Bryce Canyon National Park UT Bryce Canyon National

Bryce Canyon National Park, UT

Today’s topics • Intro to Fractures • Faults and Fault classification

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 Joints 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 lt u Fa

$Fractures What is a fracture? Subplanar surface or discontinuity across which cohesion has been$

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

$Fracture Types Extensional fractures • joints, fissures, veins • displacement perpendicular to walls Shear$

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: s 1 (maximum) s 2 (intermediate) s 3 (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.

$Conjugate Shear fractures/faults © Cambridge University Press 2011$

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: 1) Orientation of the fault surface (strike/dip of the surface) 2) 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/Structural. Geo. Book Emodules. html). © Cambridge University Press 2011

Net slip of a fault Net slip (or displacement) The line on the fault surface that connects two points that were together before deformation Footwall Hangingwall 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 Dip-slip Oblique-slip Normal Reverse Strike-slip Rotational 2 endmember classes: Dip-slip (2 types) Strike-slip (2 types) 2 other special classes:

Fault systems in the Western U. S. Cenoz t faults Active thrus ic zo so Me ults rust fa oic Th Cenozoic & active normal faults overprinting Mesozoic thrust faults st ru Th fau ic zo no lts Ce & e tiv Ac ip Sl e- rik St ts ul fa

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

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 arrowheading towards you Block moving away from you like seeing the feathers of an arrow