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Extentional Tectonics Extensional Tectonics & Rift systems Where do normal faults occur? i.e. what plate boundaries? What are major sources of stress? (stress drives extension, compression, and transpression!) Extensional Tectonics & Rift systems Where do normal faults occur? i.e. what plate boundaries? • Mid-ocean ridges (divergent plate boundaries) • Continental rift (divergent plate boundaries) i.e. East African Rift, Basin and Range, U.S., à How else could we break up supercontinents! • Convergent plate boundaries i.e. Andes, Himalaya What are major sources of stress? (stress drives extension, compression, and transpression!) • Lithostatic load (vertical!) • Plate tectonics (horizontal) • Fluid pressure • Thermal effects • Buoyancy (vertical) Stress vs. A. B. Map View C. Strain http://earth.leeds.ac.uk/learnstructure/index.htm From: www.uoregon.edu/millerm/LVSS.html From: http://www.nygeo.org/foldedrock.jpg Refresher D. E. F. From: www.fault-analysis-group.ucd.ie From: structuralgeo.files.wordpress.com From: http://www.alexstrekeisen.it/english/meta/boudinage.php Stress Regime Compression Tension Shear Brittle Type of Strain Type Ductile A. B. Map View C. http://earth.leeds.ac.uk/learnstructure/index.htm From: www.uoregon.edu/millerm/LVSS.html From: http://www.nygeo.org/foldedrock.jpg D. E. F. From: www.fault-analysis-group.ucd.ie From: structuralgeo.files.wordpress.com From: http://www.alexstrekeisen.it/english/meta/boudinage.php Compression Tension Shear Reverse Fault Normal Fault Strike-Slip Fault Map View HW FW HW Brittle FW From: structuralgeo.files.wordpress.com From: www.fault-analysis-group.ucd.ie From: www.uoregon.edu/millerm/LVSS.html Folds Boudins Ductile Shear Ductile From: http://www.nygeo.org/foldedrock.jpg From: http://www.alexstrekeisen.it/english/meta/boudinage.php http://earth.leeds.ac.uk/learnstructure/index.htm Compression Tension Shear Reverse Fault Normal Fault Strike-Slip Fault Map View HW FW HW Brittle FW From: structuralgeo.files.wordpress.com From: www.fault-analysis-group.ucd.ie From: www.uoregon.edu/millerm/LVSS.html Folds Boudins Ductile Shear Ductile From: http://www.nygeo.org/foldedrock.jpg From: http://www.alexstrekeisen.it/english/meta/boudinage.php http://earth.leeds.ac.uk/learnstructure/index.htm Compression Tension Shear Reverse Fault Normal Fault Strike-Slip Fault Map View HW FW HW Brittle FW From: structuralgeo.files.wordpress.com From: www.fault-analysis-group.ucd.ie From: www.uoregon.edu/millerm/LVSS.html Folds Boudins Ductile Shear Ductile From: http://www.nygeo.org/foldedrock.jpg From: http://www.alexstrekeisen.it/english/meta/boudinage.php http://earth.leeds.ac.uk/learnstructure/index.htm Compression Tension Shear Reverse Fault Normal Fault Strike-Slip Fault Map View HW FW HW Brittle FW From: structuralgeo.files.wordpress.com From: www.fault-analysis-group.ucd.ie From: www.uoregon.edu/millerm/LVSS.html Folds Boudins Ductile Shear Ductile From: http://www.nygeo.org/foldedrock.jpg From: http://www.alexstrekeisen.it/english/meta/boudinage.php http://earth.leeds.ac.uk/learnstructure/index.htm Compression Tension Shear Reverse Fault Normal Fault Strike-Slip Fault Map View HW FW HW Brittle FW From: structuralgeo.files.wordpress.com From: www.fault-analysis-group.ucd.ie From: www.uoregon.edu/millerm/LVSS.html Folds Boudins Ductile Shear Ductile From: http://www.nygeo.org/foldedrock.jpg From: http://www.alexstrekeisen.it/english/meta/boudinage.php http://earth.leeds.ac.uk/learnstructure/index.htm Compression Tension Shear Reverse Fault Normal Fault Strike-Slip Fault Map View HW FW HW Brittle FW From: structuralgeo.files.wordpress.com From: www.fault-analysis-group.ucd.ie From: www.uoregon.edu/millerm/LVSS.html Folds Boudins Ductile Shear Ductile From: http://www.nygeo.org/foldedrock.jpg From: http://www.alexstrekeisen.it/english/meta/boudinage.php http://earth.leeds.ac.uk/learnstructure/index.htm Compression Tension Shear Reverse Fault Normal Fault Strike-Slip Fault Map View HW FW HW Brittle FW From: structuralgeo.files.wordpress.com From: www.fault-analysis-group.ucd.ie From: www.uoregon.edu/millerm/LVSS.html Folds Boudins Ductile Shear Ductile From: http://www.nygeo.org/foldedrock.jpg From: http://www.alexstrekeisen.it/english/meta/boudinage.php http://earth.leeds.ac.uk/learnstructure/index.htm Continental rifting Continental lithosphere undergoes regional horizontal extension. At left, a region of lithosphere initially 80 km wide is stretched to 160 km wide. The Stretch = 2 ( recall S =ld/lu) [note: they call stretch b]. As a result, the lithosphere is thinned. Extension in the crust is accommodated by normal faulting. In addition, isostatic adjustments cause subsidence and deposition of sediment. So, rift systems are often characterized by syn-rift sedimentation. Extensional