FOLDS and thrust Whichdeformationand stresses cause theses structures ? Folding of Shale-Sandstone sequence, Kings Canyon, California
The fold is like a Thrust defined by its orientation Parts of a fold: • axial plane • limbs • “hinge” Hinge
Types of Folds • Anticline = arch • Syncline = trough • Monocline = stair step • Dome or Basin Chevron Fold, Laurel Mt., CA
Syncline Youngest rocks in middle syncline, Israel
Anticline Oldest rocks in middle Virgin anticline, southern Utah
Domes & Basins Chernicoff and Whitney Think of an Egg Carton!
Map from Topozone.com Grenville Dome: Sinclair, WY
Syncline- Anticline Pairs + Domes: Zagros Mts, Iran NASA “Earth as Art” web page
Domes are not produced by horizontal but vertical compression Little Shuteye, Sierra Nevada, CA David Rogers Casey Moore, UCSC
Pressure Foliation • pressure flattens and/or aligns minerals in a rock • platy or sheet-like structure reflects the direction in which pressure was applied • Slate, schist, and gneiss foliated 1 mm Microscope Image of Foliated Garnet Schist, VT
Foliation In every case, the foliation is: In the direction of least resistance at right angles to the direction of greatest compression.
How Geologists Use These Clues • Here's an outcrop that might be seen in the field.
Pay attentionit migth look like folds • K = Cretaceous • J = Jurassic • Tr = triassic • At X, we have younger rocks surrounded by older rock
But it is produced by differential errosion on a thrust • Dark green is older rock, thrust over the younger yellow rock • The Klippe (K) • W is a Fenster or Window
Folds and and Thrust are often associated The overturned fold in the upper diagram may break, yielding an overthrust fold or thrust fault
ReverseFault Small thrust fault, Las Vegas, NV, Source: M. Miller, U. of Oregon
Folds and thrust are both responsible for the orogens fold-thrust complex developed in Upper Jurassic limestones in the Haut Giffre area of the Subalpine thrust belt (Morcles nappe in France) R.W.H. Butler
Folded Appalachians Near Harrisburg, PA, Source: NASA
Oil and Gas Concentrate in Domes Chernicoff and Whitney
Small Scale Structures Mimic Large Scale Structures!! Bohlen et al., 1987 Folded Amphibolite in Marble, Warrensburg (K. Hollocher, NYSGA field photo)
Foliation Macroscopic and Microscopic Feldspar (strong) Quartz (weak) Foliated Slate, Shelburne, VT, UVM Mulwaree fault zone, Australia Mulwaree fault zone, Australia, Tullis et al. Foliated Gneiss, Nunavut,S. Tella Photo.
Pressure Cararra MarbleDeformationExperiments Brittle Ductile K. Hamblin A. Kronenberg
Continental Extension Chernicoff and Whitney
Basin and Range Nevada Bureau of Mines and Geology
Extension • Crust Thins Ductile • Faulting Brittle
Shearing • lateral slip creates faults • common at transform boundaries
Faults Hanging Wall • result from brittle deformation • rocks offset across fault • Sides referred to as “hanging wall” and “footwall” • 3 types of fault Footwall
Strike & Dip • Describe fault orientation • Direction of slip determines kind of fault: “dip-slip” or “strike-slip” Chernicoff and Whitney
NormalFault Normal Fault, Lamb Canyon, CA
Strike-SlipFault Strike-slip fault near Las Vegas, NV, Source: M. Miller, U. of Oregon
Joints • Brittle “cracks” in rocks • Form near surface • Regular spatial distribution • No offset
Preferential weathering of joints in Sandstone; Calcite veins in joints of marble, Laurel Mt., CA