Geological StructuresToday we will consider the structures in rocks produced by deformation.Sideling Hill, I-68, Washington County, MD

Wherever you see sedimentary rocks that are not lying horizontally, these rocks have been deformed in some large-scale process.

It is important to try to imagine the scale of the entire structure to which a single area or outcrop belongs. Deformation Processes

As a result of plate tectonics, the crust is constantly under stress. Rocks respond to stress by deforming. Deformation may be brittle, in which rocks will tend to break, or ductile, in which they tend to flow or bend.

To an extent, brittle behavior characterizes the upper part of the crust, since it is relatively cold. However, if the rate at which a material is stressed is small enough, even rigid materials may deform ductilely. Deformation

Response to StressTemperature has a role in the response of a material to stress, but so does composition. In general rocks with more water in them and which contain more platy minerals (micas, clays) are more prone to ductile deformation. That is why sedimentary rocks in the shallow crust frequently form fold belts, large provinces dominated by folded strata.

Types of StressThere are three principal types of stress (as are demonstrated on large scale by the three plate tectonic boundaries): compressive, tensional and shear.

Compression occurs when material is squeezed, when bodies are pushed straight together.Tension (or extension) is when material is pulled apart. Shear deformation occurs as two bodies slide past one another.

Types of Stress

Traces of Stress in RocksBy measuring objects of know undeformed dimensions, we can estimate the nature and magnitude of deformation.

Traces of Stress in RocksOrientation of slaty cleavage is another tool to estimate the nature and magnitude of stress on a rock.

Rocks that respond brittlely to stress break. Where a rock breaks and no movement takes place is called a fracture or joint.

Fractures in the shallow crust are commonly evidenced by quartz veins, where fluid once flowed and later crystallized.Brittle Deformation

A fracture along which movement takes place is a fault. FaultsWe classify faults based on direction of movement of individual blocks, with reference to a horizontal plane.acute angle = hanging wallobtuse angle = foot wallfault plane

Dip-Slip Faults-- primary movement is vertical

reverse fault: old rocks are brought up in hanging wall

normal fault: old rocks are brought up in foot wallfoot wallfoot wallhanging wallhanging wall

Reverse (Thrust) FaultsReverse faults form as a result of compressional stresses, which dominate in convergent plate tectonic margins.Thrust faults can be crustal-scale, as shown in the lower diagram.Thrust fault: reverse fault with shallowly dipping fault plane.

Normal faulting is a result of tension (or extension). Extension is the dominant stress at divergent plate boundaries.The most classic normal fault valleys are present in the axes of the mid-ocean ridges. Normal Faults

How a fault is manifested on the surface depends on which is greater: the rate of fault movement or the rate of erosion. Faults that have not penetrated to the Earths surface are blind faultsparticularly dangerous from an earthquake hazard standpoint, since they are hard to detect and map out.Surface Expressions of Dip-Slip Faults

Strike-Slip Faults-- primary movement is horizontal, not vertical, meaning no old rocks are brought up or moved down relative to the Earths surfaceStrike-slip faults result from shear stress, like what we see at transform plate margins. The San Andreas system is a big strike-slip fault

Identify the Fault

Identify the Fault

Reverse Faultolder rocks moved up in hanging wall

Identify the Faults

Normal Faultsolder rocks moved down in hanging walls

Ductile DeformationThe most obvious imprint of ductile deformation on rocks are folds. Although rocks may be brittle at the Earths surface, ductile features like folding occur partially because of elevated temperature and pressure at depth in the Earth, partially because the stresses are applied at very small rates: the same rocks that respond brittlely when stress is applied rapidly will tend to deform ductilely when the same stress is applied over a long time. Folds are generally produced by compression, and so characterize convergent plate tectonic situations.

Types of FoldsHow do you know which way is up?

In nature, folded areas commonly contain folds whose axes are not horizontal: they plunge. These make the familiar fold shapes on topographic maps and satellite images.Plunging Folds

The Valley and Ridge Province of the central Pennsylvania Appalachians is a classic example of folding.Plunging Folds

Scales of FoldingFolding can take place on highly variable scales.1 meter1 millimeter

Soft Sediment DeformationNot all deformation has to occur very deep in the Earth. Sediments frequently show familiar fold structures. These come from loading of material on water-rich layers of sediment.

Soft Sediment DeformationHere, layers of basaltic ash were deposited on a shallow lake shore. Later material loaded on top of it caused the wet ash layer to slump and contort. 15,000 yr later this is barely lithified into a sedimentary rock, yet it preserves spectacular folds.

Ductile and Brittle Deformation FeaturesDuctileBrittle

Many deformation structures are composite in nature. Fold belts (compressional) commonly contain numerous reverse faults. Composite Brittle + Ductile Features

A vein such as this may have formed in a fracture, but subsequent ductile deformation caused its wiggly appearance. Brittle + Ductile Features (small scale)

Topographic Reliefp.359

IsostasyThe balance is based on the contrast in density of the materials involved. The crust is lower density than the mantle. Isostasy is the gravitational balance of masses at the Earths surface.

Icebergs and IsostasyWhy did the Titanic sink? Partly to blame is isostasy: sea ice is much more massive below the water line than above. What ripped into the Titanics hull was probably not visible from on deck.

Discovery of IsostasyGeorge Airy determined that high mountains indeed have massive roots as a result of deflection of an engineers plumb line during the first survey of India.verticalverticalWhen surveying close to the Himalayas, the angle between the plumb line and vertical was much greater than surveys farther from the mountain core.

IsostasyThis hypothesis has been further tested using seismology, which reveals thick crustal roots beneath mountain belts.Due to the low density of the crust relative to the mantle, mountain belts will have deep roots. These protrude into the upper mantle and provide an isostatic counterbalance to the mass above the mantle surface.

Epeirogeny: epirojenyEpeirogeny refers to broad flexing motions of the crust: slow up- and down-warping of large areas without particular plate tectonic drive and little deformation.

These slow changes in topography are basically isostatic adjustments: the crust going up or down in response to more or less passive forces from above or below.

Isostatic AdjustmentsSediment is less dense than consolidated rock, but still provides significant mass when removed from one area and deposited in another.

Sediment LoadingThis model is consistent with what we see in the modern Mississippi delta, for example.Sediment erosion and loading will result in epeirogenic uplift and subsidence.

Other forms of Crustal LoadingIce is less dense than rock, but pile enough up on the crust, and it will have an isostatic impact.

Glacial ReboundMost of Scandinavia is experiencing rapid uplift, as a result of the removal of glacial ice over the past dozen millennia.

Crust Goes Up... Crust Goes DownOcean crust cools and sinks as it ages and diverges from the spreading center, causing epeirogenic subsidence. p.499Formation of a divergent plate boundary will cause regional uplift, as will the impinging of a hot mantle plume on the base of the continental crust.

Mountains can result from a variety of different forces and processes. Himalayas or central AppalachiansThere are Mountains and there are MountainsPacific NorthwestSierra Nevada or RockiesBasin and Rangewestern Appalachiansp.495

p.366Negative FeedbackA negative feedback mechanism results in consequences that work against the process.

Why is the erosion rate higher as relief increases?

The Constant Battle Between Tectonic Uplift and Erosionp.370