Forces In Mountain Building Chapter 11

Tectonic Structures Tectonics is the study of crustal deformation and structural behavior. Plate Tectonics is the deformation and structural behavior of crustal plates http://www.data.scec.org/larse/images/Figure12.gif

Stress Stress is any force which acts to deform rocks. Compression - a stress that acts to press or squeeze rocks together. Tension - a stress that acts to stretch a rock, or pull a rock apart. Shear - a stress which acts tangential to a plane through a body, causing two contiguous parts to slide past each other

Structural Behavior As a general rule: 1) Rocks tend to have a relatively high compressive strength 2) Rocks tend to have a relatively low tensile and shear strength

Strain When a stress is applied, deformation may occur, depending on the amount of stress Strain is the change in shape or volume of a body as a result of stress; deformation. There are Brittle and Ductile deformation.

Ductile Deformation During ductile deformation rocks bend or flow. Folding or bending of material without breaking. Specifically defined as a rock that is able to sustain, under a given set of conditions, 5-10% deformation before fracturing. Folds can be microscopic in size or kilometers in extent.

Ductile Deformation http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif

Anticlines Folds which arch up

Anticlines http://www.es.ucsc.edu/~es10/classnotes/images/W.R.Olig.bedsoverMeso.d.jpg

Synclines Folds which sink down

Syncline Fold http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif

Folded Mountains http://www.es.ucsc.edu/~es10/classnotes/images/W.R.Olig.bedsoverMeso.d.jpg

Monoclines Folds in which rock layers on both sides of the fold are horizontal but at different levels.

Domes Folds which are equivalant to anticlines, but are comprised of layers which are shaped like an inverted bowl.

http://www. es. ucsc. edu/~es10/classnotes/images/W. R. Olig http://www.es.ucsc.edu/~es10/classnotes/images/W.R.Olig.bedsoverMeso.d.jpg

Basins Folds which are equivalant to synclines, but are comprised of layers which are shaped like a bowl. http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif

Brittle Deformation During brittle deformation rocks break or fracture. Two main styles of fracture: Joints and Faults. Both are the result of relatively rapid stress. For example: modeling clay will break if stress is applied rapidly, but will bend if stress is applied slowly.

http://ic. ucsc. edu/~casey/eart150/Lectures/BrittleDef/CorinthHrstMod http://ic.ucsc.edu/~casey/eart150/Lectures/BrittleDef/CorinthHrstMod.jpg

Joints Joints are fracture surfaces along which there has been no displacement. Joints can form from compressional, tensional and shear stress, and can range in size from microscopic to kilometers in length. Joint sets and jointing has a major influence on landform development. Erosion is able to occur at a faster rate along joints. http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif

http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif

Faults Faults are fractures along which there has been displacement of the material on either side of the fault. Faults are classified based on: 1) the sense of movement (the direction in which the blocks on either side of the fault move) - this is controlled by the type of stress that is applied. 2) the orientation of the fault surface (the angle of the plane of fracture)

Fault Terminology Fault Plane - the plane along which the rock or crustal material has fractured. Hanging Wall Block - the rock material which lies above the fault plane. Footwall Block - the rock material which lies below the fault plane.

Strike-Slip Faults Fault plane is generally vertical. Movement is horizontal due to shear stress. 1) Left-Lateral Strike-Slip - displacement is such that the material on the other side of the fault appears to be displaced to the left. 2) Right-Lateral Strike-Slip - displacement is such that the material on the other side of the fault appears to be displaced to the right.

Strike-Slip Faults

Normal Faults Fault plane is oriented between 30 and 90 degrees (measured from horizontal) Movement has both a horizontal and vertical component. Normal faults result from tensional stress and results in the hanging wall moving down relative to the footwall.

Normal Faults http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif

Reverse Faults Fault plane is oriented between 30 and 90 degrees (measured from horizontal) Movement has both a horizontal and vertical component. Reverse faults result from compressional stress and results in the hanging wall moving up relative to the footwall.

Reverse Faults http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif

Thrust Faults Fault plane is at less than 30 degrees Movement is more horizontal than vertical due to the low angle of the fault plane. Develop due to compressional stress. http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/DetachmentFault.gif

Horsts and Grabens Horsts are up thrown blocks bounded on either side by parallel normal faults. Grabens are downthrown blocks bounded on either side by parallel normal faults.

Measurement of Orientation Strike - compass direction of the outcrop - the line formed by the intersection of a horizontal plane with the structure. Dip - the angle between the horizontal plane and the planar surface being measured. - Dip is always perpendicular to Strike

Fault Map Symbols

References http://facweb.bhc.edu/academics/science/harwoodr/Geog102/study/tecton1.htm