METAMORPHISM INRELATION TO PLATE

TECTONICS

• Metamorphic rocks result from the forces active duringplate tectonic processes. The collision of plates,subduction, and the sliding of plates along transformfaults create differential stress, friction, shearing,compressive stress, folding, faulting, and increased heatflow.

• The tectonic forces deform and break the rock, creatingopenings, cracks, faults, breccias, and zones ofweakness along which magmas can rise. Generallyspeaking, the greater the tectonic forces, the higher thepressures and temperatures affecting a rock mass andthe greater the amount of resulting structuraldeformation and metamorphism.

Dynamo thermal Metamorphism, Beforecollision, in subduction zone

• In convergent margin settings where ocean basins close,continental lithosphere can approach and eventually collide withcontinental lithosphere on the opposing plate.  As the two plateswith continental lithosphere begin to collide, subduction willeventually cease as it becomes difficult to subduct low densitycontinental lithosphere.

• Compressional stresses generated in the collision zone will resultin folding and thrust faulting of the rocks to ultimately create afold-thrust mountain belt.  Some rocks will be thrust upward tocreate the high peaks of the mountain range and some will bepushed downward to higher temperatures and pressures andbecome metamorphosed.  

Such collisional events are currentlytaking place along the Alpine Himalayanbelt which extends from western Europeto southern Asia and has resulted in themountain ranges known as the Alps andthe Himalayas.

Continental-continental collision

An overview of metamorphism in relation to tectonicregimes:

• Areas of crustal thickening and mountain building:greenschists, amphibolites granulites and eclogites(particularly if there are magmatic intrusions).

• Subduction zones: Characterized by low geothermalgradients: zeolite pumpellyite-actinolite facies /lawsonite albite facies blueschist facies, eclogites

• Ridges and rift valleys: characterized by highgeothermal gradients contact and ocean floormetamorphism.

• Areas of magmatic activity; volcanic - plutoniccomplexes: greenschists, amphibolites, granulites.

Metamorphism and Plate Tectonics• At present, the geothermal gradients observed are

strongly affected by plate tectonics.  • Along zones where subduction is occurring, magmas

are generated near the subduction zone and intrudeinto shallow levels of the crust.  Because hightemperature is brought near the surface, thegeothermal gradient in these regions, and may be inthe range of 50 to 70°C/km, and contactmetamorphism (hornfels facies) results.

• Because compression occurs along a subductionmargin (the oceanic crust moves toward thevolcanic arc) rocks may be pushed down to depthsalong either a normal or slightly higher thannormal geothermal gradient  25°C/km.  Actuallythe geothermal gradient is likely to be slightlyhigher than B (30°C/km), because the passage ofmagma through the crust will tend to heat thecrust somewhat.  In these regions we expect to seegreenschist, amphibolite, and granulite faciesmetamorphic rocks.

• Along a subduction zone, relatively cool oceaniclithosphere is pushed down to great depths.  Thisresults in producing a low geothermal gradient(temperature increases slowly with depth) of 10 -15°C/km.  This low geothermal gradient ("C") inthe diagram above, results in metamorphism intothe blueschist and eclogite facies.Examples of this type include the Franciscan (withthe Sierra Nevada) in the western U.S.A., and theSanbagawa (and Abukuma) belts in Japan.

• B- Stable continental interiors anddeep sedimentary basins:

These are characterized by burial metamorphism withclockwise P-T paths, and peak T in the greenschist toepidote amphibolite facies.

• C- Divergent Plate Boundaries:

Ocean Floor Metamorphism:

Continental rifts: In such settings, it is very common to find“metamorphic core complexes”, defined as areas that aretopographically high and that consist of igneous andmetamorphic rocks that display anomalous deformation and

metamorphism relative to the surrounding rocks.

Conservative Plate Boundaries:

Cataclasis and Mylonitization