Faults II Lecture 28 Gly 326 Faults and

Faults II Lecture 28 Gly 326

Faults and forces BRITTLE DUCTILE Relationships between deformation and force/stresses reverse faults normal faults strike-slip faults

Faults and forces Anderson’s tectonic classification

Normal faults • Dominantly dip-sip Hanging wall moves down relative to the footwall • Typically related to extension and crustal thinning • Recall: Missing units in core and map pattern Fault slip and scarp formation After erosion

Normal faults • Normal faulting: Common in extensional regimes – Mid ocean ridges, rift zones, etc. • Listric faulting common in normal fault systems Transition from high to low angle • In the lower, ductile crust, extension is accommodated along low-angle shear zones • Tectonic extension includes crustal scale extension in the horizontal plane, parallel to the surface of the Earth

Normal fault systems The domino model • A series of rigid fault blocks that rotate simultaneously in a uniform sense • Many domino systems rest on an underlying detachment low angle (<30) faults that nucleate preferentially in mechanically weak zones (brittle-ductile transition; clayrich strata; salt) • Rotation of strata prior to faulting sometimes results in domino faulting

Normal fault systems • Domino model assumptions: – No block-internal strain – Faults & layers rotate simultaneously & at the same rate – All faults have the same dip – Layers and faults are planar

Normal fault systems Domino model • Low angle fault underneath the Hornelen Devonian basin in the Scandinavian Caledonides • Devonian sandstones and conglomerates in hanging wall; mylonitized basement in footwall

Normal fault systems Domino model • Gullfaks Domino System (North Sea Gullfaks oil field) • Cross section from well and seismic data (interpreted below) • Domino systems of 4 to 6 main blocks, each subdivided by smaller faults. Dipping layers show a systematic westward decrease in dip within each block

Normal fault systems Conjugate Fault Sets Antithetic: in a direction opposite to the prevailing sense of vergence (vergence: sense of shear or transport direction deduced from the asymmetry of a structure) Synthetic: descriptive of a structure or fabric with the prevailing orientation or vergence Conjugate: a pair, or two sets of faults, shear zones, asymmetrical folds, kink bands, with opposing dips

Normal fault systems Accommodating extension: How do we avoid gaps?

Normal fault systems Rollover fold Antithetic faults in hanging wall

Normal fault systems Synthetic faults in hanging wall

Normal fault systems Rollover fold with synthetic faults

Normal fault systems • Imbrication fan: a series of wedge-shaped faults or fault blocks developed above a ramp-flat-ramp fault, where the faults either die out upward or reach the surface • Extensional duplex: a series of lenses (horses), arranged in a piggy-back fashion, bounded by an upper (roof) fault and a lower (sole or floor) fault Horse: tectonic sheet bounded by fault on all sides. Smallest tectonic unit!

Normal fault systems • Pure shear: involves symmetrical stretching of the lithosphere in the horizontal direction and thinning in the vertical direction • Simple shear: involves asymmetric thinning of the lithosphere by a throughgoing shear zone in the lower crust and mantle-lithosphere, and by brittle faulting in the upper crust

Normal fault systems Metamorphic core complex: Special simple shear case, common in extended areas • During deformation the upper crust is thinned and MOHO elevated aided by erosion of the upper plate… Exposes footwall rocks! Deep metamorphic (mid-lower crustal) rocks exposed at the surface as the footwall of a lowangle detachment

Normal fault systems