Assignments

• Geomorphology From Space: Tectonic Landforms (optional)
• Earth Revealed. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view #7 Plate Dynamics,  and #8 Mountain Building

Explore

• This Dynamic Earth: the Story of Plate Tectonics, Kious and Tilling, U.S. Department of the Interior, U.S. Geological Survey, URL: http://pubs.usgs.gov/gip/dyna
• Provinces of the United States (USGS NPS)
• USGS Tapestry of Time and Terrain
• Geologic History of Arizona (Northern AZ Univ)
• The Geology of Virgina (The College of William and Mary) Excellent, well-illustrated summary of the Appalachain Mts.

Terms: geoid, hypsometry, oceanic and continental crust, shield, platform, orogen, subduction zone, divergent boundary, convergent boundary, transform boundary, spreading center, continental margin, shelf, slope, rise, abyssal plain, active and passive margins, mid-oceanic ridge, rift valley, triple junction, forearc basin, volcanic arc, isostasy, Pratt and Airy Models, flexural isostacy, eustatic and isostatic sea level changes, orogen, hinterland, foreland basin, foreland fold and thrust belt, normal fault, thrust fault, aulacogen, craton, platform, shield, physiographic province

Tectonic Map

Review of the Earth's Structure

Significance of Plate Tectonics

There is a good reason why Plate Tectonics is the most important unifying theory in geology. In addition to determining orogenic processes, the assembly of continents, the formation and destruction of ocean basins, and the distribution of most earthquakes and volcanoes, Plate Tectonics also strongly influences:

• global climate by affecting
  1. Ocean and atmospheric circulation
  2. Degree of continentality
  3. rates of weathering related to uplift - (weathering removes CO₂ and promotes cooling)
• tidal range, and wave energy reaching the coast - influenced by the width of the continental shelf
• the size, orientation, and gradients of large drainage basins (e.g. South America)
• large scale transgressive and regressive phases, which are determined by rates of sea-floor spreading. Regression is associated with slower  rates of spreading.
• glaciation by positioning continents at high latitudes

Gravity and isostasy

Isostasy is  the theory that the lithosphere floats on the denser asthenosphere, and that surface elevations are largely a function of  variations in crustal density and thickness.  Isostacy accounts for elevation differences between continents and ocean basins and between the lower order features they contain.

• Pratt Theory: Crustal density block model
• Airy Theory: Crustal thickness block model
• Composite model: Elevation is related to both crustal thickness and density
• flexural isostasy: Crust behaves like a flexible sheet that warps in response to changes in stress. (e.g. peripheral bulge, foreland basin, glacial rebound, etc)

Isostatic anomalies resulting in uplift or subsidence are caused by loading or unloading (e.g. glacial expansion and retreat, or erosion and deposition), sub-lithospheric flow, and crustal thickening through compression or underplating, and thinning through extension. Rather than acting like a series of blocks that bob up and down the crust behaves more like a semi-rigid sheet that warps in response to changes in stress, this is know as flexural isostasy.

Animation Experiment with isostasy by changing the block height and density, and the liquid density. From Discover our Earth, Cornell University. URL: http://atlas.geo.cornell.edu/education/student/isostasy.html

Why Topography. From the Dynamic Earth website at the University of Leeds, URL: http://earth.leeds.ac.uk/dynamicearth/topo/index.htm simple explanation of the Pratt and Airy theories

Global (Eustatic) and Local (Isostatic) Sea Level Fluctuations

Erosion and deposition are intricately linked to base level, which is ultimately controlled by sea level.  Decreasing sea level initiates erosion whereas rising sea level drives deposition. 

Eustatic, or world-wide changes in sea level will occur if

1. there is a fluctuation in the volume of water in the ocean, or
2. there is a change in the size of the oceanic basin

Volume changes are climate driven, whereas basin size is driven by rates of plate motion and isostasy.   The most important climatic event influencing sea level is glaciation. Less water is held in the oceans when continental glaciers are at their maximum.  As glaciers melt sea level rises.   Eustatic rises in sea level are also linked to rapid rates of sea floor spreading and subduction, leading to more buoyant crust on the ocean floor and shallower basins.  On the other hand, slower rates of crustal turnover leads to a higher percentage of older, denser oceanic crust and deeper basins.

