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Stretching: • Stretching causes three types of isostatic readjustment: • 1) Isostatic readjustment to changes in the lithosphere thickness (initial subsidence); • 2) a thermal perturbation, which produces (by ) h l b h h d (b removing the excess heat out of the lithosphere) a long term thermal subsidence; long term thermal subsidence; • 3) deviation of basin depth from a local isostasy will cause the lithosphere to be in an up‐ or down will cause the lithosphere to be in an up or down‐ ward state of flexure. Depth of the Moho • Pure shear: this model assumes that deformation on the scale of the lithosphere is homogeneous the lithosphere is homogeneous. The crust deforms more or less brittle, whereas the mantle deforms in a ductile mode. The crustal thickness distribution over crustal thickness distribution over the rift zone gives a measure for the stretching factor • Simple shear: this model assumes that thinning of the lithosphere occurs by shearing along a master fault that cuts the entire lithosphere. In this way an asymmetric rift develops. Characteristics of this mode of deformation are: 1. Flank uplift takes place only in the hanging wall of the master fault. 2 Thermal subsidence is largest 2. Thermal subsidence is largest on the location of the mantle upwelling. • • Following McKenzie (1978): Uniform stretching model Uniform stretching model • Crustal Crustal stretching causes subsidence, whereas stretching causes subsidence whereas thinning of the upper mantle causes uplift so in order to calculate the isostatic in order to calculate the isostatic compensation we need to make the following assumptions: ‐T distribution is linear ‐stretching is instantaneous hi i i ‐deformation is homogeneous Initial subsidence • Crustal Crustal stretching causes subsidence wherears stretching causes subsidence wherears thinning of the mantle lithosphere produces uplift Assumptions: uplift. Assumptions: 1) T distribution of the lithosphere is linear 2) S 2) Stretching is instantaneous hi i i 3) Deformation is homogeneous 4) Initial heat production is zero. Density effects 1) Thinned crustal lithosphere is replaced by 1) Thinned crustal lithosphere is replaced by denser mantle lithosphere 2) Thinned mantle lithosphere is replaced by 2) Thinned mantle lithosphere is replaced by less dense mantle astenosphere 3) C 3) Crust is replaced by water (or sediments). The i l db ( di ) Th height of the water column is the amount of subsidence. b id Following McKenzie ( • In time 2 the li h h lithosphere will ill subside because of isostatic compensation compensation (you replace denser mantle lid with less dense asthenosphere. • In time 3 the lithosphere cools p and thickens as warm asthenosphere converts in cool t i l lithosphere (thermal subsidence) In isostatic equilibrium (i.e. local compensation) • The The summed masses of any column of the earth above summed masses of any column of the earth above an equipotential surface in the astenosphere must equal that of all other colums: ‐if two locations are compared the sum for all the layers of the individual mass differences must be zero; i.e. An equipotential surface in the astenosphere exists because it has a low enough viscosity to flow i Δ( h) 0 i 0 i Δh Δ elevation i 0 (differences in thicknesses of layers in a column produce differences in elevation) 0‐i: levels of different densities; : difference between colums; : density; h: thickness of each level • B By balancing the forces acting on the b l i th f ti th lithospheric columns before and after stretching for values of: t t hi f l f ‐crust: 2.8 g/cm3 ‐lithospheric mantle: 3.3 g/cm3 you will see that a thickness of 15km corresponds to 0 subsidence; i.e. >15km positive subsidence and <15km negative subsidence (uplift) Based on b and the dip of master fault Based on b and the dip of master fault • Discrete Discrete continental rift (e.g. Rhine Graben), continental rift (e g Rhine Graben) normal thickness crust, extend slowly (<1mm/yr) over long periods of time (1—30 (<1mm/yr) over long periods of time (1 30 Myr) with low total extensional strain (<10km) Master fault angles (45‐70 (<10km). Master fault angles (45 70o) • Supradetachment basins, occurred on thickened crust extend quickly (<20 mm/yr) thickened crust, extend quickly (<20 mm/yr) over short periods of time (5‐12 Myr) with higheer amount of total strain (10‐80km). amount of total strain (10 80km) Different factors Finite strength of the lithosphere d during rifting: Depth of Necking f h f k • Necking=large scale thinning of the lithosphere caused by mechanical extension. • Zneck is defined as the depth of the lithosphere that remains horizontal during thinning if the effects of remains horizontal during thinning if the effects of sediment and water loading are removed (in McKenzie model is 0km). • (i.e the level of zero vertical motion for lithosphere stretching