Week 12 Flashcards
Orogeny =
structural processes at convergent plate boundaries
Most continental basement = old orogenic belts
How to old orogenic belts achieve their high elevations and topographic relief?
High elevations = isostasy
Topographic relief = erosion
- loading and more subsidence
- thrust propagation
Plate tectonics
= rigid plates moving on a sphere
Doesn’t explain patterns of behaviour in continents
Thrust =
originally low angled contractional fault where one rock body moves over another via fault surface
= net shortening/thickening/strata duplication
e.g. Moine thrust zone, Scotland
Hinterland characteristics
Penetrative deformation
Thick-skinned
Increased shortening/deformation/metamorphism towards suture
Leucogranites
Foreland characteristics
Localised deformation
Syn-collisional sediments in flexural trough
- loaded = flexes = ‘forebulge’ + foreland BASIN
Thin-skinned i.e. no basement
Allochtonous =
not in original position
Autochthonous =
in place of deposition
What are allochthonous and autochthonous sediments separated by?
Décollement
How are klippen/fensters formed?
When allochthonous thrust sheets/nappes are eroded
In what direction do brittle kinematic indicators form?
Mainly in dip direction
N.B. On fault SURFACE
What controls the duplex type?
Amount of slip vs horse length
SMALL = ‘hinterland’ dipping = normal
LARGE = ‘foreland’ dipping
INTERMEDIATE = ‘antiformal’ stack
Foreland basin =
sedimentary basin between mountain chain and adjacent carton
- form along continental interior flanks of continental margin orogenic belts
Processes in the foreland basin
- STRUCTURAL THICKENING
= thrust stacks and tectonic subsidence
e.g. Zagros, Iraq - FLEXURE of foreland basin
= marine/non-marine
Types of collision boundaries
- Continental-continental collision
2. Andean type active continental margin
Types of folds formed by thrusts
Fault-bend-fold
Fault propagation fold
Detachment fold
Trishear folds
Detachment fold =
layers above deform more than substrate
Piggy-back formation
When younger horses thrust over older horses
What is the tip of a fault/fold?
Where displacement = 0
The edge of the fault plane
Where do back thrusts form?
Common at FRONT of foreland thrust belts in ‘triangle zone’
= thrusts in opposite direction to general thrusting direction
Soft vs hard linkage
Hard linkage = tear/transfer faults
Displacement along single faults decreases along its strike to 0, but overall shortening constant due to TRANSFER ZONES
Sedimentation in thrust belts
Foreland = oldest deposits = fine-grained, turbiditic
- Mountain belt not established = low sediment supply
- Later = deposition of shallow water/continental = abundant
N.B. Front of thrust belt = ?barrier to sedimentation e.g. S Pyrenees deposited laterally // to thrust belt front
Molasses =
conglomerates/sandstones deposited as alluvial fans/lacustrine deposits
e.g. post-tectonic flysch = interbreeds of marine shales/greywacke if submerged under water
Example of petroleum in thrust belts
Oligocene-Pliocene E Venezuela, Columbia
Canadian Rockies
Source rock
(for petroleum)
from pre-compressional rift successions
Reservoir
Foreland basin clastics
Trap
Compressional fold
What can compromise petroleum in thrust belts?
Front of thrust belt = ?barrier to sedimentation e.g. S Pyrenees deposited laterally // to thrust belt front
Uplift impacts maturation
Fault reactivation impacts seal integrity
Ophiolite =
oceanic lithosphere fragment emplaced/obducted on land
Commonly pre-dates initial continental collision
Accretionary prism =
thrust belt of scraped oceanic sediment/basement at subduction zones = new continental crust (!)
- accreted onto NON-subducted plate
- underplating
Underplating =
Emplacement beneath prism = accretionary prism
What kind of metamorphism takes place in the Hinterland?
Classic Barrovian metamorphism
- major ductile fold nappes and contractional shear zones
Leucogranites =
generated by crustal melting “anatexis” due to thickening etc
- distinct from mantle-derived melting products
Andes
examples of continental thrusting during oceanic subduction
Principles/assumptions in line length restoration
Plane strain
- deformation in plane of section
Constant bed length
Constant area
Simple shear
Trishear/kink-folds
No inversion
Projected beds above eroded section
Top basement restores to horizontal
% shortening, e =
(l-lo)/lo
Crossed fault planes in focal mechanism =
oblique
Normal vs inverse progression
Normal = ramps younger in transport direction
Inverse = ramps younger opposite to transport direction
