Presentation on theme: "Announcements Field trip to Tanque Verde this Saturday"— Presentation transcript:
1Announcements Field trip to Tanque Verde this Saturday Announcements Field trip to Tanque Verde this Saturday! 8:00 AM departure; loading dock
2In brittle regime: joints, tensile fractures, shear fractures (faults In brittle regime: joints, tensile fractures, shear fractures (faults!), pressure solution (cleavage development)- deformation mechanisms depend on pressure! What about deformation in the deeper crust?
3Today: Foliations, stretching lineations, and tectonites- deformation in the deeper crust (D&R, pp ; )
4"General" definition: Foliation: Penetrative (at outcrop and microscopic scale) and parallel planar fabric elements in a rock. "Structural geologist's" definition: Planar fabric is secondary and due to mineral recrystallization and/or plastic behavior during deformation at elevated temperatures
13Mylonitic foliation: Forms due to grain-size reduction by a mix of brittle and plastic deformation in shear zonesbrittle deformation of feldspar porphyroclastsplastic deformation of quartz "ribbons" and mica
14Coarse-grained mylonitic augen gneiss Coarse-grained mylonitic augen gneiss. The large porphyroclasts are called augen ("eyes")
15A strongly mylonitized rock- note the extremely fine grain size due to "pulverization" during shearing
16Lineation: penetrative linear fabric Lineation: penetrative linear fabric. We will focus on those that are related to deformation.How does it differ from other linear structures we have talked about, like slickenlines on a fault surface?Types of lineations:1) Intersection2) Crenulation3) Mineral4) Stretching
17Intersection lineation: Intersection of two planar features- an "apparent" lineation in that there is no fabric that is linear.e.g., intersection between cleavage and planar surface
18Crenulation lineation: Intersection between fold hinges and foliation
19Mineral lineation: preferred alignment of minerals due to deformation and/or recrystallization during deformation
20Stretching lineation: elongation of minerals due to "stretching" deformation
22Lineation defined by stretched pebbles in a conglomerate
23Tectonites: Rocks that are pervaded by foliation and/or lineation- flowed in solid state S: Schistosity (foliation) only due to flattening- no lineationL: Lineation only, due to unidirectional stretching/ constrictionLS: Foliation and Lineation, related to noncoaxial strain- shearing
25What kind of tectonite is this? Coaxial (pure shear) or non-coaxial strain?L-tectoniteCoaxialWhat is it?
26What kind of tectonite is this? Coaxial vs. non-coaxial?L-S tectonitenon-coaxial
27S-C fabrics- occur in L-S tectonites and serve as excellent sense-of-shear indicators S-Surfaces- planes of schistosity/foliation (flattening)C-Surfaces- planes of maximum shear "shear bands"C comes from cisaillement, French for shear
28When studying S-C fabrics- must look perpendicular to lineation!