Cambridge University Press 2011

Faults: an introduction!!Slip vs separation!!Stratigraphic cutoffs!!Fault growth, or scaling!!Repetition or omission in boreholes!!Fault linkage!

!Reading: Chapter 8 in Fossen!

Fault planes plotted on a stereonet with slip vector. Looking at the stereonet diagrams, how can you tell is the slickenlines lie on the fault surface?

Nojima Fault Rupture Kobe Earthquake

Kobe Earthquake Fault Rupture Museum!

Fault Components!

Footwall

Hanging wall

Fault plane, fault surface Footwall

Hanging wall

Faulting Styles Oblique slip!

Oblique-slip: a combination of dip-slip and strike-slip Here displacement of a drainage ditch offset by the Kobe Earthquake

Offset of the ground surface (note how topography changes) from an Earthquake in Baja in 2010. The slip along the fault is constant here, so why does the amount of vertical displacement occur?

Offset of the ground surface (note how topography changes) from an Earthquake in Baja in 2010. The slip along the fault is constant here, so why does the amount of vertical displacement occur?

Oblique slip continued Offset of the ground surface (note how topography changes) from an earthquake in Baja in 2010. The slip along the fault is constant here, so why does the amount of vertical displacement occur?

The key is the orientation of the ground relative to the rupture. Ground surface perpendicular to rupture = tall scarp, parallel = lower scarp

Cambridge University Press 2011

Slip versus Separation!

Pre-faulted block Faulted block Eroded block

Slip - actual relative displacement between two points that occupied the same location before faulting

Separation - apparent relative displacement between two points that may have occupied the same location before faulting

Slip Separation

Types of Faults!

Normal

Thrust

Strike-slip

Dip-slip

Dip-slip

Strike-slip

Note how the blocks create apparent separation

Cambridge University Press 2011

Determining slip is not a trivial exercise, note how you need a cross!Section oriented such that it contains the slip vector and offset points

Cambridge University Press 2011

Stratigraphic cutoffs allow you to match line of intersection of hangingwall with fault plane with intersection of footwall with fault plane. Both intersections were the same line before it was separated across the fault

Cambridge University Press 2011

Vertical boreholes appear to !either omit or repeat strata!!For the former, this occurs where!a normal fault cuts out or removes!stratigraphic section!!Alternatively thrust faults repeat!stratigraphic section.

Cambridge University Press 2011

Displacement to length scaling how faults grow in 3D

Arguments for how faults grow vary, but recent data suggest most D:L!profiles are triangular shaped and grow where slip rates are higher in the !center. Plateau shaped displacement faults !can be associated with reactivated faults

An important issue is how individual slip events (earthquakes)!add up to make the displacement to length profile

Faults develop damage !zones then localize slip !onto a slip surface.

Fault arrays also link up to form simpler single faults. These accommodate most of the subsequent strain

Hard linked vs soft linked

A soft-linked !relay ramp

Fault growth in a layer bounded by stronger rocks

The following exercise in Google Earth is designed to explore scaling relationships in extensional fault networks.!!Scaling is a natural result of how faults grow. They accumulate displacement, yet grow laterally with time. As shown earlier, these displacement profiles can take different shapes, triangular, bell shaped or plateau shaped. Triangular profiles seem to be more prevalent for dip slip faults based to recent research!!Mmax : Fault length records the end result of the history of growth for a fault. Earthquake geologists and seismologists are interested in scaling because it tells us something about the distribution of slip that might occur during a seismic event (which ultimately add up to make the D:L profiles).!!Petroleum geologists are interested because many oil and gas fields are formed in normal fault systems and faults can form pathways for oil, or seals depending on fault geometry, the amount of slip and the sediments they deform, especially clastic sediments.!

