Lecture 16 Strike-Slip Tectonics Earth Structure (2 nd Edition), 2004 W.W. Norton & Co, New York Slide show by Ben van der Pluijm © WW Norton; unless noted otherwise 11/10/2014 © EarthStructure (2 nd ed) 2 11/10/2014 © EarthStructure (2 nd ed) 3 Strike-slip Tectonics EPR’s Clipperton TF and FZ • A strike-slip fault, in the strict sense, is a fault on which all displacement occurs in a direction parallel to the strike of the fault (slip lineation on a strike-slip fault are horizontal). • Thus, strict strike-slip displacement does not produce uplift or subsidence, but strike-slip movement is usually accompanied by a component of shortening or extension. • Transpression occurs where there is a combination of strike-slip movement and shortening, and can produce uplift along the fault. • Transtension occurs where there is a combination of strike-slip movement and extension, and can produce subsidence along the fault. • Flower structure - An array of faults in a strike-slip fault zone that merges at depth into a near-vertical fault plane, but near the structure ground surface diverges so as to have shallower dips. • A positive flower structure has a component of thrusting on faults • A negative flower structure has a component of normal faulting. 11/10/2014 © EarthStructure (2 nd ed) 4 Distributed, complex structures in Strike-Slip Zones Fig. 19.12 Arrays of subsidiary structures associated with dextral shear. Subsidiary strike-slip faults Riedel (R) and R’ shears en echelon folds, and en echelon thrusts en echelon folds which formed and then were later En echelon normal faults and veins. offset by shear on a strike-slip fault 11/10/2014 © EarthStructure (2 nd ed) 5 Distributed, complex structures in Strike-Slip Zones Fig. 19.15 Strain models exemplifying subsidiary structures along a strike-slip fault. • A map view of dextral simple shear in which a square becomes a parallelogram, and a circle in the square becomes an ellipse. • A detail of the strain “Riedel shears” ellipse showing that folds and thrusts form perpendicular to the shortening direction, while normal faults and • R and R ′ shears form • Note that R and R ′ are similar to veins form at an acute angle to conjugate shear fractures formed perpendicular to the the shortening in rock cylinder subjected to an extension direction direction. axial stress. 11/10/2014 © EarthStructure (2 nd ed) 6 Transform Faults Sec. 19.2 • J. Tuzo Wilson introduced the term “transform faults” to the geologic literature in the early 1960s for plate boundaries that are not distinctly convergent or divergent • Transform fault can be applied more broadly to describe any strike-slip fault that has the following characteristics: • The active portion of a transform fault terminates at discrete endpoints that intersects other structures • The length of a transform fault can be constant or vary over time . Transform length increases as the two triple Transform length stays junctions ( T1 and constant if spreading rates T2) defining the on ridge segments at both endpoints move endpoints are the same. apart. Transform length decreases if the spreading rate at one endpoint is less than the subduction rate at the other. 11/10/2014 © EarthStructure (2 nd ed) 7 Oceanic and Continental Transform Faults Sec. 19.2 A continental transfer fault linking two rift Transform faults along the Mid-Atlantic segments can evolve into an oceanic Ridge in the South Atlantic Ocean. transform offsetting mid-ocean ridges SAP = South American Plate, NAP = North American Plate, AFP = African Plate, ANP = Antarctic Plate, NZP = Nasca Plate, PCP=Pacific Plate, SP = Scotia Plate, MAR = Mid-Atlantic Ridge. 11/10/2014 © EarthStructure (2 nd ed) 8 11/10/2014 © EarthStructure (2 nd ed) 9 Continental transform fault: San Andreas Fault (US) Fig. 19.1 North American and Pacific Plate Plate boundary 11/10/2014 © EarthStructure (2 nd ed) 10 San Andreas Fault (38-0Ma) T. Atwater, UCSB 11/10/2014 © EarthStructure (2 nd ed) 11 Transform Faults Fig. 19.5 • The amount of Displacement at X is displacement remains the the same as the same along the length of a displacement at Y. transform if the length of the transform stays constant or decreases. Displacement at X, in the middle of the fault, • If the transform length is greater than changes with time, then the displacement at point amount of slip varies along Y, near an endpoint. the length. (ex.) At time 1, the fault is fairly short. At time 2, the length of the fault is longer. 11/10/2014 © EarthStructure (2 nd ed) 12 Continental transform fault: Alpine Fault (NZ) Fig. 19.2 The Alpine Fault in New Zealand links the Macquarie Trench (M) with the Tonga-Kermadec Trench (TK) 11/10/2014 © EarthStructure (2 nd ed) 13 Transcurrent Faults Fig. 19.5 • Die out along their length , do not terminate abruptly at another fault, but either splays into an array of smaller faults ( horsetails ), or simply disappears into a zone of plastic strain. • Depending on the direction of fault-tip curvature and sense of displacement, movements will be accompanied either by folding and uplift where there is a thrust component, or by tilting and subsidence where there is a normal component. 