Structures and Landscapes

ASSIGNMENTS

  1. Geomorphology From Space: Tectonic Landforms (useful)
  2. film Earth Revealed. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view #8 Earth's Structures (useful)
  3. Hanson: Deformation and Structures GLS100 lecture (for review)
wiki
Terms: orogenic, isostacy, anticline, syncline, anticlinorium, synclinorium, salt dome, diapir, horst, graben, klippen, window, strike ridges and swales, dip slope (resequent), flatiron, scarp slope (obsequent), plunging fold, hogbacks (fairly symmetrical ridge)-steep dip slope, cuestas (asymmetrical ridge)-gentle dip slope, water and wind gaps, topographic inversion, anticlinal ridge, anticlinal basin, synclinal ridge, synclinal basin, Drainage patterns: trellis, annular, rectangular,dendritic, fin, arch, natural bridge

Realms of Deformation (Diastrophism/Tectonism)

Orogenic Deformation

  • Orogenic deformation is folding, faulting and metamorphism related to the collisional events that that create orogenic belts.  Orogenic deformation is restricted to the mobile belts of continents.

Epeirogeny and isostatic deformation

  • Epeirogeny refers to broad uplift or subsidence unaccompanied by folding or thrust faulting that occurs within a craton. Isostatic deformation is a type of eiprogeny resulting from local loading and unloading, such as that caused by the advance and melting of ice sheets.   Deformation is related to isostatic adjustments or mantle upwelling.  Recall that isostacy is the concept that the buoyant rocks of the lithosphere maintain a floating equilibrium over the denser rocks of the mantle beneath.

INTRODUCTION

Significance of Geologic Structures: Geologic structure controls the grain of the topography; the size, spacing, orientation, and shape of the hills, ridges, valleys, basins and drainage systems.

Tectonic features vs. structurally controlled erosional features (Bloom, 1998):

  • Tectonic features: active or youthful features--the present relief is related to recent or active tectonic activity (tectonic scarps, triangular facets on a fault scarp, fault block mountains, rift valleys, etc.)
  • Structurally controlled erosional features: features shaped by differential weathering, erosion and deposition (minor) acting on a structure (ancient lineaments in deeply eroded fractured rocks (faults and joints), fault line scarp, cuesta, hogback, etc)

Types of deformation

Elastic

  • recoverable
  • examples: seismic waves; isostatic depression and rebound
  • evidence: indirect--recorded by sediments

Plastic/Ductile

  • permanent bending resulting from flowage of rock
  • evidence: folding of rocks; glacial flow

Brittle

  • fracturing
  • evidence: joints and faults
Factors Controlling Deformation
  • Confining pressure
  • Time (rate at which a force is applied)
  • Heat
  • Properties of the deforming material (e.g. sandstone vs shale)
  • Rate that stress is applied

GEOLOGIC STRUCTURES

This lecture covers features and landforms related to the following structures:

  1. Folds: those produced by ductile deformation
  2. Faults and Joints: structures produced by brittle deformation (See weathering lecture for joint description)
  3. Unconformities:
    • Buried erosional surfaces produced by uplift (epeirogeny or orogenic), erosion, and deposition. Unconformities can also be the result of erosion and deposition driven by eustatic changes in sea level.
  4. Horizontal Strata: flat or gently dipping sedimentary rocks
Mapping structures and classifying structures

In order to map and classify geologic structures you must be able to define the orientation of planes and lines in space

  • Plane: strike, dip direction and amount of dip
  • Line: direction and amount of plunge

Folded Rock Structures

Fold terminology: limbs, axis (hinge), crest, axial plane

Associated features

  • Cleavage forms parallel to the axial plane of a fold
  • Joint

Fold classifications:

  • Classification based on relative position of older and younger rocks within the fold
    • Anticline
    • Syncline
  • Classification based on form only
  •  Subdivisions based on inclination of axial plane
  •  Subdivisions based on relative dips of opposite limbs
    • symmetrical--dipping equally in opposite directions
    • asymmetrical--dipping different amounts, but in opposite directions
    • overturned--both limbs dipping in the same direction, but not necessarily the same amount (direction of overturning is term vergence)
    • isoclinal--limbs are parallel
  •  Subdivision based on degree of fold closure (interlimb angle)
    • gentle (120-180)
    • open (70-120)
    • close (30-70)
    • tight (0-30)
  • Subdivisions based on the plunge of the fold axis
    • plunging
    • doubly plunging; elongate dome or basin (turtleback)
    • domes
    • basins

Large-scale folded structures

Anticlinorium and synclinorium are complex structures within in a regional fold belt that often reflect different tectonic terranes.

