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GG303 Lecture 25 9/4/01 1 FAULTS (I) I Main Topics A Why are faults important? B Observations of faults C Geologic classification of faults I I Why are faults important? A Faults generate earthquakes B Faults reveal how the earth has deformed through time C Faults (including deformation bands) play critical roles in fluid transport in the earth's crust (e.g., water, magma, petroleum & natural gas, and hydrothermal fluids [ore minerals]) D Faults are zones of weakness to account for in engineering projects III Observations of faults A Gross geometry 1 Thin relative to their in-plane dimensions 2 Bounded in extent 3 Grossly planar (usually) B Relative displacement (slip) of opposing fault walls is parallel to the fault C Structural details of faults 1 Overall structure: ~planar segments Affects internal structure & fault behavior 2 Internal structure: complicated fracture pattern 3 Common for faults to be paralleled by other fractures. Typical a s s u m p t i o n is that fault slip causes fault-parallel fractures to form (note that cross-cutting relationships don't help out). 4 Common for hydrothermal/geothermal activity to occur near ends of fault segments (San Andreas, Wasatch faults) D Composition 1 Breccia 2 Fault gouge 3 Mineralization Stephen Martel 25-1 University of Hawaii GG303 Lecture 25 9/4/01 2 E Kinematics 1 Cut adjacent material; faulting post-dates the host rock 2 Relative (not absolute) d i s p l a c e m e n t (slip) of originally neighboring points is parallel to the fault; relative displacement may or may not be small relative to fault length F Surface textures of faults 1 Slickensides (polished surfaces) 2 S l i c k e n l i n e s (striations); parallel to most recent(?) slip vector IV Geologic classification of faults (see Fig. 25.1) A Geologic classification 1 Based on orientation of slip vector relative to the strike and dip of a fault 2 Slip determined by the relative displacement of piercing points that were originally neighbors on opposite faces of a fault. 3 Piercing points mark intersection of a line with a fault 4 The slip vector connects offset piercing points 5 Slip is not the same as "movement" or "displacement" B Strike-slip fault: slip vector is predominantly horizontal (i.e., parallel or anti-parallel to the line of strike) 1 Right lateral: in map view across a fault, a marker is offset to the right 2 Left lateral: in map view across a fault, a marker is offset to the right C Dip-slip fault: slip vector is parallel (or anti-parallel) to dip 1 Normal fault: hanging wall moves down relative to footwall 2 Thrust fault: hanging wall moves up relative to footwall *Deeper (older) rocks thrust over shallower (younger) rocks* D Oblique-slip: combination of strike slip and dip slip E Slip vs. Separation (see Fig. 25.2) 1 Slip: True relative displacement of originally neighboring points 2 Separation: Apparent relative displacement of an offset feature as seen in a map or a cross-section F The amount and direction of slip can change with time and/or position along a fault! Stephen Martel 25-2 University of Hawaii GG303 Lecture 25 9/4/01 3 Fig. 25.1 Geologic Classification of Faults Strike-slip Faults Left lateral Right lateral Piercing points Slip Vector y wa ive Dr y wa ive Dr Slip Vector Folded unit, with sharp hinge Dip-slip Faults Pure normal slip Pure reverse (thrust) slip w ive Dr ll wa ot ll Fo wa ot Fo Slip vector ay wa ive Dr y l al gw in l al gw in ng ng Slip Vector Ha Ha Folded unit, with sharp hinge Stephen Martel 25-3 University of Hawaii GG303 Lecture 25 9/4/01 4 Contrast between slip and separation Fig. 25.2 Pure dip slip. Slicklenlines plunge directly down-dip. Piercing point Fault Scarp Slip vector y wa ive Dr After slip, Before erosion Folded unit, with sharp hinge Apparent right-lateral offset (right-lateral separation) Separation Separation Apparent left-lateral offset (left-lateral separation) After slip, After erosion, After driveway removal, After new home construction Stephen Martel 25-4 University of Hawaii