Transcript Mountain Belts and Continental Crust

Continental
Crust and Mountain Belts
• Continents are composed of two basic types of terrain
–Cratons, where there hasn't been significant structural deformation lately
• Cratons are composed of areas where igneous-metamorphic are exposed (shields) and areas where
igneous-metamorphic rocks are overlain by sediments (platforms) and are usually pretty flat
–Mountain belts, where there has been intense structural deformation (within the last billion years), which
come in three basic flavors:
• block-faulted (Basin and Range),
• volcanic (Cascades, Andes), and
• folded and thrust-faulted (Appalachians, Alps, Himalayas).
• Mountain belts are long, narrow zones of intense structural deformation, which might or might not have
topographic mountains.
• Some mountains are not part of mountain belts; some mountain belts do not have mountains.
• Many mountain belts have very thick accumulations of sediments that accumulate at plate margins where
the added weight of sediments causes additional subsidence, and thus more sedimentation.
v 0026 of 'Mountain
Belts and Continental
Crust' by Greg Pouch at
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21Mountains.ppt
Continental Crust and Mountain Belts
3 Cratons
4 Cratons > Shield and Basement
5 Cratons > Shields > Archean
6 Mountain belts > Overview
7 Mountain belts > Extensional
8 Mountain belts > Volcanic
9 Mountain belts > Compressional
10 Map of North America
11 The Wilson Cycle
12 Accreted Terrains
13 Origin of Continents
15 Cratons (single, crowded slide)
16 Mountain belts (single, crowded slide)
Cratons
Cratons (Normal continental crust) Most land area has not undergone significant structural deformation in
over a billion years and consists of intensely deformed igneous-metamorphic complex exposed at the surface
(shields) or flat sediments overlying intensely deformed igneous metamorphic complex (sedimentary
platforms) in which case we call the underlying igneous-metamorphic complex basement.
• Cratons are the combination of shields and platforms.
• Vast majority of continental area is cratons, that are more or less in isostatic equilibrium.
• Deformation and igneous activity might occur at the edges, and there can be igneous activity at hotspots.
• Usually, flat or gently rolling.
• Opposite of mountains, and part of what gets deformed in mountain building.
• In this map, everything in pink and gray is cratons (pink is shield, gray is platform) and brown is mountain
belts.
Cratons
>
Shield
and
Basement
Most crustal rocks are igneous and metamorphic, and there might be a thin veneer of sediments on top.
When the igneous-metamorphic rocks in a craton are exposed, we call it a shield (in profile, it's high in the
middle and slopes away from center, like ancient Greek or Roman shield). When the igneous-metamorphic
rocks are overlain by sediments, we call the underlying igneous-metamorphic rocks basement or basement
complex, but they're the 'same' thing (like coastal plain and continental shelf), and if the all the sediments
erode from platform area, it goes back to being a shield.
The book's cross-section of a continental interior is wrong. There are not a few discrete, huge batholiths
embedded in a sea of metamorphic rocks. There are a large number of greenstone belts and granite-gneiss
terrains in Archean areas, and younger deformation zones that are transitional from Archean-style to
modern-style mountain belts.
•Basement complex occurs in provinces (100s-1000s km) of similar structural style and grain, with similar
ages for deformation. Archean deformation does not look like a modern fold belt (dominated more by
elevator tectonics than plate tectonics). Most younger complexes show at least relict dates going to 2.5 Ga,
and you can often trace very old features into and through younger mountain belts, like greenstone belts in
the Grenville Province.
•Platforms are stable areas, underlain by basement complex, overlain by flat-lying sediments deposited in
shallow seas, sedimentary basins, or passive margins. Sediments are usually very mature.
–Sediment thicknesses on platform are often less than 1000 m. In basins, this may go to 3000 m.
Cratons
> Shields > Archean
•Greenstone belts are huge syncline-like downwarps of thick sequences of supra-crustal rocks (sediments and
volcanics). The volcanics are bi-modal (ultra-basic to basic and rhyolites, but no andesite). The sediments are
dominated by turbidite greywacke, shale, and immature conglomerates. These are very important economically, so well studied
–Greenstone belts are intensely deformed (folded, faulted, very low-grade metamorphism) and the
deformation occurred at the same time as the deposition/eruption. Field relationships are frustratingly
complex, but they seem to have extruded onto continental crust, and are intruded by continental granitoids.