Isostatic sea level changes are local relative fluctuations related to some isostatic adjustment of the land.  For example glacial rebound may cause relative sea level to drop even though eustatic sea level is rising.

Review of plate tectonics

Classification of plate boundaries:  A boundary is classified by its relative plate motion and the type of crust in contact across it.

• Stresses are generally not purely tensional, compressional, or shear (e.g. transtensional and transpressional -- both are associated with the San Andreas fault)

Driving mechanisms of plate motion

• slab pull - most important
• ridge push
• gravity sliding 

Plate Tectonics reviews on the Internet

• The Dynamic Earth USGS
• Plate Tectonics: USGS National Parks Service
• The Wilson Cycle: Lynn S. Fichte, James Madison University, Harrisonburg

Convergent plate margins

Oceanic-Oceanic Convergence: (Intra-oceanic arc) When two oceanic plates converge the older, denser plate will typically subduct.

Morphological elements of intra-oceanic arcs 

• Backarc zone (backarc basin)
  • Formation of backarc basins
    • entrapment: oceanic crust behind the arc is old
    • backarc spreading: spreading is initiated by upwelling along a secondary convection cell (e.g. Sea of Japan) 
• Volcanic arc zone: composed of active region and frontal anticline
  • Andesitic volcanism predominates unless arc basement is continental
  • volcanic zone is located 80 to 150 km above and 75-175 km landward of the Wadati-Benioff zone. Distance is a function of subduction angle (30-50°)
  • volcanism results partial melting of overlying mantle
•  Forearc zone:
  • frontal arc: older uplifted accretionary prism rocks
  • forearc basin (flysch--turbidites)
  • accretionary prism (outer-arc ridge): imbricate thrust packages
  • trench: the thickness of turbidite fill (flysch) is dependent on proximity to sediment source and sediment entrapment by neighboring basins
  • peripheral bulge or outer swell (linear upwarp caused by an upward flexure of the crust fronting the trench)

 Oceanic-Continental Convergence

Oceanic crust invariably subducts beneath continental crust which is too buoyant to sink. Thickening along the orogenic belt is related to shortening by plastic flow, underplating of magma, and thrust faulting.

Figure 4. Morphological elements of a continental - oceanic orogenic belt.

Continental margin orogen: Cordilleran-Type orogen 

• Morphological elements
  • Backarc zone
    • Foreland basin: epicontinental basin (Great Plains and Appalachian Plateau)
    • Foreland fold-thrust belt (e.g. GFS-Appalachian Blue Ridge and Valley and Ridge, GFS-Rockies)
  • Hinterland: central uplifted portion of the orogen
  • Volcanic arc zone: dominated by felsic to andesitic magmatism (Cascades Range, Sierra Nevada, etc)
  • Forearc zone: forearc basin (longitudinal valley), accretionary prism (Pacific coastal ranges), trench, and peripheral bulge
•  EXAMPLES: Pacific Northwest (Juan De Fuca--North American); West Coast of Mexico (Cocos--NA); West Coast of SA (Nazca--NA); Sunda Arc (Indian-Australian--Eurasian); Ancient margin of western CA (Farallon--NA; Pacific Coastal Range, San Joaquin-Sacramento Valleys, Sierra Nevadas)

One continental block underthrusts beneath the other; generally governed by the polarity of earlier subduction

Note: emplacement of ophiolites, melange; controversy concerning their significance 

Morphological elements

• Peripheral foreland basin (mollasse deposits)--developed on margin of underthrusting plate parallel to suture
• Suture zone (region of extensive thrust faulting)
• Volcanic arc (characterized by emplacement of granitic plutons; Little surface volcanism)
• Retroarc foreland basin (mollasse deposits) Basin landward of the foreland, or backarc thrust belt formed by flexural subsidence.
  • Structures: recumbent folds, nappes, decollement
  • Discussion: thin-skin vs thick-skin tectonics

Examples (Mediterranean orogenic system; African--Eurasian Plate; Eastern Sunda Arc, Indian-Australian--Eurasian; Zagros Mts. African--Arabian Plate; Himalayas and Tibetan Plateau, Indian-Australian Plate--Eurasian Plate. Ancient orogenic belts: Appalachians; Urals)

 2. Continental - island arc collision (similar to c-c convergence)

Examples: NW Australia-Indonesia (Timor), Ancient systems: Taconic orogen and portions of the Western Cordilleran 

Transform Boundaries and related features

These features will be cover in depth during the structures lecture.

