Geology 15 (Faults, Folds, and Joints)

The paragraph introduces the lecture topic of folds, faults and joints in geology. It explains that these are ways in which the earth's crust deforms and changes shape due to plate tectonic forces. An image shows massive folds in rocks to demonstrate the scale and impressiveness of these structures.

This paragraph explains key concepts related to rocks deforming, including: stress, strain, types of stress (confining, differential, compressional, tensional, shear), plastic vs. elastic deformation, time's effect, and how igneous vs. sedimentary rocks tend to deform.

The paragraph elaborates on three modes of continued rock deformation: elastic (returns to shape), brittle (breaks permanently), and ductile (bends/flows). It also covers how crystalline rocks tend to fracture while sedimentary rocks fold more ductily over time.

This paragraph introduces folds as ductile rock structures formed by compression, explaining the differences between anticlines (upfolds) and synclines (downfolds). Terms like plunge, axial plane, symmetrical, asymmetrical, and overturned folds are also covered.

Using an image of impressive chevron folds in Ireland, the parts of a fold outcrop are labeled: angular unconformity separating units, drawing bedding planes to follow structure, marking multiple fold axes showing variety of fold types (overturned anticline, syncline).

The paragraph shows other examples of folds like anticlines and synclines. It also covers related structures - domes (upwarped anticlines) and basins (downwarped synclines) - using the Black Hills dome and Michigan Basin as examples.

This covers monoclines which are step-like folds where strata is largely horizontal. The famous East Kaibab monocline is used as an example, formed by vertical basement faulting while overlaying sedimentary layers ductilely drape over the structure.

A transition is made to brittle rock structures that break instead of fold. Faults and joints are introduced as formed this way. Faults show displacement while joints are fractures without separation along them.

The paragraph covers the anatomy and types of dip-slip faults, including key terms like hanging wall, footwall, and fault scarps. Normal faults (hanging wall down) and reverse faults (hanging wall up) are explained, noting thrust faults are low angle reverse faults.

Strike-slip faults with horizontal motion are introduced, distinguishing left-lateral and right-lateral types. Related features like sag ponds and deflected drainages are noted, using the San Andreas fault as an example. Large strike-slip faults forming transform plate boundaries are also covered.

Further strike-slip related features like shutter ridges, stepovers, releasing and restraining bends forming small basins and pop-up structures are explained. Images of sag ponds and deflected drainages along the San Andreas fault are shown.

The paragraph examines fault zone rocks up close, pointing out fault gouge (pulverized rock in the fault plane) and slickensides (grooves showing fault motion direction). This indicates past earthquake activity.

Joints are introduced as fractures without fault offset. Tectonic joints associated with stresses and folding/faulting are the focus, but hydraulic, exfoliation, unloading and cooling joints are also noted.

Tectonic joints often appear in matching parallel sets, either orthogonally (90 degree) or conjugately (acute angles). Their prevalence and orientation relates to folding and faulting stresses, which is useful for analyzing geologic structures.

Additional joint types are shown like non-systematic, parallel sheeting, columnar basalts, and unloading joints. Mineral-filled joints (veins) and magma-filled joints (dikes) are also explained.