FAULTS I. GENERAL TERMS AND INFORMATION A. Fault and fault zone are brittle B. Ductile shear zone is generally plastic, but might be microscopically brittle. Ductility is the appearance of plastic flow, but it is scale dependent. C. Hanging wall and foot wall are general classifications based on two end member types of slip 1. Dip slip - (normal or reverse) 2. Strike slip - (right or left lateral) 3. Oblique slip - ( a combination of dip slip and strike slip) 4. Slip versus separation (handout) a. Slip = direction and amount (slip vector) of displacement of formerly adjacent points b. Separation = distance between traces of a displaced marker plane 5. Special cases a. Hinged faults b. Pivotal faults c. Listric faults = fault dip shallows out at depth D. Faults and topography 1. Low angle dip a. Teeth on "upper plate" = hanging wall b. Mimics topography 2. High angle dip a. Strike-slip faults generally are steep b. Cut across topography II. TO CLASSIFY FAULTS USING SLIP - we need 2 types of information: A. Slip line, which is only a direction, says nothing about relative movement of blocks B. Sense of movement or rotation along the fault C. How to determine slip 1. If you have an offset linear marker it is relatively easy: intersecting dikes, fold axes, stream channels, volcanic necks, etc... 2. But what if we don't have linear markers, (which is the usual case)? D. Slip lines within rocks are of 3 types: 1. Slickensides - as we have already discussed; beware however that they show only the latest movement a. Normal fault b. Followed by left-slip 2. Grooves or mullion structures - corrugations in the fault surface. Real fault surfaces are not perfectly planar. 3. Elongation lineations - stretched and elongated grains (feldspar and quartz), ossils, pebbles, etc... (cigar shaped strain ellipses)