Lets look at some spectacularly exposed normal fault systems in Arches National Park. First, fire up GE.!!Start off by looking for normal faults in the Delicate Arch area, the iconic landform of Utah. Type in the words Delicate Arch, then find the Delicate Arch Parking area.!!Once you have found the Delicate Arch Parking area, follow the hiking trail northeast through the red-colored Morrison Formation. The trail will disappear as the trail starts to move up a ramp-like bedding surface of the underlying Entrada Formation. !!Use the 3D perspective tool to identify faults in this area and distinguish them from stratigraphic contacts. The light pink rocks are Entrada Sandstone, the red, grey and green rocks are younger overlying sediments (Morrison, Dakota, etc). Strata in this region generally dip southward.!!Determine what type of contact the trail crosses. (offset fault contact, stratigraphic contacts, or ???).!

Now look in nearby areas for faults. These can be identified by contacts that cut across stratigraphic contacts.!!When you find nearby faults, determine in which direction the displacement on them dies out.!!You can use the File, Save, Save image to obtain a 3D image, then use the resulting jpg as a graphic tool to annotate features of interest (not now, but this is a handy tool for making presentations of geology).!!You can make a topographic profile by making a path, make sure the path is highlighted in the menu on the left, then go to Edit in the menu bar and Show elevation profile.!!Now lets go to a spectaular array of normal faults in the Needles District of Canyonlands. You should be looking at a set of horsts and grabens aligned on the east side of the Colorado River Canyon, south of its confluence with the Green River.!

Look for faults that dont have sand infilling them. Can you find any where the displacement profiles are obviously tapered? It may help to vertically exaggerate the scene, say to 1.5. !!Do they always taper down to no displacement, or do some have displacement profiles that go up, then down and then up again along their lengths.!!Why do you think this is? !!Can you identify faults that have become linked together? What evidence tells you that this might have occurred?!!Note that the bigger, deeper grabens are partly filled with eolian sand, so displacement profiles may not represent the actual displacement along the fault. !!

Relay ramp on trail to Delicate Arch!!

The following exercise in Google Earth is designed to explore scaling relationships in extensional fault networks.!!Scaling is a natural result of how faults grow. They accumulate displacement, yet grow laterally with time. As shown earlier, these displacement profiles can take different shapes, triangular, bell shaped or plateau shaped.!!These represent the end result of the history of growth for a fault. Earthquake geologists and seismologists are interested in scaling because it tells us something about the distribution of slip during a seismic event (which add up to make the D:L profiles).!!Petroleum geologists are interested because many oil and gas fields are formed in normal fault systems and faults can form pathways for oil, or seals depending on fault geometry, the amount of slip and the sediments they deform, especially clastic sediments.!

Lets look at some spectacularly exposed normal fault systems in Arches National Park.!!Start off by looking for normal faults in the Delicate Arch area, the iconic landform of Utah. Type in the words Delicate Arch, then find the Delicate Arch Parking area.!!Once you have found the Delicate Arch Parking area, follow the hiking trail northeast through the red-colored Morrison Formation. The trail will disappear as the trail starts to move up a ramp-like bedding surface of the underlying Entrada Formation. !!Use the 3D perspective tool to identify faults in this area and distinguish them from stratigraphic contacts. The light pink rocks are Entrada Sandstone, the red, grey and green rocks are younger overlying sediments (Morrison, Dakota, etc). Strata in this region generally dip southward.!!Determine what type of contact this is. (offset fault contact, stratigraphic contacts, or ???).!

Now look in nearby areas for faults. These can be identified by contacts that cut across stratigraphic contacts.!!When you find nearby faults, determine in which direction they dip and the direction displacement on them dies out.!!You can use the File, save, save image function to obtain a 3D image, then use the resulting jpg as a graphic tool to annotate features of interest (not now, but this is a handy tool for making presentations later on in your career).!!Now lets go to a spectaular array of normal faults in the Needles District of Canyonlands. You should be looking at a set of horsts and grabens aligned on the east side of the Colorado River Canyon, south of its confluence with the Green River.!

Look for faults that dont have sand infilling them. Can you find any where the displacement profiles are obviously tapered? It may help to vertically exaggerate the scene, say to 1.5. !!Do they always taper down to no displacement, or do some have displacement profiles that go up, then down and then up again along their lengths.!!Why do you think this is? !!Can you identify faults that have become linked together? What evidence tells you that this might have occurred?!!Note that the bigger, deeper grabens are partly filled with eolian sand, so displacement profiles may not represent the actual displacement along the fault. !!