Bryce Canyon National Park, UT

Bryce Canyon National Park, UT

Today’s topics Intro to Fractures Faults and Fault classification

Fractures Fractures are the main structures associated with “Brittle” deformation. Found almost everywhere at solid Earth’s surface. Most characteristic of deformation in upper crust. Important for reconstructing tectonic evolution of geologic areas, and impact fluid flow (e.g., water, oil, gas) in the subsurface  can act has permeable channels or impermeable barriers depending on nature of host rocks and fault rocks Joints Fault

Fractures What is a fracture? Subplanar surface or discontinuity across which cohesion has been lost (although cohesion can be later healed) Typically very localized (narrow) but they all have some finite thickness. Two main families of fractures Extensional fractures joints, fissures, veins displacement perpendicular to walls Shear fractures displacement parallel to walls if displacement is large, we call these FAULTS

Fracture Types Extensional fractures joints, fissures, veins displacement perpendicular to walls Shear fractures displacement parallel to walls if displacement is large, we call these FAULTS

Fractures Fracture formation in response to principal stresses We’ll spend a lot more time on stress later in the semester, but the image to the left depicts the main types of fractures and their most common geometric relationship to the 3 principal stresses: s1 (maximum) s2 (intermediate) s3 (minimum) Stresses are compressive when pointing inward Note that this figure (from Ch. 7 in Fossen textbook) could also be useful when you do the analysis of 6-mile fold in labs 4, 5, and 6.

Conjugate Shear fractures/faults © Cambridge University Press 2011

Areal Photograph of high angle joints in Permian Cedar Mesa Sandstone from Canyonlands National Park, UT

Joint planes in Esplanade Sandstone

Intro. to Faults

Fault classification Based on two basic observations: Orientation of the fault surface (strike/dip of the surface) Nature of the relative displacement across the fault (sense of shear) (trend/plunge of slip or displacement vector)

Fault terms and classification For non-vertical faults ,there is a “footwall” and a “hanging wall” A brief note on stereonets. This is a graphical method for projecting 3-D orientation for planes and lines onto a 2-D plane (paper). You will much more with sterenets in lab, but you could get a head start by looking at Appendix B in your textbook or Fossen’s e-module on stereographic projection (http://folk.uib.no/nglhe/StructuralGeoBookEmodules.html). © Cambridge University Press 2011

Net slip of a fault Net slip (or displacement) Hangingwall Footwall Net slip (or displacement) The line on the fault surface that connects two points that were together before deformation It is a vector (direction and magnitude), and within the fault plane has strike-slip and dip-slip components

A and A’ were adjacent before faulting

Fault Types 2 other special classes: 2 endmember classes: Dip-slip Oblique-slip Normal Reverse Strike-slip Rotational 2 endmember classes: Dip-slip (2 types) Strike-slip (2 types)

Fault systems in the Western U.S. Active thrust faults Cenozoic & active normal faults overprinting Mesozoic thrust faults Cenozoic Thrust faults Mesozoic Thrust faults Cenozoic & Active Strike-Slip faults

Common map symbols for faults Normal faults -hatches or balls on “downthrown side” If dip unknown, U-up, D-down Reverse/Thrust faults “teeth” on hanging wall Strike-slip faults -arrows indicate Sense of slip Common map symbols for faults

Black Mountains, Death Valley East West

Black Mountains, Death Valley East West Footwall Hanging Wall

Chief Mountain, Glacier National Park klippe “allochthonous” “autochthonous”

Relative motion (sense of slip) of strike-slip faults

Right-Lateral (Dextral)

Left-Lateral (Sinistral)

Arrow Heads ( ) and Tails ( ) Block moving toward you like an arrowhead heading towards you Block moving away from you like seeing the feathers of an arrow

Arrow Heads ( ) and Tails ( ) Right-lateral or left-lateral?

Arrow Heads ( ) and Tails ( ) Left-lateral

Arrow Heads ( ) and Tails ( ) Sinistral or Dextral?

Arrow Heads ( ) and Tails ( ) Dextral

Basic Fault Types Right-lateral (dextral) strike-slip fault Izmit, Turkey Left-lateral (sinistral) strike-slip fault Guatemala

Oceanic Strike Slip fault: Transforms

Some other general characteristics of faults (of all types)