• General features associated with transform (strike slip) boundaries
  • linear fault valley, laterally offset topography, sag ponds, fault scarps, shutter ridges, etc.
• Additional features related to oblique-Slip
  • Transtension
    • Features: pull-apart basin
  • Transpression
    • Features: Transverse orogen

Divergent Plate Boundaries (Continental)

• Rift Valley: Tectonic valley bound on one or both sides by normal faults.  A sea or ocean basin will form with further rifting and the initiation of sea-floor spreaded. An aulacogen is  failed arm of a rift system.

Explore

1.  Regional Paleogeographic Views of Earth History, Ron Blakey- paleogeographic and plate-tectonic reconstructions illustrating broad patterns of Phanerozoic Earth history.
2. Geological History of the Central Mediterranean: Outline of Cenozoic Events, Amanda Kolker, Oberlin College
3. Pacific Hemisphere Plate Tectonic History by Tanya Atwater (Quicktime Movie)

The continental craton

The craton is the Precambrian core of the continent.  It is largely stable except in areas were it is being torn by divergence or compressed by periperal collisions.  The craton of North America was assembled through a number of accetionary event ranging in age from ~4 ga to 1.2 ga.  Then contiental shield is that portion of the craton where erosion and isostatic uplift have striped away all cover rocks.  The continental platform maintains a sedimentary cover that lies unconformably over the ancient core (fig 5).

Figure 5.  Local structure of the continental craton exposed in the Grand Canyon, AZ.  (Click to enlarge.)  The dark Vishnu Schist exposed  along the inner canyon wall forms the local   Precambrian basement, part of the Yavapai system of volcanic arcs added to the craton around 1.7 ga.  Draped over the schists are Paleozoic sandstones, limestones, and shales deposited in a passive margin cratonic setting nearly a billion years later.  Incision of the canyon by the Colorado River is driven by recent uplift.

Framework of the Physiographic (Geomorphic) Provinces of the United States

The United States can be divided in to several provinces characterized by distinct elements of structure, lithology and topography that reflect a shared geologic history. In some cases, the Basin and Range for example, morphology reflects active tectonic processes.  In other cases morphology reflects events long inactive but leaving a cohesive imprint on the landscape.

The Craton   The craton is the relatively flat stable core of the continent that has not been subjected to orogenic activity for over a billion years. The Superior Uplands lies in the exposed Precambrian core of the craton know as the continental shield. The shield extends beneath a veneer of sandstones, limestones and shales laid down by epicontinental seas that periodically flooded the continental platform.   The sedimentary cover thickens towards the margins and is greatest in the plateaus that sit snuggly against Phanerozoic orogen belts.

Figure 6. Geomorphic (Physiographic) provinces of the United States.  Rollover the image to see the unlabeled provinces.  Can you visualize the distinct topography of each region?

  The Great Plains, Ozarks, Interior Low Plateaus and the Appalachian Plateau are all composed of sediment shed from adjacent orogenic belts.  The plateaus contain horizontal to gently dipping strata.  Karst is well developed in areas underlain by thick carbonate deposits, such as Mammoth Caves in the Interior Low Plateaus.

Peripheral Orogenic Belts (Mobile Belts)

The Western Cordillera and the Appalachian Mountains comprise the deformed margins of the continent.  The Southern Appalachian provinces (Piedmont, Blue Ridge, and Valley and Ridge) are Paleozoic in age, assembled by the island arc and continental collisions that created Pangea. The New England province (Northern Appalachians)is a mangled extension of the Southern Appalachians. The parallel belts are compressed into a single mess unraveled by geochemistry and radiometric dating, not by topography. The Ouachitas are a southwestward extension of the Appalachians, separated by Mesozoic rifting, subsidence, and the formation of the Mississippi embayment.