in the absence of gravity or isostasy. t t hi i th b f it i t • For level of Zneck deeper than the level of compensation (CD), an upward load acts on the lithosphere and an upward state of flexure results with flexurally supported rift flexure results with flexurally supported rift shoulders • For level of Zneck shallower than the level of compensation (CD), a p downward load acts on the lithosphere and a downward state of flexure results. Growth of Normal Faults Growth of Normal Faults Isolated fault growth by radial tip propagation Plan view Plan view Fw uplift generally < Hw subsidence Area affected by footwall uplift Area affected by hangingwall subsidence Isolated fault growth by radial tip propagation: D L scaling propagation: D‐L scaling •Fault development with characteristic bell shaped characteristic bell‐shaped D‐L (displacement‐length) profiles •Idealised fault growth pattern After Cartwright, Trudgill, Mansfield, 1995 Wasatch Fault, Utah Wasatch Fault, Utah D L Wasatch Fault, Utah D L But, it is unrealistic to consider a single fault in isolation........ Wasatch Fault array Wasatch Fault array Wasatch Fault array Wasatch Fault array How do fault segments interact? Segment linkage model 1. Isolated radial propagation Segment linkage model Relay ramp 2. Overlapping interaction Segment linkage model Sediment supply Sediment supply Relay ramps can be important in sediment supply sediment supply Segment linkage model Breached relay ramp Breached relay ramp Abandoned splay 3. Through‐going linkage Interactions = Skewed displacement profiles fil Bell shaped D L profile Bell‐shaped D‐L profile 1. Isolated propagation Asymmetric D L profiles skew toward each other Asymmetric D‐L profiles – skew toward each other 2. Interaction D‐L profile aiming toward idealised bell shape 3. Through‐going linkage Tectonics builds relief and generates a b i basin Extension ‚half graben‘ Footwall Hangingwall Tectonic subsidence Normal Fault Growth • Many studies (field, sub‐surface, analogue, numerical) suggest that extensional systems are complex interplay of linkage and propagation North Sea Mcleod et al., 2000 Volcanic Tablelands Volcanic Tablelands Dawers et al., 1993 Numerical modelling Cowie et al., 2000 Integrated fault growth models ( (Cowie et al. 2000) ) Lots of small faults A few large faults Distinctive horst and graben structure of the canyonlands Sedimentation and Normal Faults Sedimentation in rift basins • Rift basin fill normally consists of continental deposits: – – – – – Fluvial; L Lacutrian; i Alluvial fans; Deltas; Turbidites. • Evaporites may form in rift valleys: may form in rift valleys: ‐episodic invasion of the sea; ‐closed basin. l db i • Volcanic rocks may be present. Passive margins Passive margins • Strongly Strongly stretched crust over a distance of 50 stretched crust over a distance of 50‐ 500 km; • seaward thickening sedimentary prism; seaward thickening sedimentary prism; • they are normally characterized by shallow marine deposits (shelf deposits either clastic i d i ( h lf d i ih l i or carbonate). Erosion and subsequent condensed section deposited onto footwall high Asymmetric syn-rift fill POST-RIFT SYN RIFT SYN-RIFT FOOTWALL FOOTWALL HANGINGWALL HANGINGWALL PRE-RIFT SERIES OF DOMINO - STYLE HALF GRABENS • Hangingwall basins Sediment dispersal system is focused in the overlap (relay zone) A common feature of hangingwall basins is the vertical stacking of delta progradation units whose architecture depends on: 1) Tectonic subsidence; 2) Glacio-eustatic fluctuation; 3) Sediment supply fluctuation to the delta front. Schematic delta architecture Schematic delta architecture Role of fault interactions in controlling synrift p p sediment dispersal patterns ‐Fault Fault linkage lead to drainage linkage lead to drainage reorganization. ‐Changes in displacement control accommodation space. After Gupta et al. (1999) Stratigraphy of the Gulf of Suez half graben Delta complex Eastern basin margin Basin deepening Onset of extension Onset of extension Individuate: •main faults; i f lt •pre‐rift and syn‐rift strata; •and syn‐rift unconformity. ENE WSW Bedrocks 50km Stratigraphy of the Gulf of Suez half graben Eastern basin margin Basin deepening Onset of extension Onset of extension Individual delta wedges (Gilbert delta type) are 15‐30m thick and comprise: •forest unit, steeply dipping, medium‐ to thick‐ bedded conglomerates. These facies show rapid downtransport facies transition inot poximal pro‐delta bottomset sandstone turbidite and conglomerate debris flows. The Gulf of Suez example comprises: •At least 10 vertically stacked Gilbert delta; •At least 10 vertically stacked Gilbert delta; •The vertically stacked Gilbert deltas represent repeated episodes of delta progradation d d fd l d punctuated by db abrupt transgression of the delta top and drowning of the prograded p g delta. They are the result of basin y accommodation due to fault growth.