, Q3b, and Q3c we do not have enough information to determine a best estimate of slip rate. However, we do have enough data to say something about the maximum average slip rates. For each of these three surfaces, estimate the maximum average slip rates, and their uncertainties

Question 5: Are these estimates for maximum slip rate consistent with the your estimated slip rates from steps 1 and 2?

5) Recreate the graph from step #2, expanding your vertical axis to 50 m. On this graph, you will draw three boxes that indicate the possible range of ages and offset for surfaces Q2a, Q3b, and Q3c . Each box will extend from the maximum age to the minimum age of the surface, and from the maximum offset to the minimum offset of the surface.

Question 6: The boxes you drew can help to constrain the average slip rate. Are they consistent with the slip rates you have already calculated? Briefly discuss (1 or 2 sentences)

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To hand in: All your calculations for each step above, answers to questions above, and all your plots

_____________________________________________ R eferences :

Le, K., Lee, J, Owen, L.A., & R. Finkel, 2007. Late Quaternary slip rates on the Sierra Nevada frontal fault zone, California: Slip partitioning across the western margin of the Eastern California Shear Zone-Basin and Range Province. GSA Bulletin, 1 1 9 , 240-256; doi 10.1130/B25960.1