Structural mini-atlas for Physical Geology lecture.


This is a photo of bedding plane of slightly dipping Cretaceous limestones in stream bed for Ernst Tinaja in Big Bend National Park in Texas. Note the very regular array of fractures within the rock. These are known as joints, and are a brittle structure. Here they represent a very small bit of extension perpendicular to the joint plane, and they may be related to the tilting of the rock. Joints are important for many reasons including as reservoirs and conduits for fluids moving through rock bodies.


This is a photo of a small stream bed wall for Ernst Tinaha, and displays some folds in the Cretaceous limestones and red shales. Note that the layer thickness stays more or less the same, and that the geometry of the folds changes in a regular way. Note also that some layers are folded more than others, necessitating slip along some of the layering. This is a style of folding known as flexural slip, and it occurs at relatively shallow levels in the crust, above the ductile-brittle transition. The orientation of the folds tells the geologists something about the directions of shortening and patterns of deformation, that in turn can be related to plate motions.


This is a fold of some Dalradian impure marbles near Glencolumbkille, Ireland. Note that the folds are a bit more 'fluid' in style, as evidence changes in layer thickness. These folds formed at mid crustal levels during metamorphism. Note also the white vein of quartz, which represents a fracture that opened and then filled with quartz that grew out of hydrothermal fluids. Fluids can aid brittle behavior. Careful inspection shows that near the top the vein has been offset by slip along the layering. We then have evidence of multiple phases of deformation - fold formation, then vein formation, and then offset of the vein. Such complex histories are typically of rocks in the cores of mountain belts which have undergone tens of millions of years of deformation within an evolving plate boundary.


These are some more folds in the same marbles as picture above. Note the coin for scale. A careful look shows changes in layer thickness, loss of layer continuity, and refolded folds, all indicative of the complex folding history. So these layers have been folded into at least two if not three different patterns in addition to the history outlined above. Note how the different layers behave differently. Marble has the viscosity of about butter under metamorphic conditions.


This is a specimen from the Bruce limestone unit above Lake Huron in Canada. While called a limestone it is actually dark, black chert layers interlayered with the light colored rock that was limestone, but is now more marble like. Since the chert is relatively unchanged this rock has only been subjected to very low grades of metamorphism. Most interesting here is the very different ways the two different materials have deformed under the same conditions. The chert layer has been broken up by a series of discrete slip surfaces in a brittle fashion, but the marble has just flowed around it. One might imagine chocolate tablets deforming in a butter matrix. In this case we refer to the chert as competent and the marble as incompetent. Such behavior might be expected near the brittle-ductile transition in the earth's crust at the time of deformation.


This is an outcrop in a quarry near Morton, Minnesota of Archean migmatites. The light colored layers are granitic in composition (coarse feldspar and quartz). The dark colored layers are amphibolites and more mafic gneiss. Note the chaotic character of the layering. The geometry might be somewhat akin to the swirls in a multi-hued cake batter. At the time of deformation temperatures were elevated and some of granitic material was still molten, creating these flow folds. Such migmatites are only expected in the lower part of thickened continental crust. Crust with such roots will be fairly weak, and indeed it is these types of rocks that may very well control the maximum height to which mountains can rise.

 

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