–A plausible explanation: Start with continental crust above. Ultra-mafic volcanoes erupt (due to mantle
plume, asteroid impact, rifting, … your pick). Sediments are eroded from the volcano chain. Added weight
causes sagging of the thin, hot lithosphere, leading to partial melting and eruption of rhyolites and intrusion
of granites, along with the deformation.
–Typical size would be 6-20 km thick sediments, 50 km wide, 500 km long.
•Granite-gneiss terrains consist of small granite plutons, gneisses, and very high grade metamorphic rocks,
lots of migmatites and veins. The whole configuration suggests that these were not intruded in a single episode
or short series (like modern batholiths), but that the rocks kept getting re-mobilized. The sharp changes in
metamorphic grade might be due to strong influences of water (on effective pressure and temperature), or that
these are more heavily faulted and deformed than we think. They might represent lower continental crust
under modern conditions, or continental crust subject to more vertical motions and higher geothermal
gradients.
Mountain
belts
>
Overview
• Mountain belts long, narrow zones of intense structural deformation, which might or might not have
mountains. (Some mountains are not part of mountain belts; some mountain belts do not have mountains,
but mountain belts always have uplift—which might or might not be faster than erosion can wear them
down—and consequently shed sediments.)
• Mountain belts come in three basic flavors:
–block-faulted (Basin and Range), [extensional]
–volcanic (Cascades, Andes), [volcanic, and largely neutral]
–folded and thrust-faulted (Appalachians, Alps, Himalayas). [compressional]
• Many mountain belts have very thick accumulations of sediments that accumulate at plate margins where
the added weight of sediments causes additional subsidence, and thus more sedimentation.
Mountain
belts
>
Extensional
• Block faulted (Basin and Range NV-AZ-UT-NM, East
African Rift Valley) This can happen to/in almost any geologic
setting, but is most classically associated with continental
rifting.
• Parallel normal faults separate the landscape into downdropped
grabens (basins) and uplifted horsts (ranges).
• Thick sediments, some immature and often including
evaporites and limestones, accumulate in the grabens.
• Adjoining faults may not have the same throw, resulting in
tilting of the blocks.
• Usually, there are very early basalts, followed by rhyolites once
the lower crust gets heated enough to generate granitic melt,
then followed by no igneous activity.
• Structures are generally extensional (normal faults).
• Shallow or no earthquakes.
• Little or no metamorphism.
• Gravity indicates thin crust.
• Origin of passive margins and some coastal mountains
Mountain belts > Volcanic
• Volcanic chain (Andes, Indonesia, Cascades)
• Sierra Nevada is root of a volcano belt.
• Partial melting of subducting oceanic crust gives rise to andesitic volcanoes.
• Central part is completely dominated by volcanism and deeper plutons.
• The weight of the volcanoes and resulting immature sediments cause downwarping of adjacent crust and thick
accumulation of sediments, with thrust faulting and folding.
• Metamorphism consistent with intrusions and eruptions: very steep geotherm.
• Structurally neutral, deep earthquakes (some shallow quakes as well), minor secondary extensional features.
• Active margin
Mountain belts > Compressional
• Collision Zone (Alps, Himalayas, Appalachians)
• Deformation increases towards center-line, which may be fairly
simple crystalline terrain (simple batholiths of uniform age, and
simple metamorphic grades decreasing from center, like Maine)
• Outer parts are thick sequences of sediments and volcanics, thrust
faulted and folded (like Pennsylvania)
• Deformation dies out as you move outward.
• Sediments may accumulate in basins flanking the mountains.
• Might have topographic mountains or might not (Himalayas vs.
Tibet). Erosion and isostatic rebound can cause uplift of peaks.
• Structurally compressive, with minor, secondary extensional
features.
• Shallow earthquakes.
• Simple metamorphism and intrusions.
• Geotherm indicates fairly gentle increase of temperature with
depth.
• Granitic intrusions, minor rhyolite volcanics.
• Gravity indicates doubling of crustal thickness.
The Wilson Cycle
: The Official Plate Tectonics Explanation of Origin of Mountains and Almost Everything Else
• Rifting of continent gives block-faulted mountains. One basin may eventually become an ocean.
• The ocean gets old, cold and starts to subduct.
