Ketobe Knob UT Can you trace faults and

Ketobe Knob, UT

Can you trace faults and stratigraphic contacts across this outcrop?

Fault Damage zone characteristics Fault splay patterns can be systematic and sometimes used as slipsense indicators R – Riedel fractures, R’ – anti-Riedel fractures, P – P-fractures

Piercing points on a fault Piercing pt 1 Recall: Net slip is The line on the fault surface that connects two points that were together before deformation Piercing pt 2 Footwall Hangingwall These two points are called “piercing points”

Slip verses Separation Slip Pre-faulted block Faulted block Separation Eroded block • Slip – actual displacement between two points that occupied the same location before faulting (these are piercing pts) • Separation – apparent displacement between two points that may have occupied the same location before faulting

Another view of Slip versus Separation A pure dip-slip fault cuts dipping beds After the upthrown block has eroded, it looks like a right-lateral strike-slip fault, but it is not! So, if your only observations come from the last diagram (which is usually the case in reality), then it could be a rightlateral strike-slip fault OR it could be a right-side-down dip-slip fault. Or an infinite variety of oblique slip faults. You don’t have enough information to tell what the true sense of slip is on the fault.

Separation terminology So, you can’t determine net slip from only one set of planar beds But that doesn’t mean that it is not worth saying anything about the fault. Convention is to use a set of terms that are specific to separation – this is still quite useful.

Another view of Slip verses Separation © Cambridge University Press 2011

Qualitative evaluation of fault slip Rake: angle between a line on the plane and that plane’s strike

Piercing points on faults What are some examples of piercing points? Piercing “points” are actually offset lines. Examples of useful “lines” in geology are not particularly common, but could include the intersection of two planes, say an angular unconformity and tilted underlying beds, or an intrusive contact with a bedding plane, the hinge line of a cylindrical fold at a specific bedding contact, etc. The most common piercing points in structural geology/active tectonics include linear features such as fences, stream channels, roads, etc that are offset by surface ruptures in earthquakes. Planes offset by a fault, such as those shown on typical block diagrams are NOT piercing points and hence cannot be used to measure slip. Break up into groups of 5 -6 people and answer the following questions.

Qualitative evaluation of fault slip • The maps present a set of problems dealing with slip on faults. For all maps, the faults (marked with F’s) strike due north and dip 90°. The second map has topography and the first one does not. Movement on the faults is younger than ALL other events except erosion. For each map, determine: • Whether it is possible to calculate the magnitude of the net slip (“yes” or “no”)? A “yes” here means that the magnitude is determinable, but it could require time-consuming construction or calculations. • The sense of the net slip (for example, east side-up and north relative to the west side). • The direction of the net slip; that is, the orientation (direction and angle) of the slip vector. What is the approximate trend and plunge of the slip vector? • For the first problem, it is useful to use five notebooks (with each student holding a notebook). Arrange all the notebooks relative to north to match the diagram (4 for planes, 1 for fault), then estimate the trend and plunge of the slip vector with a pencil. Sort of like the game Twister, but for structural geologists. Except in this exercise, try not to come into physical contact with your fellow students.

Qualitative evaluation of fault slip

1. Can net slip be determined? NO, not from offset planar beds alone. 2. Sense of net slip? Lower & upper contacts of conglomerate are higher on W, so the vertical sense of movement is W-side-up (or E-side-down). What if fault dipped 85 °E? 3. Direction of net slip? Can’t tell direction of net slip. BUT, beds are horizontal (from map pattern) and contacts are offset, so vertical component of net slip is determinable. 4. Magnitude of net slip? Top of conglomerate at 1010’ on W, but only at 970’ on E-side. So, vertical component of net slip is 40’

Qualitative (and semi-quantitative) evaluation of slip on faults Some summary tips: 1. Offset horizontal or vertical markers are special cases. If a horizontal marker is offset vertically, then you can at least quantify the vertical component of net slip, even if that is the only marker you have. Likewise, if a vertical marker is offset horizontally, you can quantify the horiz. component of net slip… 2. For dip-slip faults, dipping planes “advance” in the direction of dip on the upthrown block after erosion. 3. If you know that a fault with appreciable slip cuts a plane, but the plane is not offset, then the slip vector must be very nearly contained in that plane. 4. The net slip vector, by definition, must lie within the fault plane.

Similar to Folding Line Method cover half the fault (here East block). Now imagine this is a cross sectional “Fault Plane view”

Rotated to better draw contacts.

Draw the contacts of the West and East side of both units.

1. Extend the East side contacts downwards to a line parallel to the fault. 2. Along the far contacts of each unit, draw a line at a (-) dip angle.

Where these two lines intersect is a piercing point! Now we have a piercing point for the east side of the fault!

Do the same thing for the units on the west side (shown in green). Where these (-) dip angles intersect in the west piercing point.

Vector between these two piercing point is the Net slip movement along the fault! 1. Is magnitude of slip determinable? Yes 2. Right-lateral oblique, west-side down fault (if fault dipped 85°E, then R-lateral oblique reverse-slip fault 3. Direction of net slip – steep to the North