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Back to plates. Tectonics principles. The basics. First order processes governed by plate tectonics have observable geological consequences at regional scale. These consequences are decipherable in the geologic record.
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Back to plates Tectonics principles
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
Rule #1 - Wilson’s 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.
Rule #2- Continental drift • Continental breakup leads to the development of an oceanic basin. Continental extension is a precursor to breakup.
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.
Rule # 4- The architecture of the oceanic lithosphere (Steinman’s trinity) • The oceans are short-lived, <200 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.
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/270
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.
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.
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.
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.”
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.
Collision Subduction Arcs The Insubric lsuture in the Alps, typical for continental collision and Barrovian sequences.
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.
Schematic cross section through the Himalayan collisional belt (from Kapp and DeCelles)
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.
Zircons are used to date the age of the orogenic event; Zircons do not grow away from high grade continental events
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.
Terranes accreted to the North American Cordillera (from Coney et al., 1980)
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.
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.
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.
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
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
