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3/19/15& Normal faults • Accommodate extension • Andersonian orientation is 60° dip • Measure displacement, throw heave Normal faults • Multiple faults produce characteristic structures – Horst – Graben – ‘Master faults’ 1& 3/19/15& Listric normal faults Gulf&of&Oman& North Sea faulting styles…. 2& 3/19/15& Normal faults • Displacements often measured with fault cutoff lines • In map view, the fault is defined as a gap Normal faults • Extension typically causes missing sections in stratigraphy • Unless fault dips more steeply than wellbore 3& 3/19/15& Faults don’t continue on forever • Block diagrams are convenient… but schematic • Fault displacement dies out near fault tips 1m Big Hole fault, Utah 4& 3/19/15& Big Hole fault, Utah Big Hole fault, displacement profile Shipton&and&Cowie&& (2001)& 5& 3/19/15& Fault growth Growth of an ideal normal fault As faults accumulate displacement they propagate Fault growth (porous rock) Shipton&and&Cowie,&2001& 6& 3/19/15& Fault growth (crystalline rock) Martel&(1990)& Damage zone processes • As soon as the slip surface nucleates, the early deformation bands become the “damage zone” 7& 3/19/15& Fault populations 8& 3/19/15& Normal fault systems East Pacific Rise North Sea Basin and Range, USA Brazil 9& 3/19/15& How do faults interact? Relay ramps 10& 3/19/15& Displacement profiles and linkage Linkage in the slip direction • Faults propagate in slip direction too • Overlapping, underlapping equivalents exist, but complicated because restraining/ releasing geometries • Linkage facilitates growth 11& 3/19/15& Fault linkage and hydrocarbons Fault linkage and hydrocarbons Walsh & Watterson (1991)& 12& 3/19/15& Growth faults • Normal faults create accommodation space for sedimentation • The extent and thickness of a given unit represent the size and displacement in an interval of time Growth faults 58 0 40'N Great Glen Fault Wick Wick Fault • Example from the Morray Firth, North Sea cross-section Smith Bank 20 km Fault Helmsdale Fault Beatrice 57 0 35'N 40 W NW Smith Bank Fault 5 km 30 W 20 W SE 1 km Helmsdale Fault 13& 3/19/15& How do fault populations grow through time? Final fault system Active faults early stage (b)& N Active faults late stage From the Inner Moray Firth Beatrice Field (Walsh et al. 2003) 2 k m (c)& Evolution of fault populations involves the death of small faults and the localization of displacement (strain) onto larger faults. Fault communication and sealing • Faults affect fluid flow! • Predict their effects (in space and time) 14& 3/19/15& Movie&by&Schmatz&and&Urai&(RWTH&Aachen&University)& Normal faults in Anderson’s world • If σ1 is vertical, normal faults should dip ~60° 15& 3/19/15& Common features of range fronts uplifted by normal faults 16& 3/19/15& Domino model • Rigid fault-bounded blocks, all rotate • Predictions: – No block internal strain – Faults and layers rotate simultaneously at same rate – Faults have same offset, dip 17& 3/19/15& Evolution of domino systems • Geometric problems with dominos • When blocks rotate too far, new “normal” normal faults form Soft domino model • Soft dominos allow internal strain – System of faults of all sizes and displacement amounts – Folding can occur 18& 3/19/15& Dominos or horsts/grabens? • Dominos promoted by presence of a weak basal layer (detachment) • Regional tilting (of detachment) The problem of Low Angle Normal Faults 1: they exist • Dip ≤ 30° – NOT Andersonian • Slipped 5–50 km • Cut mid-crust rocks so not surficial (landslides) 19& 3/19/15& The problem of Low Angle Normal Faults 2: they appear to have formed at low angle • Reconstructions show low angles between detachment and footwall bedding, preexisting thrusts • Often not cross-cut by later, steeper normal faults so not dominos The problem of Low Angle Normal Faults 3: they are not seismically active • V. few earthquakes on faults dipping ≤ 30° are observed • Mechanically unlikely to be reactivated (slip more) 20& 3/19/15& LANF’s form as low angle normal faults? • Pre-existing thrusts at low angles could be weak and fail in extension if the tectonic regime changes LANF’s and metamorphic core complexes • Extension at the scale of the crust 21& 3/19/15& LANF’s and metamorphic core complexes • Model predicts – Faults of different ages – Possible opposing dips – Heat flow higher in footwall – Transition from ductile to brittle deformation Turtlebacks in LANF footwall Photo&from&Casey&Moore& 22& 3/19/15& LANF’s form shallow? • Some LANF’s cross-cut steeper dipping normal faults • Requires a weak detachment / high pore fluid pressure Extensional systems: rifting • Active • Passive • Pre-, syn-, post accompany thinning and subsidence 23& 3/19/15& Rift types: McKenzie model • Pure shear, symmetric • Thinning in response to horizontal extension (Lister(and(Davis,(1989)( Rift types: Wernicke model • Simple shear, asymmetric • Controlled by a shallowly dipping zone of localized simple shear (Lister(and(Davis,(1989)( 24& 3/19/15& Which is it?? A note on fault populations • Power law distribution for number of faults in a rift system • Given an observed number of faults of size X, can we predict all the others? 25& 3/19/15& Extensional systems • Mid ocean ridge spreading centers Ultramafic oceanic core complexes AtlanQs&Massif& • • • Large striated domal feature located along a non-volcanic section of the Mid-Atlantic Ridge Thought to be an exposed detachment fault that is bringing peridotite and gabbro to the surface Fault displacements in these detachments can be too large to structurally restore. New crust must have been created at the base of the uplifting block during life of the detachment fault. 26& 3/19/15& Extensional systems throughout the Wilson cycle 27&