Room: 407 Tel: Patrice Rey

Lecture 3: Fractures and Faults Aim: To characterize the structures that accommodate the deformation of rocks. Faults occur at all scales in the lithosphere, and geologists study them for many reasons: Faults control the spatial arrangement of rock units, so their presence creates puzzles that challenge even the most experienced geologist. Faults affect topography and modify the landscape. Faults affect the distribution of economic resources (e.g., oil fields and ore bodies). Faults control permeability of rocks and sediments, properties which, in turn, control fluid migration. Faulting creates deformation (strain ± rotation ± translation) in the lithosphere during plate interactions and intraplate movements. Faulting may cause devastating earthquakes. Thus, fault analysis plays a role in diverse aspects of both academic and applied geology. This lecture introduces the terminology used to describe fault geometry and displacement, and shows you how to recognize and interpret faults at the surface and in the subsurface.

Attitude of planes and lines Description folds, faults, or any others structures, requires the specification of the attitude of planar and linear elements associated with theses structures. Plane: Strike-dip-dip direction Line: Plunge-plunge direction or Pitch (if line carried on a plane)

Joints and fractures Joints: Planar cracks caused by tension, no movement perceptible across a joint. Fractures: Planar cracks showing movement across the fracture plane. Tension fractures=>movement perpendicular to the fracture plane. Shear fractures=>movement parallel to the fracture plane.

Joints and fractures En echelon tension fractures (tension gashes), open in a direction parallel to that of the minimum applied stress. Sigmoïde en echelon tension fractures (tension gashes), indicate the formation of an weak zone (incipient fault). “S” sigmoide show anti-clockwise shearing, “Z” sigmoide show clockwise shearing. Mode 1: extension, relative motion perpendicular to the plane of the fracture. Mode 3: Scissors, relative motion parallel to the fracture surface. Mode 2: sliding, relative motion parallel to the fracture surface.

A fault is a fracture plane that accommodates the relative displacement of a pair of blocs. Dip slip and strike slip faults are the two main type of faults. Fault types In dip slip faults, the motion of the hanging wall relatively to the footwall occurs in a direction parallel to the dip of the fault plane. The footwall block stands underneath the fault plane, the hanging wall block stands above the fault plane. In strike slip faults, the motion of a pair of block relatively to each others occurs in a direction parallel to the strike line of the fault plane.

Fault types Dip slip faults: Reverse faults, normal faults, thrust, and detachment. Reverve fault: Hanging wall moves upward relatively to the footwall. Normal fault: Hanging wall moves downward relatively to the footwall. A thrust is a gently dipping reverse fault. A detachment is a gently dipping normal fault.

Normal faults in sand-box experiments McClay et al. More at: Physical models (=analogue experiments) allow to investigate how simplified geological systems react to simplified boundaries conditions. Using colored layers of sand, as an analogue for the upper crust, and applying various boundaries conditions, McClay’s team can successfully create, in their laboratory, structures observed in nature. Pictures on the rigth represent various geometry of sedimentary basins, from symmetrical extension (top), to highly asymmetrical extension (bottom). Listric fault

Thrust and reverse faults in sand-box experiments More at: McClay et al. From Schreurs The movie on the right is a digitized version of a physical experiment involving the shortening of a sand-box. It shows the development of reverse faults and the progressive stacking of several units. A similar experiment from McClay’s team. A numerical experiment from Beaumont’s group, illustrating the development of a thrust.

Fault types: Strike slip faults Since strike slip faults are vertical, there is neither footwall nor hanging wall. The blocks on each side of the fault plane are referred from their geographic position with respect to the fault plane (e.i. NW block and SE block across a NE-SW fault) Two types of strike slip faults can be distinguished depending on the relative motion of the two blocks: Dextral and sinitral strike slip faults Dextral strike slip: Relative motion toward the right. Sinistral strike slip: Relative motion toward the left.

Strike-slip in a sand- box experiment. (McClay) The top right movie shows a plan view of a sand-box experiment involving the shearing of a plate underneath a layer of sand. In a first stage, a set of en echelon tension fractures develop. Each fractures is oriented WNW. In a second stage, EW fractures develop bridging the gap between each tension fractures. A strike-slip fault develops.

Faults related structures

Striations = slickensides -> vector displacement along fault planes

So boys and girls... strike slip or dip slip fault? Dextral or Sinistral?

Mineral steps give the sense of movement along fault planes

Thrust in the Alps. A gouge (crumbled rock!) occurs in the footwall

Faults from space: A fault in the Kimberley region (WA) N

Faults from space: Halls Creek fault in the Kimberley (WA)

N

A B C Type of faults?

Lecture 3 at a glance Plane: Strike (3 digits number)-Dip (2 digits number)-Dip direction (letters) Line: Plunge (2 digits number)-plunge direction (3 digits number) Line on a plane: Pitch (2 digits number) Two types of faults: Dip slip (footwall, hanging wall): Normal fault, detachment, Slickensides: mechanical striation give the displacement direction Kinematic criteria such as mineral steps give the kinematic of faults (i.e. the sense of movement across the fault). Strike slip: Dextral Reverse fault, thrust.Sinistral