  • anticlinorium: a belt typically containing older rocks indicative of a broad upward flexure.  Typically they are deformed highlands, arcs, or continental fragments.
  • synclinorium: A synclinorium contains younger rocks relative to adjacent anticlinoria; rocks are often sedimentary or volcanic in origin suggesting a deform basin or oceanic terrane.

Example: A volcanic arc collides with a continent; the forearc basin would form the synclinorium and the arc the anticlinorium.

Examples of geomorphic features formed from folding

FRACTURES: Faults :

Fault Terminology:

  • strike and dip
  • hanging wall/headwall
  • footwall

 Types of faults

Dip Slip Faults

Strike Slip Faults

  • left lateral
  • right lateral
  • Pull-apart Basins and transverse Mountains: form where a transform fault bends

Examples:

Figure 1. Secondary features related to a transform bend.  Examples: Death Valley, CA (pull-apart basin), Transverse Range, southern CA.

 

 

Structural Landforms

Tectonic Landscapes related to Faulting

Fault block landscapes are composed of multiple basins and mountains separated by faults.  The most widely studied and best known fault block terrain is the western Basin and Range.  Introductory texts traditionally decribe the region as horst and graben topography, like that illutrated if figure 1B. The upthrown blocks are horst, the down thrown blocks are graben. However, the structure is more like that shown in 1B.  Upwarping and stretching has split the crust into overlapping half graben that slide down along listric faults.  Fault block landscapes can also be related to reverses faults (1C), the original Rockies formed during the Late Cretaceous Laramide Orogeny are an example.  Low angle thrust faults are less likely to form traditional fault block terrains.

 

faultblocks

 

Figure 2. Tectonic landscapes related to faulting. Diagram B approximates the structure of the Basin and Range.  Diagram C illustrates the structure of the Laramide uplifts and Sierra Pampean Ranges in Argentina.

Fault-block landscapes and climate

Streams in an arid climate rarely leave the basin.  Instead they cut across the scarp to the valley floor where they release their sediment, either on the dry basin floor or in an ephemeral playa.  The basin floor is base level so features are largely depositional.  Even the scarp will progressively be subsumed beneath basin fill.  In humid climates streams have the flow and energy to carve  passages from the basins to the ocean.  Sediment that accumulates at the base of scarps is carried away and the basin floor is an active alluvial valley. 

Table 1. Characteristics of fault-block landscapes in varing climatic regions.  (See Bloom, 2004)

Landscape

New Zealand Type
Basin and Range Type
climate Humid Arid - Semiarid
drainage exterior interior
basin erosional: alluvial valley depositional: playa, salt pans, bolson, bajada fringe, alluvial fans
scarp


scarp retreat through erosion


scarp is covered by basin deposits

triangular faucets rounded divides and subdued faucets sharp divides and well developed triangular faucets
scarp terraces fluvial terraces offset bajada and fan deposits
Examples Anwatere Fault NZ (Quicktime panorama) C.Jones Death Valley (Quicktime Panorama) D.Bains

Fault block landscapes defined by the internal structure of fault blocks

Rift Valleys

 Rift Valleys: Long anastomosing rift valleys -One or more dominating central grabbens

Table 2.  Examples of rift valleys. Links are to images from NASA's Visible Earth image database.
Active
Inactive
Afro-Arabian Rift System /Red Sea Rhine Graben
East African Rift System Mississippi embayment
Rio Grande Rift, NM Connecticut and Hudson Valleys (Mesozoic)
Lake Baikal, Russia  

Other Fault related Features

scarps are related features

scarps

Features associated with strike slip faults

wineglass

Figure 3. Scarps and related features.

thrust sheets and nappes and related features

drainage patterns: rectangular, centripetal (or basinal)

Fold-Related Landforms

Monoclines are single flexure folds that flank large upwarps. The are typically cored by a blind faults.  As with any fold structure, it's surface morphology is determined by the rates of erosion relative to uplift (if still active), the fold geometry, and the relative strength of the individual layers.  Eroded monoclines exhibit strike ridges and swales in the flextural zone and plateaus and basins in the adjoining flats.

