Regional Tectonics

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Regional Tectonics

• Geos 425/525
• Fall 2009

Tectonics principles
LECTURE 4, September 1 2009
41 slides, 60 mins
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The basics

• First order processes governed by plate tectonics
  have observable geological consequences at
  regional scale. These consequences are
  decipherable in the geologic record.
• The geologic record is four-dimensional in that
  it records a time-integrated sequence of events.
  For more than 200 years, geologic research was
  governed by a couple of simple principles (the
  superposition of strata, uniformitarianism). We
  still obey to these rules in much of what we do
  in geology.
• However, with the advent of plate tectonics the
  basic rules available for tectonic and geologic
  interpretation have become more complex. Below
  are some of the most commonly used principles in
  tectonics they are rooted in the plate tectonic
  framework but apply to regional analysis

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Rule 1 - Wilsons Cycle

• Continents break up and rejoin multiple times
  during the evolution of the Earth. A corollary
  would be that any continental crustal segment has
  a time-integrated record of several break-ups,
  passive margin developments, subduction and
  collisions (each cycle in that order). A modern
  or ancient belt is always more complicated than
  one of the stages that may be prevalent in its
  history e.g. the Himalayas are the result of
  Indo-Asian collision but they must also carry
  record the subduction that preceded that.

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Rule 2- Continental drift

• Continental breakup leads to the development of
  an oceanic basin. Continental extension is a
  precursor to breakup.

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formation and features of passive margins
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Rule 3- The principle of orogenic linearity

• Orogenic belts that form as a result of plate
  interactions are linear or arcuate reflecting the
  nature of plate boundaries.
• Most sedimentary, thermal and deformational
  patterns are parallel to the belt but equivalent
  stages can be diachronous along the strike of the
  belt.

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Rule 4- The architecture of the oceanic
lithosphere (Steinmans trinity)

• The oceans are short-lived, lt200 Ma, develop a
  lithosphere that is progressively thicker away
  from the ridge, and a crust about 5-7 km thick.
• The oceanic crust and uppermost mantle is made of
  a basalt-gabbro-peridotite trinity that has
  depleted isotopic signature.
• This trinity (named ophiolite) is recognized as
  reflecting a former oceanic realm, when found
  obducted onto a continent.
• Any two continental regions currently adjacent to
  each other could have been separated by an ocean
  at some point in the past.

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oceanic crust forms at mid-ocean ridges by
seafloor spreading
partial melting of mantle peridotite (high Mg and
Fe) mafic magma (basaltic composition)
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
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Rule 5- The geoclinal rule for sedimentary
basins

• Marine basins without volcanic input
  (miogeoclines) represent passive margins, whereas
  the presence of volcanic material (eugeoclines)
  requires a nearby active (subduction) margin.
• Passive margins are quartz and carbonate-dominated
  , whereas an active margin sediment is richer in
  feldspar.

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Passive Continental Margins
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Rule 6- The architecture and polarity of
subduction

• Subduction zones are characterized by the
  presence of an accretionary wedge, a forearc, a
  magmatic arc and a backarc region (progressively
  further from the trench).
• Corollary the subduction plane dips toward the
  back arc.

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Rule 7- Calc-alkaline arcs

• Intermediate composition magmatism, usually
  distributed along the strike of an orogenic belt
  is a product of oceanic subduction
• They loosely mark the surface location above
  where the top of the slab is located at a depth
  of 100-125 km.
• Only one arc forms at a subduction zone at any
  given time.
• Corollary the migration of arc location in a
  direction perpendicular to the strike of the
  orogenic belt in time reflects a change in
  subduction dip for a given geographic region.

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Rule 8- Blueshists subduction zones

• High-pressure low temperature metamorphism, best
  represented by blueschists and eclogites
  fingerprints subduction.
• These rocks are most commonly found in a chaotic
  mix of wedge sediments and oceanic crustal/mantle
  rocks, named mélange.

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Rule 9- Barrovian metamorphism continental
collision

• Regional metamorphism following a clockwise PTt
  pattern is indicative of continental collision
• The lower plate follows a counterclockwise PTt
  pattern and inverted metamorphic gradients are
  common near major reverse faults.

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Red line - geotherm for barovian metamorphism
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Collision
Subduction
Arcs
The Insubric lsuture in the Alps, typical for
continental collision and Barrovian sequences.
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Rule 10- Convergence and crustal thickening

• Subduction and collision lead to crustal
  thickening.
• The primary mechanism of crustal thickening on
  earth is the development of fold and thrust
  belts, which have a determinable sense of
  development as a function of time.
• Corollary fold and thrust belts allow
  determination of the polarity of
  subduction/collision, and the rate and magnitude
  of shortening.

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Schematic cross section through the Himalayan
collisional belt (from Kapp and DeCelles)
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Rule 11- The rule of pressure gaps

• A vertical section through a crustal column
  should yield a continuum of metamorphic
  pressures.
• Pressure gaps, common in orogenic belts, reflect
  extensional collapse.

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Rule 12 - Isotopic genealogy

• Aged continental masses have a very different,
  more evolved radiogenic isotopic composition than
  oceanic rocks.
• The approximate age of a crustal mass can be
  determined by isotopic tracers and directly by
  geochronology.

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from http//earth.leeds.ac.uk/dynamicearth/
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Zircons are used to date the age of the orogenic
event Zircons do not grow away from high grade
continental events
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Rule 13 - The principle of allochtony

• Continental breakup and drift are responsible for
  rafting buoyant continental masses and the
  shuffling together of domains that have
  experienced a different geologic history.
• A mass that is exotic to its larger surrounding
  is allochtonous, and if defined by clear
  structural boundaries, a paleomagnetic record and
  ophiolitic sutures, is a terrane.
• Terranes accrete to continental interiors because
  they are unsubductable. There are juvenile
  terranes, i.e. new crust formed in an oceanic
  realm (island arcs, oceanic plateaus) as well as
  evolved continental terranes.

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Terranes accreted to the North American
Cordillera (from Coney et al., 1980)
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Rule 14 - The rule of lateral assembly

• Large-scale lateral transport can significantly
  alter orogenic belt linearity and the standard
  architecture of plate margins.
• Allochtony can be lateral as well as frontal.

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Example of lateral assembly central California.
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Rule 15 - The principle of cratonization

• A craton is a region that has not been deformed
  for several Wilson cycles. A craton does not have
  to be old, but if it is (e.g. Archean), it most
  likely will be close to a continental interior.
• The lack of young deformation is commonly
  demonstrated by the presence of an undeformed
  sedimentary cover (platform).
• The more time a fragment of continental
  lithosphere escapes deformation, the thicker a
  lithosphere it develops subsequent breakup is
  more difficult.
• Corollary of the cratonization principle Wilson
  cycles are not entirely plate independent.

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Major cratons and shields on continents.
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Rule 16 - Continentalization

• Wilson cycles open and close oceans but also form
  new island arcs and plateaus that are
  unsubductable and add continental mass as
  terranes through time.

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geologic map of the United States
Basin and Range (rifting)
craton shield and platform
shield
Paleozoic to Recent active margin
Paleozoic orogenic belts (Appalachians)
platform
Mesozoic to Recent passive margin
Paleozoic to Recent orogenic belts
Paleozoic orogenic belts
from http//pubs.usgs.gov/publications/text
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result is that continental crust is heterogeneous
ages of continental crust
cratons pink, yellow, red, green areas
orogenic belts (sites of collision) brown and
light blue (continents)
note older areas in interiors younger along
edges
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Next lecture

• Tectonic rates of motion
• The oceanic lithosphere, part I