11/10/2014 © EarthStructure (2 nd ed) 14 Transcurrent FaultsFault Fig. 19.9 • Initiate at a point and grow along their length as displacement increases • Fault displacement measured in map view is proportional to fault length. Rule of thumb: Displacement = 0.03 or Length (L = 30 D) • Displacement across a transcurrent fault is greatest near the center of its trace and decreases to zero at the endpoints of the fault • The displacement on a transcurrent fault must always be less than the length of the fault. 11/10/2014 © EarthStructure (2 nd ed) 15 Distributed, complex structures in Strike-Slip Zones Fig. 19.11 Stepover along strike-slip faults. At a restraining stepover, compression and thrusting occur. • A stepover occurs where fault slip is relayed from one fault to another At a releasing stepover, extension and subsidence occur. 11/10/2014 © EarthStructure (2 nd ed) 16 Transcurrent fault evolution Fig. 19.14 Laboratory model of strike-slip fault development. As deformation begins, Riedel shears develop • A clay cake rests on two wooden blocks that A mature were pressed together. transcurrent fault system showing a • The clay represents the weak uppermost through-going crust, and the wood blocks represent the fault, in which stronger lower crust. Riedel shears have been linked • The vertical boundary between the two by P fractures. blocks represents the strike-slip fault. 11/10/2014 © EarthStructure (2 nd ed) 17 Transcurrent fault evolution Fig. 19.14 A side-scan radar image from the Darien Basin in eastern Panama showing an array of en echelon An example of a clay-cake experiment for left-lateral shear anticlines. The field of view is about 50 km. 11/10/2014 © EarthStructure (2 nd ed) 18 Block Rotation in Strike-Slip Zones Fig. 19.16 Mechanisms of block rotation in a right-lateral strike-slip zone. • Map view of a grid • The grid lines rotate and • Alternatively, smaller, less being subjected to fault-bounded blocks may organized fault blocks may dextral simple rotate intact as slip on form with varying directions shear. bounding faults increases and amount of rotations. (bookshelf model). • Paleomagnetic declination data give rotaitons for each block (solid arrows) realtive to a reference direction (dashed line). 11/10/2014 © EarthStructure (2 nd ed) 19 Continental Strike-Slip Faults: Anatolia (Turkey) Fig. 19.25 Lateral escape in response to the northward movement of the Arabian Plate is squeezing Turkey out to the west. 11/10/2014 © EarthStructure (2 nd ed) 20 Continental strike-slip faults: Red River and Altyn Tagh faults (E Asia) Sketch map showing India colliding with southern Asia. • Strike-slip faults have developed in several settings here. • The Chaman Fault (CF) is a transform boundary that delimits the northwestern edge of the Indian subcontinent. • Strike-slip faults also form due to oblique collision, oblique convergence, and lateral escape. • Small rifts have developed just north of the Himalayas. 11/10/2014 © EarthStructure (2 nd ed) 21 Transtension, Transpression, and Fault Bends • Restraining • Transpression occurs where bend at which there is a combination of strike- thrust faults slip movement and shortening, form transverse and can produce uplift along the uplifts. fault. • Transtension occurs where • Releasing bend there is a combination of strike- at which normal slip movement and extension, faults form a and can produce subsidence pull-apart basin along the fault. Simple wood block model illustrating the concept of transpression and transtension. When blocks shear When blocks and squeeze shear and pull together sand is apart sand pushed up sags. 11/10/2014 © EarthStructure (2 nd ed) 23 Transtension, Transpression, and Fault Bends San Andreas Fault north of Los Angeles (LA) Releasing bend Restraining bend Transverse mountain range Is where a fold-thrust belt forms at a restraiing bend causing uplift 11/10/2014 © EarthStructure (2 nd ed) 24 Transpressions and Transtension Pressure ridge in a road cut across the San Andreas Fault near Palmdale, Ca. Structures along SAF near Palmdale, CA 11/10/2014 © EarthStructure (2 nd ed) 25 Flower Structure - An array of faults in a A positive flower strike-slip fault zone that merges at depth into a near- structure has a component of thrusting vertical fault plane, but near the structure ground on faults surface diverges so as to have shallower dips. Seismic-reflection profile across strike-slip fault in Ardmore Basin, Oklahoma, showing positive flower structure A negative flower structure has a component of normal faulting. 11/10/2014 © EarthStructure (2 nd ed) 26 Strike-Slip Duplex Fig.