Fault Systems

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South Carolina Geological Survey Educational Series #4 Fault Systems Rocks respond to stress (being squeezed or pulled apart) near the Earth’s surface by breaking. When rocks break and there is no offset along either side of the break, the break is called a fracture or joint. Rocks can also break as a result of thermal expansion and contraction, the effects of fluids freezing, or when rocks are squeezed together or pulled apart. When the rocks move past each other along fracture surface, it is called a faulting. Fault surfaces are often nearly planar, and that planar surface is referred to as a “fault plane.” There are four types of faulting -- normal, reverse, strike-slip, and oblique. A normal fault is one in which the rocks above the fault plane, or hanging wall, move down relative to the rocks below the fault plane, or footwall. A reverse fault is one in which the hanging wall moves up relative to the footwall. When rocks on either side of a nearly vertical fault plane move horizontally, the movement is called strike-slip. An oblique-slip fault is special type fault that forms when movement is not exactly parallel with the fault plane. Oblique movement occurs when normal or reverse faults have some strike-slip movement and when strike-slip faults have either some normal or reverse movement. It is not hard to recognize a fault when you look at an outcrop or an exposure. Usually, different rock types or rock features (such as quartz veins, mineral layers, or beds) are broken and offset along the fault plane. Faults are commonly marked by debris, or breccia, that forms when there is movement along the fault plane. Grinding of rock along the fault plane may also produce a clay-like, pulverized rock called gouge. Sometimes when the fault plane is exposed, you may see grooves, striations (scratches), and asymmetric fractures, called slickensides that provide visual evidence of movement. NORMAL FAULT (CROSS SECTION) FOOTWALL BLOCK ! HANGING WALL BLOCK DOWN REVERSE FAULT HANGING WALL (CROSS SECTION) BLOCK UP ! FOOT WALL BLOCK SINISTRAL (LEFT-LATERAL) STRIKE-SLIP FAULT (PLAN VIEW) ! DEXTRAL (RIGHT-LATERAL) LEFT-LATERAL FAULT RIGHT-LATERAL FAULT Note: Movement determined by which side of the fault moves toward an observer looking along the fault plane..

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Strike and Dip Refer to the Orientation Or Attitude of a Geologic Feature. The
Name__________________________________ 89.325 – Geology for Engineers Faults, Folds, Outcrop Patterns and Geologic Maps I. Properties of Earth Materials When rocks are subjected to differential stress the resulting build-up in strain can cause deformation. Depending on the material properties the result can either be elastic deformation which can ultimately lead to the breaking of the rock material (faults) or ductile deformation which can lead to the development of folds. In this exercise we will look at the various types of deformation and how geologists use geologic maps to understand this deformation. II. Strike and Dip Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature, is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented parallel to the strike line. Strike (or strike angle) can be given as either a quadrant compass bearing of the strike line (N25°E for example) or in terms of east or west of true north or south, a single three digit number representing the azimuth, where the lower number is usually given (where the example of N25°E would simply be 025), or the azimuth number followed by the degree sign (example of N25°E would be 025°). The dip gives the steepest angle of descent of a tilted bed or feature relative to a horizontal plane, and is given by the number (0°-90°) as well as a letter (N, S, E, W) with rough direction in which the bed is dipping.
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Introduction This volume is a general geology field guide to the San Andreas Fault in the San Francisco Bay Area. The first section provides a brief overview of the San Andreas Fault in context to regional California geology, the Bay Area, and earthquake history with emphasis of the section of the fault that ruptured in the Great San Francisco Earthquake of 1906. This first section also contains information useful for discussion and making field observations associated with fault- related landforms, landslides and mass-wasting features, and the plant ecology in the study region. The second section contains field trips and recommended hikes on public lands in the Santa Cruz Mountains, along the San Mateo Coast, and at Point Reyes National Seashore. These trips provide access to the San Andreas Fault and associated faults, and to significant rock exposures and landforms in the vicinity. Note that more stops are provided in each of the sections than might be possible to visit in a day. The extra material is intended to provide optional choices to visit in a region with a wealth of natural resources, and to support discussions and provide information about additional field exploration in the Santa Cruz Mountains region. An early version of the guidebook was used in conjunction with the Pacific SEPM 2004 Fall Field Trip. Selected references provide a more technical and exhaustive overview of the fault system and geology in this field area; for instance, see USGS Professional Paper 1550-E (Wells, 2004). San Andreas Fault: An Overview The catastrophe caused by the 1906 earthquake in the San Francisco region started the study of earthquakes and California geology in earnest.
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Faults and Joints
133 JOINTS Joints (also termed extensional fractures) are planes of separation on which no or undetectable shear displacement has taken place. The two walls of the resulting tiny opening typically remain in tight (matching) contact. Joints may result from regional tectonics (i.e. the compressive stresses in front of a mountain belt), folding (due to curvature of bedding), faulting, or internal stress release during uplift or cooling. They often form under high fluid pressure (i.e. low effective stress), perpendicular to the smallest principal stress. The aperture of a joint is the space between its two walls measured perpendicularly to the mean plane. Apertures can be open (resulting in permeability enhancement) or occluded by mineral cement (resulting in permeability reduction). A joint with a large aperture (> few mm) is a fissure. The mechanical layer thickness of the deforming rock controls joint growth. If present in sufficient number, open joints may provide adequate porosity and permeability such that an otherwise impermeable rock may become a productive fractured reservoir. In quarrying, the largest block size depends on joint frequency; abundant fractures are desirable for quarrying crushed rock and gravel. Joint sets and systems Joints are ubiquitous features of rock exposures and often form families of straight to curviplanar fractures typically perpendicular to the layer boundaries in sedimentary rocks. A set is a group of joints with similar orientation and morphology. Several sets usually occur at the same place with no apparent interaction, giving exposures a blocky or fragmented appearance. Two or more sets of joints present together in an exposure compose a joint system.
