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Maggie Ortiz, 2011
Edited by Jenny Marion and Paul Knobel, 2013
1.1What are Structural Geology and Tectonics
Structural geology & tectonics-motions & processes that build Earth’s crust
Types of Motions:
Rigid body motion-transportation, no change in size or shape; no permanent imprint
Deformation - motion that changes size and/or shape
Structural geology scale-submicroscopic to regional
Tectonics Scale-regional to global
Continuum Mechanics-describes the motion of material bodies of continuous matter, taking into account the body’s deformability. Includes kinematics and mechanics.
1.2Structural geology, tectonics and the use of models
Geometric Models
-3D interpretations of structures in Earth
-based on: mapping, geophysical data
-examples: geological maps, vertical cross sections
Kinematic Models
-prescribe motions that could have carried system from undeformed→deformed state
-not concerned with why, how or physical properties of the system
-assess validity by comparing observed motions w/ model
Mechanical Models
- models motions as the consequence of their relationship with forces and material properties.
1.3The interior of the Earth and other terrestrial bodies
Core
-dense, predominantly iron-nickel alloy
-solid inner core, liquid outer core
Mantle
-thick, much lower density than core, magnesium-iron silicates
1. Lithospheric mantle-crust and upper mantle (100 km under ocean, 200-300 km under continents)
2. Asthenosphere-closer to melting temp→weaker
3. Mesosphere-stronger, high density, crystalline phases of magnesium-iron silicates or oxides
Crust
-thin layer that surrounds mantle
-low-density materials
-igneous rocks-granitic to basaltic
-sediments and sedimentary rocks
-metamorphic equivalents of above rocks
Temperature gradient-+25oC per km in crust and mantle (this change decreases with depth)
Convection
-moves heat out of liquid core
-carries heat transferred from core & from radioactive decay w/in mantle to surface
Heat escapes earth by:
-Conduction through cold lithospheric boundary layer
- Advection of heat in magmas
- Upwelling of asthenosphere at oceanic spreading centers
1.4The Earth’s crust and plate tectonics: Introduction
Continental Crust-granodioritic composition
Oceanic Crust-basaltic composition
Surface elevation-Bimodal
-continents-w/in 100’s of meters of sea level
-ocean floor-~5 km below sea surface
Tectonics-lithosphere is divided into plates the move (rigid body motion)
-deformation of plates concentrated in belts ~100’s of km wide
Plate Boundaries
-Divergent
*plates move away from each other
*Material flows up from mantle
*horizontal stretching & vertical
thinning of crust
*normal faulting surface, ductile thinning deeper
-Convergent-subduction zones
*thrust faults w/strike-slip faulting
*continent-continent collision-folding,
metamorphism & igneous activity
-Transform-sliding (horizontal)
*strike-slip faults
*vertical zones of ductile deformation
w/sub-horizontal direction of displacement.
Factors influencing structures
-orientation and intensity of forces
-motions to which rocks are subjected
-physical conditions-pressure and temp
-mechanical properties of rock
Brittle deformation
-rock fracture, low temp & pressure
Ductile deformation
-high temp and pressure but below melting temp and low intensity of applied forces or slow imposed deformations
-flow of rock in solid crystalline state
-stratigraphic layers, stretching & thinning of layers, parallel alignment of grains
1.5Ocean basins
Ophiolites - on-land exposed rock sequences thought to represent old oceanic crust
Oceanic crust
-3-10 km (avg. of 7 km) thick
-igneous rocks of basaltic composition
-differences in elevation due to differences in density and thickness of underlying crust and mantle
Averaged layer model
Vp is seismic p-wave velocity
Layer 1-Vp=3-5 km/s
-unconsolidated sediment of pelagic, hemi-pelagic or turbidic origin
Layer 2-Vp=4-6 km/s
-predominately submarine basaltic extrusive and shallow intrusive rocks
- subdivided into 2A, 2B, 2C based on how velocity increases in depth
Layer 3-Vp=6-7.5 km/s
-mafic, ultra-mafic plutonic rocks and/or serpentinized mantle peridotite
-3A & 3B-reflect olivine quantityi. Features of Oceanic Plate Margins
Midocean ridges
-divergent plate margins-high regions
-40,000 km long, 2.5 km high (above floor), and 1000-3000 km wide
