Download
rock structure and fault activity n.
Skip this Video
Loading SlideShow in 5 Seconds..
Rock Structure and Fault Activity PowerPoint Presentation
Download Presentation
Rock Structure and Fault Activity

Rock Structure and Fault Activity

352 Vues Download Presentation
Télécharger la présentation

Rock Structure and Fault Activity

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Rock Structure and Fault Activity chapter 9

  2. What is structural geology The study of the forms of the Earth’s crust and the processes which have shaped it • analysis of displacement and changes in shape of rock bodies (strain) • reconstruct stress that produced strain

  3. Structural Deformation Rocks deform when stresses placed upon them exceed the rock strength • Brittle deformation (e.g. fractures) • ductile deformation (e.g. folds)

  4. Driving Forces • Plate tectonics – plate convergence and ridge spreading • Deep burial of sediments • Forceful intrusion of magma into the crust • Meteorite impacts

  5. Evidence of Crustal Deformation • Folding of strata • Faulting of strata • Tilting of strata • Joints and fractures

  6. Evidence of Crustal Deformation • Folding of strata • Faulting of strata • Tilting of strata • Joints and fractures

  7. Evidence of Crustal Deformation • Folding of strata • Faulting of strata • Tilting of strata • Joints and fractures

  8. Evidence of Crustal Deformation • Folding of strata • Faulting of strata • Tilting of strata • Joints and fractures

  9. Applications of structural geology • subsurface exploration for oil and gas • mining exploration • geotechnical investigations • groundwater and environmental site assessment

  10. Geological structures • Geologic bed contacts • Primary sedimentary structures • Primary igneous structures • Secondary structures

  11. Fundamental Structures Three fundamental types of geologic structures: • bed contacts • primary structures - produced during deposition or emplacement of rock body • secondary structures - produced by deformation and other process after rock is emplaced

  12. Bed Contacts Boundaries which separate one rock unit from another • two types: 1. Normal conformable contacts 2. Unconformable contacts (‘unconformities’)

  13. Conformable Bed Contacts Horizontal contact between rock units with no break in deposition or erosional gaps • no significant gaps in geologic time Book Cliffs, central Utah

  14. Unconformable Contacts Erosion surfaces representing a significant break in deposition (and geologic time) • angular unconformity • disconformity • non-conformity

  15. Angular Unconformity Bedding contact which discordantly cuts across older strata • discordance means strata are at an angle to each other • commonly contact is erosion surface

  16. Formation of an angular unconformity

  17. Disconformity Erosional gap between rock units without angular discordance • example: fluvial channel cutting into underlying sequence of horizontally bedded deposits

  18. Nonconformity Sedimentary strata overlying igneous or metamorphic rocks across a sharp contact • example: Precambrian-Paleozoic contact in Ontario represents a erosional hiatus of about 500 ma Grand Canyon, USA

  19. Structural Relations The structural relations between bed contacts are important in determining: 1. presence of tectonic deformation/uplift and; 2. relative ages of rock units • principle of original horizontality • principle of cross-cutting • principle of inclusion

  20. Principle of Original Horizontality Sedimentary rocks are deposited as essentially horizontal layers • exception is cross-bedding (e.g. delta foresets) • dipping sedimentary strata implies tectonic uplift and tilting or folding of strata

  21. Principle of Cross-cutting Igneous intrusions and faults are younger than the rocks that they cross-cut Mafic dike cutting across older sandstones

  22. Cross-cutting Relations Often several cross-cutting relationships are present • how many events in this outcrop?

  23. Principle of Inclusion Fragments of a rock included within a host rock are always older than the host

  24. Fundamental Structures Three fundamental types of structures: • bed contacts • primary structures • secondary structures

  25. Primary Sedimentary Structures Structures acquired during deposition of sedimentary rock unit Stratification- horizontal bedding is most common structure in sedimentary rocks

  26. Primary Sedimentary Structures Cross-bedding - inclined stratification recording migration of sand ripples or dunes

  27. Primary Sedimentary Structures Ripples - undulating bedforms produced by unidirectional or oscillating (wave) currents

  28. Ripple marks

  29. Primary Sedimentary Structures Graded bedding- progressive decrease in grain size upward in bed • indicator of upwards direction in deposit • common feature of turbidites

  30. Primary Sedimentary Structures Mud cracks - cracks produced by dessication of clays/silts during subaerial exposure

  31. Primary Sedimentary Structures Sole marks- erosional grooves and marks formed by scouring of bed by unidirectional flows • good indicators of current flow direction

  32. Primary Sedimentary Structures Fossils – preserved remains of organisms, casts or moulds • good strain indicators • determine strain from change in shape of fossil • relative change in length of lines/angle between lines

  33. Primary Igneous Structures Flow stratification • layering in volcanic rocks produced by emplacement of successive lava sheets • stratification of ash (tephra) layers

  34. Primary Igneous Structures Flow stratification • layering in volcanic rocks produced by emplacement of successive lava sheets • stratification of ash (tephra) layers

  35. Primary Igneous Structures Pillow lavas - record extrusion and quenching of lava on sea floor

  36. Importance of Primary Structures 1. Paleocurrents - determine paleoflow directions 2. Origin – mode of deposition, environments 3. Way-up - useful indicators of the direction of younger beds in stratigraphic sequence 4. Dating - allow relative ages of rocks to be determined based on position, cross-cutting relations and inclusions 5. Strain indicators - deformation of primary structures allows estimates of rock strain

  37. Secondary Structures Secondary structures - deformation structures produced by tectonic forces and other stresses in crust Principle types: • fractures/joints • faults/shear zones • folds • cleavage/foliation/lineation

  38. Fractures and Joints Fractures – surfaces along which rocks have broken and lost cohesion Joints - fractures with little or no displacement parallel to failure surface • indicate brittle deformation of rock

  39. Fractures and Joints

  40. Faults • Faults - fracture surfaces with appreciable displacementof strata • • single fault plane • • fault zone - set of associated shear fractures • • shear zone - zone of ductile shearing

  41. Shear Zones • Shear zone - zone of deformed rocks that are more highlystrained than surrounding rocks • • common in mid- to lower levels of crust • • shear deformation can be brittle or ductile

  42. Fault Terminology Hanging wall block- fault block toward which the faultdips Footwall block - fault block on underside of fault Fault plane – fault surface

  43. Fault Slip Slip is the fault displacement described by: • direction of slip • sense of slip • magnitude of slip

  44. Fault Types Dip-slip faults - slip is parallel to the fault dip direction normal reverse thrust

  45. Fault Types Normal fault - footwall block dispaced up

  46. Fault Types Reverse (thrust) fault - footwall block displaced down

  47. Fault Types Strike-slip – fault slip is horizontal, parallel with strike ofthe fault plane • right-handed (dextral) • left-handed (sinistral)

  48. Fault Types Oblique slip – Combination of dip- and strike-slip motion • dextral-normal • dextral-reverse • sinistral-normal • sinistral-reverse

  49. Faults What type of faults are shown here?

  50. Faults What type of faults are shown here?