GLG Physical Geology Bob Leighty Geologic Structures

These lecture notes are very similar to the ones I use in my traditional classes. You’ll find they are loaded with imagery and streamlined text that highlight the most essential terms and concepts. The notes provide a framework for learning and, by themselves, are not meant to be a comprehensive source of information. To take advantage of the global knowledge base known as the Internet, I have included numerous hyperlinks to external web sites (like the Wikipedia, USGS, NASA, etc.). Follow the links and scan them for relevant info. The information from linked web sites is meant to supplement and reinforce the lecture notes – you won’t be responsible for knowing everything contained in them. As a distance learning student, you need to explore and understand the content more independently than in a traditional class. As always, I will help guide you through this learning adventure. Remember, Dr. Bob if you have any questions about today’s lecture Leave no questions behind! Explore and have fun! These notes and web links are your primary “lecture” content in this class. Additionally, various articles are assigned each week to supplement this “lecture” information. I believe you’ll have enough information to reference without having to purchase a costly textbook.

 If rocks are subjected to stress, they may be deformed (& show strain)stressstrain undeformed rocks deformed rocks (strain) (stress) compression Stress & Strain Structural Geology

Types of Stress Stress & Strain Structural Geology

higher T (deeper)lower T (shallow) stronger rocksweaker rocks BrittleDuctile (faulting)(folding) fast strain rateslow strain rate Types of Strain Stress & Strain Structural Geology

 Form under brittle conditions by any stress  Fractures with differential offset (vertical and/or horizontal) FW How to ID a fault with vertical offset HW 1)Find matching features on either side of the fault Hangingwall (HW): rocks above the fault plane Footwall (FW): rocks below the fault plane 2) Which way does the HW move?Faults Structural Geology

 Normal faults – Tensional stress causes HW  HW  Reverse faults – Compressional stress causes HW  HW  Transform faults – Shear stress causes horizontal movementFaults Structural Geology

Faults Unnamed fault on AZ87 (a high-angle normal fault) HW FW South Mountain detachment faultdetachment fault (a low-angle normal fault)

 Thrust faults = low-angle (dip <45º) reverse faults Thrust faults HW FWFaults Structural Geology

 The San Andreas transform fault (mostly horizontal movement)San Andreas transform faultFaults Structural Geology

 Joint: a fracture where no offset has occurred JointJoints Structural Geology

 Form under ductile conditions by compressional stressesFolds Structural Geology

 Axial plane - splits the fold into two limbsFolds Structural Geology

Folds  Strike & dip – describes orientation of bedding Strike & dip

dip measuring strike & dip with a compass strike dip strikeFolds Structural Geology

 limbs dip in toward axial plane  youngest beds in center of the fold  “point” downSynclinesFolds Structural Geology

 “point” up  limbs dip away from axial plane  oldest beds in center of the foldAnticlinesFolds Structural Geology

 Folds “plunge” when the fold axis is not horizontal  Form curving outcrop patterns Plunging Folds Folds Structural Geology

 A one-limbed fold with horizontal beds on the top & bottomMonoclinesFolds Structural Geology

 Dome – A circular upwarp (a circular anticline) Dome Black Hills, SD Structural Domes Folds Structural Geology

 Basin – A circular downwarp (a circular syncline) Basin Michigan Structural Basins Folds Structural Geology

WWW Links in this Lecture > Stress - > Strain - > Faults - > Dip-slip fault - > Strike-slip fault - > Detachment fault - > Thrust fault - > San Andreas transform fault - > Joint - > Folds - > Strike and dip - > Syncline - > Anticline - > Dome - > Basin - Structural Geology