Geologische Rundschau

, Volume 80, Issue 2, pp 321–332 | Cite as

P-T-t paths of collisional orogens

  • Rebecca A. Jamieson
Article

Abstract

Pressure-temperature-time (P-T-t) paths summarize the changes in pressure and temperature imposed on a metamorphic rock during orogenesis. They provide a convenient framework for the interpretation of complex metamorphic histories and also offer insight into the thermal and tectonic factors controlling metamorphism in collisional orogens. P-T-t data are acquired through a combination of textural observations, thermobarometry, and thermochronometry, and assembled into a P-T-t path using geological constraints. One-dimensional P-T-t models, assuming instantaneous deformation and thermal relaxation by conduction, are flexible and useful for testing tectonic models, particularly where geochronological constraints are available. Two-dimensional models allow more sophisticated deformation geometries and allow the effects of advection to be incorporated. Analysis of collisional orogens in terms of critical wedge theory can yield P-T-t paths that reflect coupling between thickening, uplift, exhumation, erosion, and convergence. Where rates of erosion approach rates of tectonic uplift, as is currently happening in the Southern Alps of New Zealand, high-grade metamorphic rocks can be exhumed rapidly from considerable depth. Alternatively, rapid exhumation may reflect gravity-driven extension in an over-steepened or thermally weakened orogen.

Keywords

Metamorphic Rock Orogen Tectonic Uplift Orogenesis Instantaneous Deformation 

Zusammenfassung

Druck-Temperatur-Zeit (P-T-t) Pfade fassen die Änderungen in Druck und Temperatur zusammen, die den metamorphen Gesteinen während der Orogenese aufgeprägt wurden. Sie liefern einen passenden Rahmen für die thermischen und tektonischen Faktoren die die Metamorphose in Kollisionsorogenen steuern. P-T-t Daten werden erworben durch eine Kombination von textureilen Beobachtungen, Thermobarometrie, Thermochronologie, und werden in einem P-T-t Pfad unter Berücksichtigung von geologischen Grenzen zusammengefaßt. 1-dimensionale P-T-t Modelle die gleichzeitige Deformation voraussetzen sind flexibel und hilfreich um tektonische Modelle zu testen, insbesondere dort, wo geochronologische Grenzen zur Verfügung stehen. 2-dimensionale Modelle gestatten raffiniertere Deformationsgeometrien, und erlauben es die Advektionsprozesse zu integrieren. Analysen von Kollisionsorogenen im Sinne der kritischen Grenzwinkeltheorie kann P-T-t Pfade liefern, die eine Kopplung zwischen Verdickung, Heraushebung, Freilegung, Erosion und Konvergenz wiederspiegeln. Wo sich die Erosionsraten den Heraushebungsraten annähern, wie es zur Zeit in den südlichen Alpen Neuseelands passiert, können hochgradig metamorphe Gesteine aus erheblichen Tiefen freigelegt werden. Andererseits kann schnelle Freilegung auch gravitationsgesteuerte Ausdehnung in einem übersteilten oder thermisch instabilen Orogen widerspiegeln.

Résumé

Les trajets pression-température-temps (P-T-t) représentent les changements de pression et de température subis par les roches métamorphiques au cours d'une orogenèse. Ils fournissent un cadre adéquat à l'interprétation d'histoires métamorphiques complexes en même temps qu'ils éclairent les facteurs thermiques et tectoniques qui régissent le métamorphisme dans les orogènes de collision. Les données (P, T, t), obtenues par combinaison des observations structurales, de la thermo-barométrie et de la thermochronométrie, sont assemblées dans un trajet P-T-t qui tient compte des contraintes géologiques. Des modèles P-T-t, à une dimension, qui supposent une déformation instantanée et une relaxation thermique par conduction, présentent une certaine souplesse et sont utiles lorsqu'il s'agit de tester des modèles tectoniques, en particulier si on dispose de contraintes géochronologiques. Les modèles à 2 dimensions autorisent des geometries déformatives plus sophistiquées et permettent d'incorporer les effets d'advection. L'analyse d'orogènes de collision avec mise en æuvre de la théorie du coin critique, peut fournir des trajets P-T-t qui reflètent les connexions existant entre l'épaississement crustal, le soulèvement, l'exhumation, l'érosion et la convergence. Lorsque la vitesse d'érosion avoisine celle du soulèvement tectonique, comme c'est couramment le cas dans les Alpes du Sud de Nouvelle Zélande, des roches de degré métamorphique élevé peuvent être exhumées rapidement à partir de profondeurs considérables. Alternativement, une exhumation rapide peut traduire une extension à contrôle gravifique dans un orogène très redressé ou thermiquement instable.

Краткое содержание

С помощью термобаром етрических параметр ов удалось проследить и зменения давления и температуры, воздейс твующих на метаморфн ые породы во время ороге неза. Они создают соответству ющие обрамление для термических и тектон ических факторов, управляющих метамор физмом при орогене коллизий. Данные о Р-Т-t получают при комплексном рассмот рении наблюдений за текстурой, термобаро метрией, термохронол огией, учитывая и границы ге ологических единиц. С помощью одноразмер ных моделей Р-Т-t удается проследит ь деформацию; эти модели гибки и помога ют проверить тектони ческие модели именно там, где установлены гехроно логические границы. Двух размерн ые модели разрешают рас смотреть сложную гео метрию деформаций и вписыва ть туда процессы латерального движен ия масс мантийного материала. Рассмотре ние орогенов коллизи й с точки зрения теории з игзагов границ может указать путь для PTt, ука зывающий на связь между процессами уто лщения, поднятия, обна жения и сближения. Там, где ск орость эрозии приближается к скоро сти поднятия, как это имело место в свое вре мя в Южных Альпах Новой Зеландии, пород ы глубин с высокой степенью метаморфиз ма могут оказаться об наженными. С другой стороны, быст рое обнажение может отображать так же и расширение в сверхкрутых, или терм ически нестабильных орогенах, под влияние м силы тяжести.

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Copyright information

© Ferdinand Enke Verlag Stuttgart 1991

Authors and Affiliations

  • Rebecca A. Jamieson
    • 1
  1. 1.Department of GeologyDalhousie UniversityHalifaxCanada

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