LATE- TO POST-OROGENIC TECTONIC PROCESSES THE PRESENT DEFORMATION PATTERN HAS A STRONG CORRELATION WITH TOPOGRAPHY, AND CANNOT BE EXPLAINED ONLY FROM THE.

LATE- TO POST-OROGENIC TECTONIC PROCESSES

THE PRESENT DEFORMATION PATTERN HAS A STRONG CORRELATION WITH TOPOGRAPHY, AND CANNOT BE EXPLAINED ONLY FROM THE PLATE-MOTIONS AND AMBIENT FORCES AFFECTING THE REGION

collision

subduction

STABLE EURASIA NORTHAMERICA

PLATE

N. CHINA

OK

PACIFICPLATE

PHSP

S. CHINA

INDIA

AUSTRALIA

AF

Syn- to post-orogenic extension

Regional extension

Jolivet et al, 1999

Late - to post-orogenic tectonic processes and exhumation mechanisms (ROCKS APPROACHING THE SURFACE)

1) EROSION (MINOR ON A REGIONAL SCALE)

2) THRUST STACKING + EXTENSION AND/OR EROSION(IMPORTANT FOR BRINGING HP AND UHP ROCKS NEXT TO EACH OTHER?

3) VERTICAL CO-AXIAL SHORTENING/HORISONTALSTRETCHING

(IMPORTANT FOR MID AND LOWER CRUST AFTER EXHUMATION TO AMPHIBOLITE FACIES)

4) HINTERLAND EXTENSION FORLAND SHORTENING(IMPORTANT AT AN EARLY STAGE OF COLLISION)

• WHOLE-SALE EXTENSION BY PLATE-DIVERGENCEand/or TRANS-TENSION (IMPORTANT)

(from: Molnar and Lyon-Caen)

Normalfault-planesolution

Reversefault-planesolution

Strike-slipfault-planesolution

Map showing major earthquake fault plane solutions and the topography in the Himalayan-TibetanRegion. Notice the strong correlation betwen altitude and contractional earthquakes. Notice also theDominant NW-SE of the principal tension axes as shown by the normal fault.plane solutions.

(Molnar and Lyon-Caen)

Horizontal projections of principal stress axes directions derived from fault-plane solutions (pink-reverse, blue-normal, green-strike-slip) in the previous figure.

AMBIENT FORCEFROM PLATE MOTION

BODY FORCE FROMTOPOGRAPHY ONTHE SURFACE AND ON LITHSPHERE

THE THERMAL EFFECT OF REMOVAL OF THICK MANTLE LITHOSPHERE

Vertical stretching/lithospheric thickening

Horizontal stretching/lithospheric thinning

Modified from: England & Platt, 1994

Crust

Lithospheric mantle

Higher geotherm leads topartial melting in the lithosphere

Convective removalof thermal boundary layer

re-equilibration and extension

Partial melting in astenosphere during decompression

Adiabatic geotherm

Conductive geotherm

From late to post orogenic tectonics incontinental collision zones to rifts

The end of a Wilson cycle does not mark the end of the tectonicThe end of a Wilson cycle does not mark the end of the tectonicactivity in a mountainbelt. In many orogenic belts high-gradeactivity in a mountainbelt. In many orogenic belts high-graderocks formed by the crustal-thickening during collision getrocks formed by the crustal-thickening during collision getquickly exhumed.quickly exhumed.

In many instances the exhumation processes are too fast to be In many instances the exhumation processes are too fast to be accounted for by erosion alone. We have to resort to tectonicaccounted for by erosion alone. We have to resort to tectonicprocesses to explain the exhumation.processes to explain the exhumation.

The geology and seismic ativity in several modern orogenic beltsThe geology and seismic ativity in several modern orogenic beltshave an intimate relationship between shortening and extension. have an intimate relationship between shortening and extension.

Some definitionsSome definitions::

ExhumationExhumation --> rocks approaching the surface. --> rocks approaching the surface.UpliftUplift --> rise of the earth´s surface with respect to --> rise of the earth´s surface with respect to

reference levelreference levelSubsidence Subsidence --> lowering of the earth´s surface with respect to --> lowering of the earth´s surface with respect to

reference levelreference level

Extension gives some easily recognizable features:Extension gives some easily recognizable features:

1)1) Thermal: Narrowing of isotherms; steep geothermThermal: Narrowing of isotherms; steep geotherm2)2) Structural: Normal faults and detachmentsStructural: Normal faults and detachments3)3) Metamorphic: Metamorphic hiatus exision across structural Metamorphic: Metamorphic hiatus exision across structural

featuresfeatures4)4) Sedimentary: Creation of accomodation space for sediments Sedimentary: Creation of accomodation space for sediments

Some definitionsSome definitions::

ExhumationExhumation --> rocks approaching the surface. --> rocks approaching the surface.UpliftUplift --> rise of the earth´s surface with respect to --> rise of the earth´s surface with respect to

reference levelreference levelSubsidence Subsidence --> lowering of the earth´s surface with respect to --> lowering of the earth´s surface with respect to

reference levelreference level

Extension gives some easily recognizable features:Extension gives some easily recognizable features:

1)1) Thermal: Narrowing of isotherms; steep geothermThermal: Narrowing of isotherms; steep geotherm2)2) Structural: Normal faults and detachmentsStructural: Normal faults and detachments3)3) Metamorphic: Metamorphic hiatus exision across structural Metamorphic: Metamorphic hiatus exision across structural

featuresfeatures4)4) Sedimentary: Creation of accomodation space for sediments Sedimentary: Creation of accomodation space for sediments

An orogenic crust will, however, not go on thickening forever An orogenic crust will, however, not go on thickening forever and the topographic elevation will reach a threshold value that and the topographic elevation will reach a threshold value that depends on the rate of convergence, the strength and density depends on the rate of convergence, the strength and density structure of the orogenic lithsophere. structure of the orogenic lithsophere.

