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Rock Deformation and Geologic Structures Rock Deformation and Geologic Structures • How do we measure rock deformation? • What causes rocks to deform? • How do geologic structures relate to stress, • • • • strain, and rock strength? How does rock strength vary in the lithosphere? What is the connection between earthquakes and rock deformation? How do we detect and measure earthquakes? How are earthquakes destructive? Why study rock deformation? • Earthquakes are a powerful and violent force and cause great destruction. • Geologic structures can act as traps for economically important materials, e.g. crude oil. • Magmas often rise to the surface along structures such as faults. • Geologic structures are a record of rock deformation – which is caused by plate tectonic forces. How do we measure rock deformation? • Describing the spatial orientation of rocks with tilted or dipping beds: • Strike: The compass direction of a line formed by the intersection of a horizontal plane (at the surface) with the dipping (inclined) bed. • Dip: The angle between the surface horizontal plane and the inclined rock layer. How do we measure rock deformation? Example of determination of strike and dip. Strike and dip symbols are used on geologic maps to indicate orientation of rocks. How do we measure rock deformation? • Describing the spatial orientation of rocks that are folded: • • • • Limbs: The two dipping sides of a fold. Axial plane: Plane separating the two dipping sides. Hinge line: Line where the dipping limbs join. Plunging fold: When the hinge line plunges downward into the ground (is not horizontal). • Anticline: A folded structure that arches upward in the center (limbs dip away from axial plane). • Syncline: A folded structure that arches down in the center (limbs dip in towards axial plane). How do we measure rock deformation? Folding may occur over very different scales: folds can be very large or extremely small. How do we measure rock deformation? How do we measure rock deformation? Anticlines and synclines can be either horizontal or plunging. At the center of a fold is the Hinge Line and the Axial Plane. The sides of a fold are called Limbs. Hinge line How do we measure rock deformation? How do we measure rock deformation? How do we measure rock deformation? Anticlines: Layered rocks that have been folded into arches. Thus, after erosion the oldest rocks are at the center. Synclines: Layered rocks that have been folded into troughs. Thus, after erosion the youngest rocks are at the center. They often occur next to each other and repeat over distances. Strike and Dip Measurements Indicate Structure At Depth How do we measure rock deformation? A plunging anticline closes up in the direction of plunge. A plunging syncline opens out in the direction of plunge. How do we measure rock deformation? Anticlines and synclines which are plunging to the left (northwest in photo). How do we measure rock deformation? Structural domes and basins may also form. In a dome the strikes form a circular pattern and dips are away from the center. The oldest rocks are at the center. In a basin the strikes form a circular pattern and dips are towards the center. The youngest rocks are at the center. Aerial photo of a structural dome. Rocks dip away from the center. The Michigan Basin - Youngest Rocks At Center How do we measure rock deformation? • Faults – Planar breaks through rocks along which movement occurs. • Footwall: The rock below the fault plane. • Hanging wall: The rock above the fault plane. • Dip-slip: Movement on a fault that is primarily vertical displacement along the dip surface. These can be either normal or reverse. • Strike-slip: motion along a fault that results in horizontal rock displacement. • Oblique-slip fault: A combination of dip- and strikeslip displacements. How do we measure rock deformation? Footwall and hanging wall terminology comes from mining. It refers to the rock under a miners feet, and the wall above the miner (where you might hang your lamp). How do we measure rock deformation? A normal fault (dip-slip) - the hanging wall has moved down relative to the footwall. Produced by tensional tectonic forces. How do we measure rock deformation? A reverse fault (dip-slip) - the hanging wall has moved up relative to the foot wall. Produced by compressional tectonic forces. How do we measure rock deformation? A thrust fault is a low-angle (<45o) reverse fault. How do we measure rock deformation? Strike-slip faults and Oblique-slip faults What causes rocks to deform? • Faulting and folding occur at plate boundaries. • Compressive force: reverse faults and folding. (convergent boundaries) • Extensional force: normal faults. (divergent boundaries) • Strike-slip force (shear stress): strike-slip faults. (transform boundaries) What causes rocks to deform? • Joints: Fractures created by tension or changes in pressure in brittle rocks. • No movement has occurred. • Can form in two ways 1) Form by decompression as rock is brought to the surface, the rock expands. 2) Form by cooling, e.g. a lava flow, the rock contracts. Columnar Joints in Basalt Contraction during cooling. Joints Developed in Granite Expansion during decompression. Exfoliation Dome: A type of jointing developed in granites due to uplift and decompression of the rock. Geologic Structures And Economic Deposits Oil and gas traps: Oil, gas, and groundwater all move through pores and all have different densities that cause them to stratify. If an impermeable layer of rock stops the upward migration a crude oil and gas deposit may form. Geologic Structures And Economic Deposits The crests of anticlines or domes may accumulate crude oil and gas deposits in this way. Faults may also act as traps. Geologic Structures And Economic Deposits Hot fluids migrate along faults and deposit ore minerals. Miners often follow fault zones to recover these ores. What causes rocks to deform? • Rocks strain when stress exceeds the yield strength. • Stress - the force applied to rocks • Strain - the deformation resulting from stress • (a change in size and shape) • Three types of stress and strain. • Compressive stress – results in shortening strain • - convergent plate boundaries • Tensional stress – results in elongation strain • - divergent plate boundaries • Shear stress – results in shear strain • - transform plate boundaries - What causes rocks to deform? What causes rocks to deform? Faulting, folding, and plastic flow are all types of strain. What controls whether a rock is brittle or ductile? • Confining Pressure • Near the surface pressure is low, rocks are more brittle. • Deeper in Earth pressure is higher, rocks are more ductile. • Temperature • At cooler temperatures, rocks are more brittle. • At higher temperatures, rocks are more ductile. • Type of rock • Some rocks are naturally more brittle or ductile than others. • High strength = brittle deformation, low strength = ductile deformation. • e.g., Shale and halite are generally ductile, granite and quartzite are generally brittle in their response to stress. • Rate of deformation (i.e. applied stress) • Very slow rates of deformation (e.g. plate tectonic motions) produce ductile behavior. • Fast rates of deformation (e.g. during an earthquake) produce brittle behavior. What controls whether a rock is brittle or ductile? Ductile and Brittle Strain Diagram As stress is applied 3 different responses: Elastic strain – reversible, rock can return to original shape. Plastic strain – rock flows, strain rate increases faster as stress goes up, irreversible. Brittle strain – rock breaks suddenly as stress increases, irreversible. Simple Illustration of Elastic and Brittle Strain What controls whether a rock is brittle or ductile? Experiments show that confining pressure affects whether brittle or ductile deformation will occur. Summary of “Rock Mechanics” • Some rocks are strong and others are not. • Thus, some are prone to brittle fracture while others deform plastically – at the same stress level. • With increasing depth, temperature and confining pressure brittle fracture decreases and plastic flow increases for all rock types. • So, faults are expected at shallow depths, whereas plastic flow should occur at depth. What controls whether a rock is brittle or ductile? Horst – uplifted block between two normal faults. Graben – downdropped block between two normal faults. What controls whether a rock is brittle or ductile?