• Subduction of oceanic crust yields andesite by partial melting, causing island arcs or volcano chains.
• Continent-Arc collisions and, finally, a Continent-Continent collision occur as arcs and the continent are
carried to the subduction zone by the subsiding ocean. This results in compressional events (orogenies),
thickening of continental crust followed by erosion and isostatic rebounds.
• Secondary tension on outside of bends
• Secondary block faulting due to isostasy
• Delamination of lower part of lithosphere (cold mantle) resulting in sudden heating of crust, possibly with
igneous activity.
Accreted Terrains
• Accreted/suspect/exotic terrains: basic gist is that fragments of
plates get separated and collide.
–There have to be segments of plates in collision zones (like
the leading edge) that get displaced and would be exotic
terranes. Geometrically, this has to be true sometimes.
–I don't see many "micro-continents" on a present day map
–I've seen too many features that continue across terrains and
pre-date their supposed incorporation to believe in this as
much as some people do.
– It seems very suspicious that everything that is hard to
explain is swept away with a deus ex machina explanation.
Origin of Continents
Plate tectonics explains the origin of oceans and oceanic features quite well. It kind of explains some
deformation of the edges of continents. It fails to explain many features of continental interiors. The
following is a plausible, but not necessarily widely accepted, explanation.
In the beginning, ultra-mafic rocks (mantle) partially melt to give mafic rocks, which get partially melted
to produce andesitic rocks, which partially melt to produce granitic rocks (continental crust).
Size of an area that needs to stay in isostatic equilibrium increases with time as the planet has cooled and
the lithosphere has thickened. Early on, 'isostatic size' was very small, now it is around 100km. (Think of
ice thickening on a pond: with thin ice, any weight causes warping, as the ice gets thicker, larger objects can
be placed on the ice and are supported by the strength of the ice)
• Pre-Archean 4.6-3.8 Ga Planet coalesces homogeneous and cold from the solar nebula. Iron catastrophe
and near-melting of mantle, eruption of basaltic crust, vertical subsidence (elevator tectonics) and partial
melting moving rocks down Bowen's reaction series, leading to origin of continental crust. Size of area that
needs to stay in isostatic equilibrium very small, maybe 5 or 10km.
• Archean 3.8-2.5Ga Greenstone belts (vertically subsiding elevator tectonics of volcanic arcs or hotspot
tracks). Possible plate-style extrusion and subduction of oceans, probable drifting of continents carrying
greenstone belts. High geotherm makes granite unstable at a shallower depth, so much more generation of
partial melts. Size of area that needs to stay in isostatic equilibrium small, maybe 50km.
• End of the Archean No more ultra-mafic volcanoes. Granitic rocks go from being granite-like (quartz and
sodic plag) to true granites (quartz, K-spar, and sodic plag). If the mantle underwent some big ex-solution
event, that could explain how a lot of potassium suddenly appears in the crust, and the degree of partial
melting in the mantle suddenly drops.
• Transition to modern plate tectonics ~2.5 to 1.2 Ga
• Plate tectonics 1.2Ga-present Modern style plate tectonics. Size of area that stays in isostatic equilibrium
reaches point where horizontal plate deformation dominates. North America separates from Europe at 1.2
Ga, collides, separates again around 0.6 Ga (collides around 0.4Ga), separates around 0.2Ga… Tendency
for plates to break apart along old sutures, and odd tendency to collide with the same plate they separated
from. (Start seeing blueschist metamorphism (low T, high P). Thrust faults. Granitic intrusions are fairly
simple.)
Continents and Mountain Belts
• Continents are composed of cratons and narrow mountain belts.
• Cratons include shields where basement is exposed and platforms where it is covered by sediments. Old
(Archean) basement has greenstone belts and granite-gneiss terranes but lacks the simple metamorphic
zones and extensive related intrusions found in modern mountain belts.
• 'Modern' mountain belts come in three flavors
–Extensional (rifting phase of Wilson cycle)
–Volcanic (subduction phase of Wilson cycle)
–Compressional (continental collision phase of Wilson cycle)
• Continents might grow by accreting volcanic mountain belts, or this might just be re-working of old
continental crust.
• Collision zones are not as regular as simple diagrams, and involve at least some strike-slip motion giving rise
to exotic terranes.