Faults- Where they form Rifts evolve from continents to oceanic ridges as a natural consequence of the supercontinent cycle Stratigraphy of rifts, pre, syn, post rift sediments. How would you tell them apart? © Cambridge University Press 2011 Extensional Faults- Continental Rift - East Africa Extensional Faults - Continental Rifts – Ethiopia Note rift flanks, (i.e. thermal bulge at triple junction) This topography is generally driven by mantle bouyancy. Extensional Faults - Continental rifts - Western Rift, East Africa, Lakes occupy half grabens. D:L ratios, with the greatest amount of slip in the middle can thus produce internally drained basins (required for a natural lake). East African rift system Passive margins and extension Note two levels of normal faulting: The lower level is inactive and left over from earlier rifting (developed prior to passive margin). The upper level is active and is gravitationally driven. Because the latter faults “grow” during sedimentation, they are commonly called “growth faults” – remember your last set of labs? Some Proterozoic “failed” rifts in the U.S. Extensional fault system (rift) geometries Classic (pre-1970’s) rift geometry - follows Andersonian theory A modern view of a rift system - asymmetric with detachment Most extensional systems have normal faults that “root” into a detachment. Andersonian mechanics and principal stresses Drawing some block diagrams! 12/4/2017 Warmup: CompressionDraw the principalTension stresses of a normal fault Shear Reverse Fault Normal Fault Strike-Slip Fault Map View HW FW HW Brittle FW From: structuralgeo.files.wordpress.com From: www.fault-analysis-group.ucd.ie From: www.uoregon.edu/millerm/LVSS.html Folds Boudins Ductile Shear Ductile From: http://www.nygeo.org/foldedrock.jpg From: http://www.alexstrekeisen.it/english/meta/boudinage.php http://earth.leeds.ac.uk/learnstructure/index.htm Extensional Faults- Continental Rifts - Basin and Range - Nevada 100% extension, why is this not an ocean? Rebuilding of the crust Basin & Range – a wide continental rift system Large-scale extension initiated in mid-Tertiary time (~20-25 Ma) in the western U.S. due to extensional collapse of a previously thickened Sevier orogenic belt…but continues today due in part to transtensional movement between the Pacific and N. American plates. Rotation in normal fault systems and stratigraphic cutoffs. Footwall and hangingwall stratigraphic cutoffs are the angles between the fault and the strata it cuts. (the angles add up to 180o). As the blocks rotate, those angles remain constant. If the strata are flat to begin with, this can be used to determine how much the fault blocks have been rotated, and thus the amount of extension. Which extensional system is more efficient at stretching the landscape? Extensional fault system (rift) geometries A modern view of a rift system - asymmetric with detachment Detachment or “decollement” – a subhorizontal zone of weakness along which along which higher order and higher angle upper crustal normal faults merge at depth. Separates “deformed” rocks in upper plate from “undeformed” rocks in in lower plate. Commonly a relatively “weak” layer or horizon. Example 1 – “frictional –viscous” (or “brittle-ductile”) transition – zone where T’s hot enough for rocks to begin to flow plastically. Example 2 – a weak lithology like a shale or evaporite horizon in sedimentary rocks. Detachment faults are large, nearly flat-lying surfaces that can accommodate great amounts of displacement. They often contain arrays of synthetic faults in their hangingwalls that rotate with increased stretching. Multiple generations of normal faults above a detachment. As the earlier normal faults become rotated, they become unfavorably oriented relative to the stress field. As some point, it requires less energy to form new faults, rather than overcome friction on older, flatter faults. In short friction increases as normal stress increases. Yerington –Nevada, a region with rotated upper plate normal faults Rotated normal faults at Yerington Nevada – the first (and brilliant) documentation of rotated normal faults, here in a large copper mine How would you start to restore this cross section? Undo the most recent deformation first. How many generations of faults are shown here? Rotated normal faults at Yerington Nevada – the first documentation of rotated normal faults, here in a large copper mine. How much rotation has occurred on the early set of high angle faults? Can you rotate a normal fault so much it looks like a thrust??? Rotated normal faults at Yerington Nevada –rotated normal faults, Metamorphic Core Complexes: High magnitude extension and the rolling hinge. Extension of earlier thickened crust – Sevier thrust belt (i.e. lots of potential energy Narrow corridors
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