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

Syn- to post-orogenic extension Regional extension Jolivet et al, 1999

Late - to post-orogenic tectonic processes and exhumation mechanisms (ROCKS APPROACHING THE SURFACE) EROSION (MINOR ON A REGIONAL SCALE) THRUST STACKING + EXTENSION AND/OR EROSION (IMPORTANT FOR BRINGING HP AND UHP ROCKS NEXT TO EACH OTHER? 3) VERTICAL CO-AXIAL SHORTENING/HORISONTAL STRETCHING (IMPORTANT FOR MID AND LOWER CRUST AFTER EXHUMATION TO AMPHIBOLITE FACIES) HINTERLAND EXTENSION FORLAND SHORTENING (IMPORTANT AT AN EARLY STAGE OF COLLISION) WHOLE-SALE EXTENSION BY PLATE-DIVERGENCE and/or TRANS-TENSION (IMPORTANT)

(from: Molnar and Lyon-Caen) Map showing major earthquake fault plane solutions and the topography in the Himalayan-Tibetan Region. Notice the strong correlation betwen altitude and contractional earthquakes. Notice also the Dominant NW-SE of the principal tension axes as shown by the normal fault.plane solutions. Normal fault-plane solution Reverse fault-plane solution Strike-slip fault-plane solution (from: 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. (Molnar and Lyon-Caen)

BODY FORCE FROM TOPOGRAPHY ON THE SURFACE AND ON LITHSPHERE AMBIENT FORCE FROM PLATE MOTION

of thermal boundary layer Crust Conductive geotherm Lithospheric mantle Adiabatic geotherm Vertical stretching/lithospheric thickening THE THERMAL EFFECT OF REMOVAL OF THICK MANTLE LITHOSPHERE Convective removal of thermal boundary layer re-equilibration and extension Higher geotherm leads to partial melting in the lithosphere Horizontal stretching/lithospheric thinning Partial melting in astenosphere during decompression Modified from: England & Platt, 1994

From late to post orogenic tectonics in continental collision zones to rifts The end of a Wilson cycle does not mark the end of the tectonic activity in a mountainbelt. In many orogenic belts high-grade rocks formed by the crustal-thickening during collision get quickly exhumed. In many instances the exhumation processes are too fast to be accounted for by erosion alone. We have to resort to tectonic processes to explain the exhumation. The geology and seismic ativity in several modern orogenic belts have an intimate relationship between shortening and extension.

Some definitions: Exhumation --> rocks approaching the surface. Uplift --> rise of the earth´s surface with respect to reference level Subsidence --> lowering of the earth´s surface with respect to Extension gives some easily recognizable features: Thermal: Narrowing of isotherms; steep geotherm Structural: Normal faults and detachments Metamorphic: Metamorphic hiatus exision across structural features 4) Sedimentary: Creation of accomodation space for sediments

An orogenic crust will, however, not go on thickening forever and the topographic elevation will reach a threshold value that depends on the rate of convergence, the strength and density 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 exhumation within the thickened crust by: EROSIONAL PROCESSES Increased topography will increase the precipitation, hence increase the rate of erosion Increased topography will increase the slope instability, hence enhance landsliding and mass transport TECTONIC PROCESSES Extensonal and strike-slip faulting to transport material away from toptgraphically elevated areas

Mechanism resulting in extensional exhumation: Underplating and extension (critical taper) Slab-breakoff and orogenic collapse Diapiric rise along density contrasts Subduction roll-back Plate divergence (including transtension)

Some good actualistic examples: Himalaya - Tibet plateau Region Mediterranean Region -->Agean Sea -->Italy - Corsica section -->Alboran Sea (Spain - Morocco)

EXTENSION CHASES AFTER CONTRACTION EASTWARD MIGRATION OF EXTENSION AT THE SAME TIME AS CONVERGENCE, SUBDUCTION ROLL-BACK EXTENSION CHASES AFTER CONTRACTION EASTWARD MIGRATION OF THE EXTENSION AND COMPRESSION SINCE THE EARLY TERTIARY From Jolivet et al. 2004

From Jolivet et al. 2004

Late-to post Orogenic collapse