The western Cordillera have been bobbing up and down for the last 200 million years.   They built westward as bits and pieces of arcs, sediment, and continental fragments were swept off subducting oceanic crust and accreted to the western margin.  Although no longer along an active convergent margin California's coastal range, valley, and Sierra Nevada inherented their morphology from recent subduction. The subduction continues today beneath the Pacific Northwest forming the volcanically active Cascade Province.  The Pacific Border Province includes both the inactive (California) and active (Oregon and Washington) accretionary prism and forearc. The foreland thrust belt is located nearly 1000 km to the east in the Rocky Mountain Province.  The Rocky Mountains were raised in the Late Cretaceous Period, eroded and raised again in the Eocene and Tertiary periods, and again eroded. 

Between the Rocky Mountains and the Pacific Coastal Range lies the tectonically active Basin and Range Province.  This province is being uplifted and stretched.  Thinning crust is breaking along normal faults forming an array of tilted blocks spread out like overlapping cards. The edges form the ranges, the flats the basins. Deformation of the Basin and Range is spreading westward and eastward.  Although not included in the Basin and Range Province the Modern Sierra Nevada Range is the western most fault block, uplifted within the last 3 million years.  Even the Rocky Mountains owe their present elevation to renewed uplift in the last 5 million years;  not related to its foreland setting but to the deformation driving the Basin and Range.  The unique topography and arid climate of the Basin and Range Province create interior drainage systems characterized by alluvial fans, salt pans, playas and sediment filled basins.

Colorado Plateau

The Colorado Plateau is an elevated relatively undeformed welt surrounded by the Basin and Range and the Rocky Mountains.  Why the region has not been rent apart like the surrounding provinces is no doubt linked to the strength of it's basement.  The arid plateau is cut by canyons, dissected monoclines and exhibits an array of features formed by fluvial dissection.

Passive Margin - Atlantic Coastal Plain

During the Jurassic, approximately 150 million years ago Pangea split apart.  The Appalachian were wrenched from the European Caldonides and the eastern margin of North America splintered and sank against the newly forming Atlantic ocean.  Sediment shed eastward burying the fragmented margin to form the modern Atlantic Coastal Plain. Failed rift basins from this period create the seaward troughs occupied by the Connecticut, Hudson and Mississippi Rivers.

Volcanic Provinces

Active volcanic regions of the conterminous United States include the Cascade Range, the previously discuss volcanic arc, the Snake River Plain and the Yellowstone Plateau.  The latter two regions are part of a two-stage volcanic sequence related to the westward movement of the North American Plate over a shallow, enigmatic mantle hotspot.   The hotspot first generates a felsic caldera phase characterized by violent pyroclastic activity.  As the plate moves westward the cooled subcaldera pluton is penetrated by mantle-derived magma that covers the caldera with flood basalts. Like many calderas before it, Yellowstone will likewise be buried as a new caldera forms to the east.  The Columbia River Plateau is one of the world's largest  basalt provinces. Approximately 164,000 Km² of eastern Washington and Oregon are covered in lava up to 4000 feet thick.  The flows are largely Miocene in age and are most likely related to the initial impingement of the Yellowstone hotspot beneath the North America continent.

Exercises: Tectonic Plates Review or learn the major tectonic plates, then test your knowledge. Terminology Test your knowledge by completing the crossword puzzle (interactive web version, pdf) Learning the Provinces Learn the Province Locations; then press the button on the left to take the quiz. Once familiar with the provinces try locating and outlining them on this shaded relief map: Landforms of the Conterminous United States - A Digital Shaded-Relief Portrayal

Bibilography

Bloom, Arthur. 1998, Geomorphology, A systematic analysis of Late Cenozoic landforms, (3rd edition): Prentice Hall, Upper Saddle River, N.J., 482 p.

Chorley, R.J., Schumm, S.A., Sugden, D.E., 1984, Geomorphology: Methuen and Co. Ltd., London, 605 p.Crowell, J.C., 1984, Origin of late Cenozoic basins in southern California: in Sylvester, ed. Wrench Fault Tectonics. AAPG Reprint Series No. 28, p. 195-209.

Thornbury, William D., 1969, Principles of Geomorphology (2nd edition): Wiley and Sons, New York 594 p.   

Vigil, José F. , Pike,Richard J., and Howell, David G., 2000, A Tapestry of Time and Terrain, U.S. Geological Survey
Geologic Investigations Series I 2720, online version 1,Tapestry Main Page

Lindley Hanson/Department of Geological Sciences/Salem State College/Geomorphology/GeoIndex/QkRef