Plateau height h ≈ 3.5 km for a convergence rate of ca 5 cm/year

If convergence continues at this rate the plateau will rise to the threshold value, and then grow in width (spread laterally as indicated by pink boxes).For the avereage height (h) to increase, we either have to • increase the rate of convergence, • increase the strength of the rocks • introduce a vertical force lifting the rocks higher, by reducing their average density so that they will float higher.

Increased topography will enhance the rate of Increased topography will enhance the rate of exhumation within the thickened crust by:exhumation within the thickened crust by:

EROSIONAL PROCESSESEROSIONAL PROCESSES• Increased topography will increase the precipitation, henceIncreased topography will increase the precipitation, hence

increase the rate of erosionincrease the rate of erosion• Increased topography will increase the slope instability, henceIncreased topography will increase the slope instability, hence

enhance landsliding and mass transportenhance landsliding and mass transport

TECTONIC PROCESSESTECTONIC PROCESSES• Extensonal and strike-slip faulting to transport material away Extensonal and strike-slip faulting to transport material away

from toptgraphically elevated areasfrom toptgraphically elevated areas

Increased topography will enhance the rate of Increased topography will enhance the rate of exhumation within the thickened crust by:exhumation within the thickened crust by:

EROSIONAL PROCESSESEROSIONAL PROCESSES• Increased topography will increase the precipitation, henceIncreased topography will increase the precipitation, hence

increase the rate of erosionincrease the rate of erosion• Increased topography will increase the slope instability, henceIncreased topography will increase the slope instability, hence

enhance landsliding and mass transportenhance landsliding and mass transport

TECTONIC PROCESSESTECTONIC PROCESSES• Extensonal and strike-slip faulting to transport material away Extensonal and strike-slip faulting to transport material away

from toptgraphically elevated areasfrom toptgraphically elevated areas

Mechanism resulting in extensional exhumation:Mechanism resulting in extensional exhumation:

1)1) Underplating and extension (critical taper)Underplating and extension (critical taper)

2)2) Slab-breakoff and orogenic collapseSlab-breakoff and orogenic collapse

3)3) Diapiric rise along density contrastsDiapiric rise along density contrasts

4)4) Subduction roll-backSubduction roll-back

5)5) Plate divergence (including transtension)Plate divergence (including transtension)

Mechanism resulting in extensional exhumation:Mechanism resulting in extensional exhumation:

1)1) Underplating and extension (critical taper)Underplating and extension (critical taper)

2)2) Slab-breakoff and orogenic collapseSlab-breakoff and orogenic collapse

3)3) Diapiric rise along density contrastsDiapiric rise along density contrasts

4)4) Subduction roll-backSubduction roll-back

5)5) Plate divergence (including transtension)Plate divergence (including transtension)

Some good actualistic examples:Some good actualistic examples:

Himalaya - Tibet plateau RegionHimalaya - Tibet plateau RegionMediterranean RegionMediterranean Region

-->Agean Sea-->Agean Sea-->Italy - Corsica section-->Italy - Corsica section-->Alboran Sea (Spain - Morocco)-->Alboran Sea (Spain - Morocco)

Some good actualistic examples:Some good actualistic examples:

Himalaya - Tibet plateau RegionHimalaya - Tibet plateau RegionMediterranean RegionMediterranean Region

-->Agean Sea-->Agean Sea-->Italy - Corsica section-->Italy - Corsica section-->Alboran Sea (Spain - Morocco)-->Alboran Sea (Spain - Morocco)

EXTENSION AT THE EXTENSION AT THE SAME TIME AS SAME TIME AS CONVERGENCE,CONVERGENCE,

SUBDUCTION ROLL-BACKSUBDUCTION ROLL-BACK

EXTENSION CHASES AFTEREXTENSION CHASES AFTERCONTRACTIONCONTRACTION

EASTWARD MIGRATION OFEASTWARD MIGRATION OFTHE EXTENSION AND THE EXTENSION AND COMPRESSION SINCE THECOMPRESSION SINCE THEEARLY TERTIARYEARLY TERTIARY

EXTENSION AT THE EXTENSION AT THE SAME TIME AS SAME TIME AS CONVERGENCE,CONVERGENCE,

SUBDUCTION ROLL-BACKSUBDUCTION ROLL-BACK

EXTENSION CHASES AFTEREXTENSION CHASES AFTERCONTRACTIONCONTRACTION

EASTWARD MIGRATION OFEASTWARD MIGRATION OFTHE EXTENSION AND THE EXTENSION AND COMPRESSION SINCE THECOMPRESSION SINCE THEEARLY TERTIARYEARLY TERTIARY

From Jolivet et al. 2004

From Jolivet et al. 2004

Late-to post Orogenic collapse