CHAPTER 8

1. CHAPTER 8 FAULTS AND FAULTING Dr. Masdouq Al-Taj

2. FAULTS • A fault is any surface or zone in the Earth across which measurable slip (shear displacement) develops. • Faults are fractures on which slip develops primarily by brittle deformation processes. • Fault zone is a brittle structure in which loss of cohesion and slip occurs on several faults within a band of definable width. • Shear zone: occurs at depth without definable displacement on the surface

4. We used four relative scales of observations - Micro: optical scale (microscope or even electron microscope). - Meso: single outcrop (personal scale). - Macro: regional scale (mountain range). - Mega: continental scale (plate dimensions).

5. Fractured feldspar grain in photomicrograph Fault trace in aerial photo Mesoscopic faults in outcrop

6. Fault components • Rocks adjacent to the fault surface is the wall of the fault, and the body of rocks that moved as consequence of slip on the fault is a fault block. • If the fault is not vertical, we can distinguish between the hanging-wallblock, which is the rock body above the fault plane, and the footwallblock, which is the rock body below the fault plane.

7. Foot wall and Hanging Wall

8. HOW TO DESCRIBE THE ATTITUDE OF FAULT We need to measure: • Strike • Dip angle, dip direction and hade • Net slip vector (rake) • Strike-slip component • Dip-slip component (Heave and throw)

9. Foot wall and Hanging Wall

11. Note that the rake angle is measured from the horizontal to the direction of net-slip on the fault plane

12. Fault types The most common types of faults are: 1. Dip-slip faults • Normal (Listric) • Reverse or Thrust (if dip angle <45º) 2. Strike-slip faults 3. Oblique-slip faults 4. Other faults: Scissors (Rotational).

17. Listric fault

18. Fault Scarp

19. Misleading Scarps(Fault-line scarp) • If the fault moves rock of much different strength together, differential erosion may create a fault scarp. • Such scarps may have dips opposite to that of the underlying fault.

21. Concepts of extensional and contractional faults

30. Thrust Sheet Diagram • Window (fenster) shows of the autochthon through the eroded allochthon • Klippe is a piece of allochthon surrounded by autochthon

31. Definitions • Autochthon: A body of rocks that remains at its site of origin, where it is rooted to its basement. Although not moved from their original site. • Allochthon: A mass of rock that has been moved from its place of origin by tectonic processes, as in a thrust sheet • Many allochthonous rocks have been moved so far from their original sites that they differ greatly in facies and structure from those on which they now lie

32. Window (Fenster) • Thrust faults are often thin sheets, and erosion may open holes in them • A hole through a thrust sheet is called a fenster, or window • Fenster: An eroded area of a thrust sheet that displays the rocks beneath the thrust sheet • Triangular teeth point outward fenster are used on a map

33. Klippe • If erosion leaves an isolated remnant of thrust sheet, surrounded by exposed footwall, the remnant is called a klippe (German for cliff) • Klippe are indicated on a map by inward pointing teeth

34. DESCRIPTION OF FAULT DIP 0° →horizontal fault 0 -10 →sub horizontal fault=Detachment: a regional, low-angle, listric at depth and overthrust faults . 10-30→shallowly dipping faults 30-60→modertly dipping fault 60-80→steeply dipping faults 80-90→sub vertical fault 90 →vertical fault

35. SEPARATION • The distance between the separated parts of the marker horizon is the separation, which is not the same as the net slip unless the line along which separation is measured happens to parallel the net-slip vector.

36. Components of Separation • Separation can be divided into several components: • Stratigraphic separation • Horizontal separation ( from offset in map view) • Dip separation (Heave and Throw) • Strike separation • Vertical Separation (from cross section)

37. Stratigraphic Separation • Offset measured perpendicular to bedding

39. Dip separation has two components; 1. Heave: Horizontal component of the dip separation 2. Throw: Vertical component of the dip separation • Strike separation (S): Distance between the offset horizons measured along the strike direction.

40. Change in Fault attitude (Fault bends) • Fault bends or steps along strike-slip faults cause abrupt changes in the strike of the fault and in the associated structural features. • Where movement across a segment of a strike-slip fault results in some compression, we say that transpression (restraining bends) is occurring across the fault (form Pressure ridges); But where movement results in some extension, we say that transtension (releasing bends) is occurring across the fault (form sag pond (local area) or pull-apart basin (regional scale, example is the Dead Sea).

41. rele

42. San Andreas Fault Ridge • Ridge created by transpression along the fault • Striped white and gray rocks are basement rocks pushed up relative to dark sedimentary cover.

43. Sag Pond • Prominent scarps with sag ponds are found along the Denali fault trace. • The ground is weakened on the fault trace and has the tendancy to sag and erode more easily than surrounding land.

44. Change in attitude vertically • Fault segments may parallel bedding in either the footwall or hanging wall, but cut across bedding in the opposite block.

45. Bedding and Fault Plane Orientation Flat and Ramp

47. Pressure and Temperature Influence Faulting • Changes due to burial depth • Breccia or gouge< 5 km • thin between 3-5 km and 10-15 km • Brittle faults end at 15 km depth • more wide at depth

48. Age relationship between different faults and their termination relation • Like joints, fault must terminate, and can do so in several different ways • The Principle of Cross-Cutting Relationships can be used to determine the relative ages. • A fault may terminate where it has been cut by a younger structure, such as another fault (C & D), an unconformity (E), or an intrusion (B), or at the ground surface (A)

49. Death of a Fault • Faults can also split, to form an anastamosing array, which may merge and diverge several times along its length • A fault splay may develop, with the fault splitting and dying out – these are called horsetails (B) • A fault dies when its displacement becomes less and less, finally reaching zero near the tip, in a zone of plastic deformation (C).