-active normal faults
Transform fault boundaries
-seismically active parts of fracture zones
-sharp ridge and trough topography
-steeply dipping faults
-deformed oceanic rocks
-up to 10,000 km long, up to 100 km wide
-crust tends to be less thick here
Convergent
-Chains of volcanic islands accompanied by parallel trenches (island-arc-deep sea trench pairs)
-extend 1000’s of km
-volcanoes 70-80 km apart-rise above ridges-few 100 km wide
-trenches-up to 12km deep, ~100km wide
*landward side-active thrust faults
-crust- ~25 km thick
ii. Features of oceanic plate interiors
Abyssal plains
-vast areas of flat ocean floor
-deepest regions of ocean
-5 km below sea level
Oceanic plateaus
-broad elevated regions
-variety of origins
-100-1000’s 0f km2 area
-1-4 km above normal ocean floor
Aseismic ridges
-linear ridges characterized by high elevation, thick crust, and lack of associated seismic activity
-most cases-linear constructional ridges formed by chains of basaltic volcanoes
1.6The structure of continental crust
Continental crust
-older, thicker, less dense, lower velocity
-~35 km thick (average)
-velocity tends to increase with depth
-velocity inversions
Upper crust-metamorphosed rocks intruded in places by granitic rocks
Middle crust-migmatite
Lower crust-highly folded rocks
-moho discontinuity is less sharp
1.7Precambrian shields
-Precambrian shield-large areas where Precambrian rock >60 Ma are exposed
-Archean rocks- >2500 Ma
-Proterozoic rocks- 2500 Ma to 540 Ma
-Archean regions-greater evidence of crustal instability
i. Archean terranes
Archean Terranes
-divisible on basis of metamorphic grades
-high grade gneissic regions
*amphibolite or granulitic facies of
metamorphism
*form bulk of archean regions
*quartz-feldspathic gneisses derived
by metamorphism of felsic igneous
rocks
*complexly mixed with greenstone belts over 10-100’s of km
-greenstone belts-rocks at greenschist or lower grades of metamorphism
*Greenstone-mafic to silicic volcanic
rocks and shallow intrusive bodies
Sutures-regions of deformed oceanic material thought to be remnants of disappeared oceans
Structural features of Archean Terranes
-highly deformed and display more than 1 generation of folds
-contacts between gneissic and greenstone are complex
-sedimentary rock types fall into one of two categories:
*immature volcanogenic sediments
*quartzite-carbonate-iron-assemblages
-Similar sedimentary and tectonic conditions occurred globally during Archean times
During Archean:
-higher temps in earth
-thermal gradient 2-3x higher than present
-plate tectonics likely operated
-oceanic crust prob. thicker
-continents smaller and less numerous
ii. Proterozoic terranes
Terranes include both highly deformed mobile areas and slightly deformed stable regions
-Cratons-tectonically stable regions of crust (became abundant in Proterozoic)
Deformed belts
-multiply deformed regions rich in volcanic rocks
-thick sedimentary sequences deposited in linear troughs
- include Proterozoic dike swarms
Aulacogens-series of smaller linear sediment-filled grabens
1.8Phanerozoic regions
i. Interior lowlands and cratonic platforms
-most platform sediments are marine
ii. Orogenic belts
-thick sequences of shallow-water sandstones, limestones and shales deposited on continental crust
-crude bilateral symmetry
-form at convergent margins
iii. Continental rifts
-abundant normal faulting, shallow EQ activity, mountainous topography
iv. Modern continental margins
Passive, rifted, or Atlantic-style margins (horizontally lengthened and vertically thinned continental crust)
-initiate at divergent plate boundaries
Convergent or Andean-style margins
-where consuming plate boundaries are located along a continental margin
-abrupt topographic change from a deep sea trench offshore to a high belt of mountains w/in 100-200 km off the coast
-narrow or absent continental shelf
-mountains have chain of active stratovolcanoes
Transform or California-style margins
-sharp topographic differences between oceans and continents
- active strike-slip faulting, poorly developed shelf, irregular ridge & basin topography and many deep sedimentary basins
Back-arc or Japan style margins
-composite margins - passive margin separated by narrow oceanic region from an active island arc
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Maggie Ortiz, 2011
Edited by Jenny Marion and Paul Knobel, 2013