South Mountains field trip

Meeting location

Southern Ramadas at the Pima Canyon entrance to the South Mountains Park.
City of Phoenix South Mountains web site
South Mountain Trail guide
Driving directions from ASU:
south on Rural to west on Guadalupe Road. Continue over the I-10. Take a left at the first light. Take your first right onto S 48th street and then immediate left into the Park on E. Pima Canyon rd. Go up to the end of the road and park and meet on the southern side Ramada at 1:45 pm Tuesday, March 19, 2009.
Overview map Detailed navigation with red line on map

Information and what to bring

We will work from 1:45 pm to 5 pm along the hills to the southwest of the parking lot measuring lots of faults and fracture orientations in order to constrain the conditions under which the structures were active and infer the material properties of the South Mountains detachment fault.
Please bring:

Water and snacks
Good boots and sun protection
Brunton compass if possible
Handheld GPS if possible, set to NAD_1983_UTM_Zone_12N if you know how
Clipboard, paper for notes, good pencil and pen for mapping

Base maps

Overview
Mapping area, one page
Mapping area, eastern half and Mapping area, western half

Notes table

Table for fracture observation recording (MS Word doc)
Table for fracture observation recording (pdf)

Supplemental data

Joints	Faults (strike and dip; trend and plunge of slickenline)
295, 80	150, 41; 240, 41
305, 81	184, 45; 274, 45
347,79	235, 35; 325, 35
305,86
358,85
170,84
295, 80

Assignment

Using the class notes and the Matlab files (from the lectures page), and your observations of fault and fracture surface attitudes (supplement with above if necessary) please do the following:

Figure out a good orientation for the least compressive horizontal stress (that will orient the stress tensor). Use the joint data and if necessary compute the mean orientation using the spherical distributions approach.

Analyze your faults (compute the traction components and the CFF along them). Also compute the angular misfit between the observed or assumed slip direction and the computed shear traction orientation. Use the specified stress magnitudes for a start:

%assume that the principal stresses are appropriate for normal faulting
%conditions so maximum stress is the vertical stress
sv = -26.5.*12; %assume 26.5 MPa per km and 12 km depth
shmin = sv.*0.1; %assume the 1 direction is the minimum horizontal stress and is 10% sv
shmax = sv.*0.5; %assume the 2 direction is intermediate
S = [shmin 0 0;
     0 shmax 0;
       0   0 sv]

How does changing the least compressive horizontal stress orientation and the differential stress influence the resulting CFF and the misfits?
Given a strike of 330 and dip of 15 (towards NE) and pure dip slip for the main detachment fault, analyze its loading condition with the above stress state (or a modified one if you find one that pushes the small faults closer to failure). What is its CFF and angular misfit? How poorly oriented is it? Assuming it was active in the same stress state as the small faults you measured, what do you need to change in terms of friction or pore pressure to get it to fail?

GLG510 Advanced Structural Geology

Last modified: April 27, 2009