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GY403 STRUCTURAL GEOLOGY KINEMATIC ANALYSIS KINEMATICS • Translation- described by a vector quantity • Rotation- described by: • Axis of rotation point • Magnitude of rotation (degrees) • Sense of rotation (reference frame; clockwise or anticlockwise) • Dilation- volume change • Loss of volume = negative dilation • Increase of volume = positive dilation • Distortion- change in original shape RIGID VS. NON-RIGID BODY DEFORMATION • Rigid Body Deformation • Translation: fault slip • Rotation: rotational fault • Non-rigid Body Deformation • Dilation: burial of sediment/rock • Distortion: ductile deformation (permanent shape change) TRANSLATION EXAMPLES • Slip along a planar fault • 360 meters left lateral slip • 50 meters normal dip slip • Classification: normal left-lateral slip fault 30 Net Slip Vector X(S) 40 70 N 50m dip slip X(N) 360m strike slip 30 40 0 100m ROTATIONAL FAULT • Fault slip is described by an axis of rotation • Rotation is anticlockwise as viewed from the south fault block • Amount of rotation is 50 degrees Axis W E 50 FAULT SEPARATION VS. SLIP • Fault separation: the apparent slip as viewed on a planar outcrop. • Fault slip: must be measured with net slip vector using a linear feature offset by the fault. 70 40 150m D U 40 STRAIN ELLIPSOID X • A three-dimensional ellipsoid that describes the magnitude of dilational and distortional strain. • Assume a perfect sphere before deformation. • Three mutually perpendicular axes X, Y, and Z. • X is maximum stretch (S ) and Z is minimum stretch (S ). X Z Y Z • There are unique directions
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Precambrian Research 157 (2007) 4–25 The Penokean orogeny in the Lake Superior region Klaus J. Schulz ∗, William F. Cannon U.S. Geological Survey, 954 National Center, Reston, VA 20192, USA Received 16 March 2006; received in revised form 1 September 2006; accepted 5 February 2007 Abstract The Penokean orogeny began at about 1880 Ma when an oceanic arc, now the Pembine–Wausau terrane, collided with the southern margin of the Archean Superior craton marking the end of a period of south-directed subduction. The docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back-arc extension both in the initial arc and adjacent craton margin to the north. A belt of volcanogenic massive sulfide deposits formed in the extending back-arc rift within the arc. Synchronous extension and subsidence of the Superior craton resulted in a broad shallow sea characterized by volcanic grabens (Menominee Group in northern Michigan). The classic Lake Superior banded iron-formations, including those in the Marquette, Gogebic, Mesabi and Gunflint Iron Ranges, formed in that sea. The newly established subduction zone caused continued arc volcanism until about 1850 Ma when a fragment of Archean crust, now the basement of the Marshfield terrane, arrived at the subduction zone. The convergence of Archean blocks of the Superior and Marshfield cratons resulted in the major contractional phase of the Penokean orogeny. Rocks of the Pembine–Wausau arc were thrust northward onto the Superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 Ma in the south (Baraga Group rocks) and 1835 Ma in the north (Rove and Virginia Formations).
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Collision Orogeny
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Faulted Joints: Kinematics, Displacement±Length Scaling Relations and Criteria for Their Identi®Cation
Journal of Structural Geology 23 (2001) 315±327 www.elsevier.nl/locate/jstrugeo Faulted joints: kinematics, displacement±length scaling relations and criteria for their identi®cation Scott J. Wilkinsa,*, Michael R. Grossa, Michael Wackera, Yehuda Eyalb, Terry Engelderc aDepartment of Geology, Florida International University, Miami, FL 33199, USA bDepartment of Geology, Ben Gurion University, Beer Sheva 84105, Israel cDepartment of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA Received 6 December 1999; accepted 6 June 2000 Abstract Structural geometries and kinematics based on two sets of joints, pinnate joints and fault striations, reveal that some mesoscale faults at Split Mountain, Utah, originated as joints. Unlike many other types of faults, displacements (D) across faulted joints do not scale with lengths (L) and therefore do not adhere to published fault scaling laws. Rather, fault size corresponds initially to original joint length, which in turn is controlled by bed thickness for bed-con®ned joints. Although faulted joints will grow in length with increasing slip, the total change in length is negligible compared to the original length, leading to an independence of D from L during early stages of joint reactivation. Therefore, attempts to predict fault length, gouge thickness, or hydrologic properties based solely upon D±L scaling laws could yield misleading results for faulted joints. Pinnate joints, distinguishable from wing cracks, developed within the dilational quadrants along faulted joints and help to constrain the kinematics of joint reactivation. q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction impact of these ªfaulted jointsº on displacement±length scaling relations and fault-slip kinematics.
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2 Review of Stress, Linear Strain and Elastic Stress- Strain Relations
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Paleostress and Remote Sensing Analysis of Brittle Fractures from the Eastern Margin of the Dead Sea Transform, Jordan”
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