Transcript Late - to post-orogenic tectonic processes and exhumation

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)
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/HORISONTAL
STRETCHING
(IMPORTANT FOR MID AND LOWER CRUST AFTER EXHUMATION TO AMPHIBOLITE FACIES)
4) HINTERLAND EXTENSION FORLAND SHORTENING
(IMPORTANT AT AN EARLY STAGE OF COLLISION)
5) WHOLE-SALE EXTENSION BY PLATE-DIVERGENCE
and/or TRANS-TENSION (IMPORTANT)
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)
AMBIENT FORCE
FROM PLATE MOTION
BODY FORCE FROM
TOPOGRAPHY ON
THE SURFACE
AND ON LITHSPHERE
Crust
Conductive
geotherm
Lithospheric mantle
Adiabatic
geotherm
Vertical stretching/lithospheric thickening
Convective removal
of thermal boundary layer
Higher geotherm leads to
partial melting in the lithosphere
THE THERMAL EFFECT
OF REMOVAL OF THICK
MANTLE LITHOSPHERE
re-equilibration
and extension
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
reference level
Extension gives some easily recognizable features:
1) Thermal:
Narrowing of isotherms; steep geotherm
2) Structural:
Normal faults and detachments
3) 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:
1) Underplating and extension (critical taper)
2) Slab-breakoff and orogenic collapse
3) Diapiric rise along density contrasts
4) Subduction roll-back
5) Plate divergence (including transtension)
Some good actualistic examples:
Himalaya - Tibet plateau Region
Mediterranean Region
-->Agean Sea
-->Italy - Corsica section
-->Alboran Sea (Spain - Morocco)
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