stable cratons (green): cratons are stable and relatively cold, with ''normal'' thermal gradients of ~20 K/km. - magmatic arcs (red-orange): magmatic arcs are sites where heat is advected to shallow levels, producing low P/T metamorphism -crustal extension (orange): crustal extension via normal faulting leads to advection of heat to shallow levels, followed by cooling to a normal thermal gradient - oceanic extension--mid-ocean ridges (red-orange): convection carries heat to very shallow levels, where 7-km thick oceanic crust forms; hydrothermal circulation produces low P/T metamorphism

ophiolite soles (red): are thrust zones beneath very hot oceanic lithosphere emplaced onto passive continental margins; in contrast to other low P/T metamorphism, inverted metamorphic gradients form because the emplacement rate is rapid compared to the rate at which the extreme heat is conducted away

subduction zones (blue): rapid subduction advects cold material into the mantle, producing high P/T metamorphism

continent-continent collisions: rapid crustal thickening produces high temperatures at moderate temperatures, followed by cooling

ultrahigh-pressure metamorphism: small pieces of continental material subducted to as much as 200 km are eventually regurgitated, leading to ultrahigh P/T metamorphism

Contact Methamorphism

discernible in the decussate texture of this metavolcanic wall rock

Regional contact metamorphism is caused by pluton intrusion and cooling over a large area. Shown here is Cretaceous plutonism and cooling from the Dabie Shan of China

The shallowest levels of ocean crust are only weakly metamorphosed to zeolite facies

The hallmark of magmatic extension is sheeted dikes, which are usually partially altered to greenschist facies -sheeted

The underlying gabbros are heterogeneously altered, mostly around hydrothermal circulation zones - layered gabbro

"Tethyan-type" ophiolites are emplaced onto passive continental margins by subduction of the continental margin beneath an intraoceanic, immature arc

Specifically, intraoceanic thrusting may begin along transform faults where young and old lithosphere are juxtaposed

This process leads to the formation of a metamorphic sole with an inverted metamorphic gradient

U/Pb zircon and 40Ar/39Ar dating shows that the intraoceanic thrusting took <2 Myr

Leading to a cooling history that is very rapid (and very similar) in both the oceanic crust and the metamorphic sole

The continental margin rocks record high pressures and the ophiolite sole shows high temperatures that are easily modeled as the result of emplacement of very young (<2 Myr old) lithosphere. The continental margin underwent deep burial and minor heating followed by cooling and decompression, whereas the metamorphic sole underwent substantial heating and minor compression and then cooling and decompression P-T diagram

Subduction zone: Blueschist-facies rocks are often incompletely reacted to the high-pressure assemblage, leading to variable preservation of igneous textures and/or minerals, as exhibited by this outcrop from the Klamath Mountains

Subduction zone: Eclogites in many locations occur as blocks within a more felsic matrix

Subduction zone: The host gneiss is usually, after considerably more searching, also found to contain evidence of high-pressure metamorphism. Other localities, such as Oman, however, contain extensive intact tracts of high-pressure rocks

Subduction zone:This eclogite from Dabie Shan China contains abundant phengite, the Mg+Fe+Si-rich form of muscovite. The eclogite at Verpeneset, Norway, contains Na-pyroxene + garnet (which define an eclogite), plus rutile, kyanite, and zoisite

"Warm" subduction zones--a result of slow subduction and/or young subducted lithosphere and "cold" subduction zones-- a result of rapid subduction and/or old subducted lithosphere, have different thermal regimes, dehydration depths, mantle wedge temperatures and circulation, and styles of volcanism

Collision "continent-continent"

Collision "continent-continent''

Deep-level constraints on the present-day structure of the orogen are provided chiefly through seismic reflection and refraction data

the southern margin of the Plateau is underlain by India and the northern margin of the Plateau is underlain by Asia.

The present-day high (5 km) elevation of the plateau is believed to be related to large-scale crustal thickening, combined with delamination of the mantle lithosphere and asthenosphere reaching to unusually shallow levels

The thermal evolution during continental thickening can be modeled with a 1-D "sawtooth" thermal gradient, and attains high temperatures largely through the extreme thickness of radiogenic material

Continental thickening can lead to interesting kinds of melts and residuum that subsequently influence the deformation and evolution of the orogen

An example of the texture produced by such melting

An example of the texture produced by such melting -gneiss dike

Typical Barrovian continental metamorphism is Kangmar Dome in southern Tibet

Kangmar P-T - The calculated P-T conditions/paths of various samples indicate a large range of pressures from ~3 to 8 kbar, but a modest range in temperature from ~400 to 600�C

This group of P-T paths can be explained as part of what one would expect from the kind of burial and heating characteristic of Barrovian metamorphism

Here are the calculated P-T conditions of samples shown in the cross section Kangmar

The P-T conditions of the samples constrains a surprisingly complicated tectonic history

A modern analog for UHPM is the Hindu Kush mountain range of central Asia

earthquakes down to depths of xxx km in material that has seismic wave speeds consistent with continental crust

Cross Section

These subducted continental rocks are exhumed through the mantle following tearing of the slab (called "slab breakoff")