Strike ridges are refered to as hogbacks if steep and cuestas if gentle.  The dip and scarp slope of a hogback have a similar angle of repose, whereas a cuesta has a much gentler dip slope.  Cuestas can also be formed by the erosion of gently dipping strata, such as in the Coastal Plain Province.

Some of the best examples of monoclines are located in the Colorado Plateau Provines:

  • Waterpocket Fold, Capitol Reef National Park
  • Coobs Ridge, Monument Uplift, Utah
  • San Rafael Reef, Eastern flank of the San Rafael Swell

Figure 4. Monoclines and their eroded forms.

Domes (anticlinal) and Basins (synclinal) are folds that are round or mildly linear in planform.  Their topography does not alway reflect the structure depending on the resistance of th core. Topographic inversion is common.

dome1  dome2

Figure 5. Examples of eroded domes. 

Examples:

basin2

Examples:

Folds: Active terraines typically display basins and domes flanked by cuesta and hogback ridges.  Inactive eroded terrains reflect the relative rock strength and fold geometry. 

Examples of individual fold structures: Kaibab uplift (Grand Canyon, AZ), Moab and Salt Valley anticlines, Utah, Sheep Mountain Anticline, WY, Split Mt. Anticline, UT

Examples: Orogenic belts

 

Geomorphic features and terms related to folds

  • strike ridges and swales
    • hogbacks (fairly symmetrical ridge)-steep dip slope
    • cuestas (asymmetrical ridge)-gentle dip slope
    • dip slope (resequent)
    • flatiron: A  triangular-shaped feature created along disected dipslope strata of a fold limb or hogback.
    • scarp slope (obsequent)
  • water and wind gaps: Abandonned fluvial valleys that cut through in a strike ridge.
  • anticlinal ridge, anticlinal basin(topographically inverted by erosion)
  • synclinal ridge (topographically inverted by erosion), synclinal basin
  • Drainage patterns: trellis, annular, basinal (in arid regions like Iran)

FRACTURES: JOINTS (dilation displacement)

For discussion of joints see  weathering outline

Geomorphic Features related to excavation along joints

Learn about the formation of arches : NPS/Geology of Arches National Park

Horizontal Strata

Features formed by the erosion of horizontal strata are best developed and displayed in the arid landscapes of the Colorado Plateau Province.  Canyons in humid regions are cut by perennial streams and delicate features like hoodoos and arched are less likely to be preserved.

Canyons: slot canyons (joint controlled), box canyons (arid)

Hills and spires formed by progressive plateau erosion: tableland, mesa, butte, hoodoos (Bryce Canyon)

Slope elements: bench, esplanade (broad bench), cliff or scarp

Features form by weathering along vertical joints cutting horizontal strata: slot canyons, fins, and arches

Landscape formed on erodible and poorly permeable strata: badlands topography

Typical Drainage patterns:

  • dendritic
  • rectangular (joint controlled)

Unconformities

Types of Unconformities:

  1. Parallel (beds and unconformity are parallel)
  2. Disconformity (beds are parallel but unconformity is irregular)
  3. Angular unconformity
  4. Nonconformity (underlying rocks are igneous or metamorphic)

Geomorphic expression (if any)

  • superimposed drainage system
  • may be confused with features formed by differential weathering of faults 
  • Chili Altiplano Unconformity Nasa Visible Earth

Tectonic controls on fluvial systems

1. Size, shape and orientation of continental drainage systems.

The current and recent orogenic history of a continent controls the regional slope of major drainage systems.

    a. Continents in which the major divides are orogenic belts:

      • South America: west coast is an active collision boundary while the east coast is passive. All large drainages systems (e.g. Amazon, Sao Francisco, Uruguay, etc.) drain from west to east.
      • North America: principal drainages flow from the west or east of the Cenozoic Cordilleran (continental divide). The ancient Appalachian system deflect the Mississippi River toward the south is the principal divide for eastern streams.
      • Northern Europe-Asia: Most rivers flow north away from the Alpine-Himalayan orogenic belts.

    b. Large drainage systems are evenly distributed around the margins of continents that are bound by passive margins. (Examples: Africa and most of India)

