GEO 309 Structural Geology Exam II
Name_____KEY______
(19 pts)
1a) Of the following: (brittle failure /volume diffusion creep / dissolution creep / dislocation creep )
(3)I) Is governed by a 'power law'. ___Dislocation Creep
II) Of the diffusion mechanisms, this one occurs at higher temperatures__Volume Diffusion Creep
III) It happens faster with a small grain size than with a large grain size.__Dissolution Creep
(1)b)If xx = 13 MPa, xy = 9 MPa and xz = -7 MPa, what is yx? ___9_
(1)c)For rotation R, Rxx = .65 and Rxy = .30 Ryx = __-0.30_
(1)d)For pure strain tensor in principal axis coordinates, xx=.65 and yy= .10
xy=____0___
(1)e)If water has a density of 1 g/cc and g=10 m/s2, what is the
maximum possible value of 1 at a depth in the ocean where 3 = 7 MPa? ____7MPa___
(6)f)You are in control of a large reservoir. After construction the region experienced a 10 year draught. However, after heavy winter rains, the reservoir has filled to maximum capacity for the first time. The water level is 100 meters higher than the previous all-time high. Consider ONLY the effect of increasing the vertical stress by 1 MPa, with the horizontal stresses staying the same. Assume that 1 = 45 MPa, 2 = 32 MPa, and 3= 25 MPa.
Would the increase in vertical load make faulting (more likely, less likely, no different)
If the fault beneath the reservoir is
normal ?_____Increase__
reverse ?__Decrease______
strike-slip ? __No Change___
After some time, the pore fluid pressure, Pf, will increase.
This increase in Pf would tend to trigger ___All______earthquakes.
(normal, reverse, strike-slip, normal&reverse, reverse&strike-slip, normal&strike-slip, all, none)
(4, 2)j)Write the strain tensor in principal axis coordinates that has these properties:
10% shortening in X, 10% extension in Y, and no strain in Z
Pure Shear
[-0.1 0 0]
[0 0.1 0]
[0 0 0]
2)In an area you are studying, conjugate faults are forming by the Coulomb failure criterion,
with Byerlee friction (=0.85, =40°). The stresses are such that 2 is in the (135°, 00°) direction and 3 is vertical.
(4)a)The 1 axis is oriented (___045______00____)
(8)b)The ideal orientations for
the conjugate faults are (__135, 25S_____) and (315, 25N______)
…. And slip on them is ___Reverse______and ______Reverse______, respectively
( reverse, normal, strike-slip, can’t tell )
(4)c) Elsewhere, conjugate ductile shear zones are forming in a place with exactly the same orientations of the three principal stresses. Here, however, failure is by the von Mises criterion. These shear zones are probably
______Reverse______and are _____Deeper______in the crust than the faults.
( reverse / normal / strike-slip / can’t tell ) ( shallower / deeper / can’t tell )
(20 pts.)
3) If 1 = 10 MPa, 2 = 7 MPa, and 3= 4 MPa and Pf = 0 ,
(2)The maximum shear traction is _____3__ MPa, and it is found on a plane inclined by :
(6) _____45__ degrees from 1, ___0_____ degrees from 2, and ____45___ degrees from 3
(2)The normal traction on that plane is _____7_____ MPa
(4)Draw the Mohr stress circle below.
What would have to be if there were a preexisting fault
(2)in an orientation that is just barely slipping in friction? = _____1/2____ (to 2 digits)
Now, imagine that the fluid pressure = 5 MPa. What would the friction coefficient have to be if (2) that same fault were to be just failing under those conditions?
= __Probably a Typo, Pf 5MPa makes for a range of values, Ignore question (to 2 digits)
(2) Draw THAT effective stress Mohr circle (dashed) above, next to the other circle.
(26 pts.)
4)In your field area, there are many normal faults. There are also many nearly vertical joints with a consistent strike of: N70°W.
(4)Express that strike for the joints azimuthally (as opposed to quadrantally): _110______° or __290______° or
(6)a) • DRAW the Mohr circle for both the stress AND the fracture criterion applied to the joint formation.
• SHOW completely and CLEARLY how they either touch or miss each other in regard to joint formation.
• LABEL both axes.
(6)The simplest explanation is that the principal stresses had the following bearings and plunges in relation to the joint formation:
1: (____Vertical______), 2: (____110, 00______), 3: (______020, 00______)
(12 pts.)
5)Sketch a stereonet at right, and show all work. N
(4)
In your field area,
the 1 direction is Vertical
the 2 direction is 045°, 00°
and the 3 direction is 135°, 00°
if = 45° , then the ideal conjugate faults are a…..
(describe fully!)
(4)
____Normal______fault
that is oriented _____N45E, 67.5S
and a
(4)
_____Normal______fault
that is oriented ______S45W, 67.5__
(12 pts)
(6) Above is an outcrop that unfortunately was spray painted by vandals. The vandals have wiped out the arrows that show the sense of motion in the shear zone. (A) Draw the shear zone below, indicate whether this shear zone is right-lateral or left-lateral. (B) Indicate the approximate directions of maximum principal stress and minimum principal stress. (C) Below your cartoon of the shear zone, draw a second cartoon showing what the Reidel shear zone features would look like for such a shear zone. Label the R and R’ directions.
You are smart, Fill your own drawing in here.
(10 pts. tot.)
7)From this list: I) grain-boundary diffusion creep II) volume-diffusion creep
(2 pt. ea)III) dissolution (pressure-solution) creep IV) brittle fracture V) dislocation creep