Plummer Chapter 15: Geologic Structures PDF

Plummer Chapter 15: Strike and Dip Geologic Structures measurements of orientation of rock...

Plummer Chapter 15: Strike and Dip Geologic Structures measurements of orientation of rock layers can be v. helpful in recognizing rocks can be folded, faulted, + understanding nature of deformed folds: bends in rock strata units caused by compression strike: direction of intersection of a rock faults: ~planar surfaces of layer w/ a horizontal surface fracture in rock bodies; dip: angle at which a bed inclines from the caused by brittle failure horizontal; is measured at right angles + involving relative to strike displacement see Plummer Figs. 15.6 to 15.9 UMN Maps, Cross Sections, + Block Diagrams Deformation of Rocks 3 categories of forces that deform rock geological map: describes in “plan view” the nature + distribution of surface + near-surface bedrock 1) Compressive forces: squeeze + shorten a and/or unconsolidated materials; may provide info on body (brittle outcome: reverse faulting; unit ages, lithologies, orientations, etc ductile outcome: folding) geological cross section: diagram depicting “side 2) Tensional forces: stretch a body + can pull it view” of geological units, as if vertical slice had been made through part of crust, exposing associated units apart (brittle outcome: normal faulting; ductile outcome: crustal thinning) geological block diagram: 3d view of block of geological materials, depicting various map + cross- 3) Shearing forces: push two sides of a body in section views; useful for visualizing rock volumes opposite directions (brittle outcome: see Plummer Figs.15.9 + 15.12 + Box 15.1 strike-slip faulting; ductile outcome: shear folding) See also Plummer Figs.15.2 + 15.3 rocks bend or break depending on factors such as: 1) rock strength 2) local T’s 3) confining P’s W.H. Freeman at higher T’s + confining P’s, deformation more likely to be slow + steady (ductile response) compressive forces dominate at convergent boundaries tensional forces dominate at divergent boundaries at lower T’s + confining P’s, fracturing more shearing forces dominate at transform boundaries likely (brittle response) 1  in general, most igneous brittle outcomes: little change until sudden rocks stronger than most breaks (like glass) sedimentary rocks (but exceptions exist) ductile outcomes: smooth + continuous plastic deformation (like modeling clay) most “basement rocks” (older underlying ig or met diff rocks behave differently under different rocks) respond to forces in a more brittle manner than conditions: e.g., marble brittle if shallow, the younger seds that may but ductile at greater depths cover them nevertheless, at scale of crust, v. deep rocks respond to forces in a less S. Nelson brittle manner than surface rocks, due to higher T + P Fractures in Rocks: Joints and Faults Joints 2 kinds of fractures in rocks: joints + faults joints are fractures along which no appreciable relative movement has taken place Joints: fractures along which there has been no appreciable movement see Plummer Figs. 15.18 + 15.19 Faults: fractures w/ relative movement of joints very common in bedrock rocks on both sides, parallel to fracture joints caused by wide range of tectonic forces at local or regional scales joints also caused by nontectonic expansion / contraction of rocks (e.g., removal of J.B. Bennington confining pressure on underlying units; W.H. Freeman contraction of lavas during cooling) Faults Fault plane: surface along which formation faults are fractures along which appreciable fractures + slips relative movement has taken place Orientation of fault plane defined by strike + dip see Plummer Figs. 15.20 to 15.28 Dip-slip fault: relative movement of rocks is up or down dip of fault plane (suggests either caused by brittle responses to compressive, compression or tension) tensional, + shearing forces “Normal fault” is dip-slip fault in which hanging wall faults are common features of e.g. mtn belts, block moves down relative to foot wall block; rift valleys, + areas of horiz displacements formed as brittle response to extension betw plates (transform faults) “Reverse fault” is dip-slip fault in which hanging wall block moves up relative to foot wall block; W.H. Freeman formed as brittle response to compression 2  strike-slip fault: relative movement horizontal; Oblique-slip fault: movement is along strike + parallel to strike of fault plane; a product of also up or down dip (a product of shear + shearing; left-lateral + right-lateral types compression / tension) strike-slip fault at plate boundary is essentially a transform fault A. Makarow S. Nelson See also Plummer Fig.15.21 See also Plummer Fig.15.21 Thrust fault: a reverse fault in which dip of Overthrusts: thust faults in which one block fault is small (