2. Formation of Interior and Exterior Drainage Systems are influenced by rift zones and passive margin structures

    1. Interior drainage systems commonly develop in rift zones or related fault-block basins (Examples: Tanganykia in the East African Rift System, and Great Salt Lake, Pyramid Lake, Lake Tahoe in the Basin and Range)
    2. Interior drainages may also result from passive margin upwarping (e.g. Lake Victoria, Africa)
    3. Failed third-arm rifts (aulocogens) of Cenozoic rift systems are deep structural low areas in which the marginal trunks of major river systems have become established.  (Examples: Hudson, Connecticut, Mississippi rivers of NA., Amazon of S.A., and Niger and Congo rivers of Africa)

3. Foreland Basin Development: A foreland basin is tectonic basin filled with coarse alluvium (mollasse) shed from an adjacent orogen. Deposits may be several thousand feet deep.

Other structural controls

  • Some large rivers, such as the Upper Mekong, Yunling Shan, Bramaputra, etc. are focused along major fault zones.

Nasa's GFS Tectonics and Landforms

***USGS Tapestry of Time and Terrain: This site is a great help if you're unfamiliar with the major geologic features of the U.S. Provinces of the United States (USGS NPS)

The Geology of Virginia (The College of William and Mary) Excellent, well illustrated summary of the Appalachian Mt.se

Illustrated reviews of structures on the Internet
  1. Folds, Faults and Other Records of Rock Deformation Simon Fraser University
  2. Teaching Resources in structural Geology Rob Butler, Martin Casey, Geoff Lloyd, Andrew McCain, University of Leeds
  3. Structural Geology > outcrop patterns / folds Miller, University of Oregon
  4. flash moves of folding and faulting (McGraw Hill)
  5. Guglielmo, Giovanni, Jr., B. C. Vendeville, and M. P. A. Jackson 1998, Animation of rising and falling salt diapirs: A BEG hypertext multimedia publication on the Internet at: http://www.beg.utexas.edu/indassoc/agl/animations/AGL98-MM-005/index.html.

Exercise: Landform Identification

Pan the images below.  How many different structures can you find and recognize?

Satellite Images

  1. Visible Earth: Idaho, Montana, and Wyoming http://visibleearth.nasa.gov/view_rec.php?id=3893
  2. Visible Earth: Nevada and Utah http://visibleearth.nasa.gov/view_rec.php?id
  3. Visible Earth: Utah, Colorado, and New Mexico http://visibleearth.nasa.gov/view_rec.php?id=6266
  4. Visible Earth: S California http://visibleearth.nasa.gov/view_rec.php?id=3299
  5. Visible Earth: New York NY: World Trade Center Zoom-In http://visibleearth.nasa.gov/view_rec.php?id=11023
  6. Visible Earth: Fall colors in Northeast United States http://visibleearth.nasa.gov/view_rec.php?id=6742
  7. Visible Earth: The Mid-Atlantic States http://visibleearth.nasa.gov/view_rec.php?id=6883
  8. Visible Earth: Space radar image of Sunbury, Pennsylvania http://visibleearth.nasa.gov/view_rec.php?id=559
  9. Visible Earth: Konari, Iran http://visibleearth.nasa.gov/view_rec.php?id=3430
  10. Haro and Kas Hills ( anaglyph)in the Kachchh region of western India, which suffered the most deadly earthquake in India's history on January 26, 2001. (Earth Observatory Newsroom)

Online Images

 BIBLIOGRAPHY

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.

Easterbrook, Donald J., 1993, Surface Processes and Landforms: Macmillan Pub. Co., 520 p.

Howard, A.D., 1967, Drainage analysis in geologic interpretation: a summation: The Amer. Assoc. of Petr. Geol., v. 51, n. 11, p. 2246-2259.Jones, N. W., 1998, Laboratory manual for physical geology: WCB McGraw-Hill, Boston MA, 303 p.

Perry, W. P., et. al., 1985, North American Thrust-Faulted Terranes: AAPG Reprint Series No. 27, 466 p.

Ritter, D. F., Kochel, C. R., and Miller, J. R., Process Geomorphology (3rd Edition): Wm .C. Brown Publishers, Dubuque, IA, 544 p.

Summerfield, M. A., 1991, Global Geomorphology. John Wiley and Sons, New York, NY, 536 p.

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