Fractures “cracks along which cohesion of the material has been lost” - planes of discontinuity…most common type of geologic structure Two types of fractures: (1) Faults: surface across which the rock has been displaced in a direction that is generally parallel to the fracture

(2) Joints: a fracture with NO displacement parallel to the fracture plane…displacement is perpendicular to the fracture…will look at in more detail in subsequent lecture Very commonly filled with mineralization… Fissure: special class of joints that have dilation >20 cm

Why study fractures? zEngineering zFluid flow zEarthquake hazards zStress-strain significance

1. Engineering

2. Fractures can control subsurface fluid flow paths Courtesy of Zion National Park

3. Siesmic hazards Freeway Damage from 1994 Northridge Earthquake

4. Stress-strain signficance Fractures record tectonism Joints due to: 1.Faulting 2.Mechanical Stratigraphy 3.Fold Shape

Describing Faults Faults: a fracture surface across which the rock has been displaced in a direction that is generally parallel to the surface itself. Classification of faults: - each type of fault forms under different geologic conditions - so, if we can characterize the nature of the fault, we can conclude something about the geologic history...

Fault classifications are based on two basic observations: (1) orientation of the fault surface (2) nature of the displacement or offset across the fault Also: to a lesser extent (3) Character of the fault surface (This comes into recognizing the faults…we’ll look at this later) Fault Classification

Orientation of fault surfaces: Strike Dip strike dip Note: for mechanical reasons fault planes are seldom planar over long distances…thus, must take MANY strike and dip along a fault

Basic Fault Terminology (more to come later): Footwall – block sitting below the fault surface Hanging wall – “ “ “ “ above the fault surface Normal Fault Footwall Hanging wall Footwall Hanging wall Thrust/Reverse Fault Strike-Slip Fault Nature of slip along Faults

Nature of Fault Surfaces zDiscrete fracture or fault zone

Nature of Fault Surfaces zSlickensides zStriations zGrooves

Reidel shears (photo by M.Miller) Nature of Fault Surfaces zJoints and Microfaults zFault breccia zVein Filling zFault Gouge zPseudotachylite

Nature of Fault Surfaces zDeformation Bands zCataclastic rocks and mylonite zDuctile shear zones Adirondacks Jay, NY

Nature of Fault Shapes zFaults are not infinite Wytch Farm Oil Field, southern England (taken from Kttenhorn & Pollard, AAPG Bull, 2001)

Terminology of Slip zSlip: actual relative displacement on a fault. yOrientation of the slip line yMagnitude of displacement yRelative displacement

Drag folds as slip indicators zDistortion of layers consistent with sense of shear zReverse drag

Recognition of faults z1. Truncation and offset of rock units z2. Inconsistent stratigraphic relationships

Recognition of faults z3. Fault Scarps z4. Fault Line Scarps Photos by M. Miller

Recognition of faults z5. Triangular facets z6. Erosional traces Death Valley (photo by M. Miller)

Recognition of faults z 7. Topographic irregularities yTectonic geomorphology

Recognition of faults z8. Geophysics From the University of Leeds, UK