Week 12

Question 1

Orogeny =

Answer 1

structural processes at convergent plate boundaries

Most continental basement = old orogenic belts

Question 2

How to old orogenic belts achieve their high elevations and topographic relief?

Answer 2

High elevations = isostasy

Topographic relief = erosion

• loading and more subsidence
• thrust propagation

Question 3

Plate tectonics

Answer 3

= rigid plates moving on a sphere

Doesn’t explain patterns of behaviour in continents

Question 4

Thrust =

Answer 4

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

Question 5

Hinterland characteristics

Answer 5

Penetrative deformation
Thick-skinned
Increased shortening/deformation/metamorphism towards suture
Leucogranites

Question 6

Foreland characteristics

Answer 6

Localised deformation
Syn-collisional sediments in flexural trough
- loaded = flexes = ‘forebulge’ + foreland BASIN
Thin-skinned i.e. no basement

Question 7

Allochtonous =

Answer 7

not in original position

Question 8

Autochthonous =

Answer 8

in place of deposition

Question 9

What are allochthonous and autochthonous sediments separated by?

Answer 9

Décollement

Question 10

How are klippen/fensters formed?

Answer 10

When allochthonous thrust sheets/nappes are eroded

Question 11

In what direction do brittle kinematic indicators form?

Answer 11

Mainly in dip direction

N.B. On fault SURFACE

Question 12

What controls the duplex type?

Answer 12

Amount of slip vs horse length

SMALL = ‘hinterland’ dipping = normal

LARGE = ‘foreland’ dipping

INTERMEDIATE = ‘antiformal’ stack

Question 13

Foreland basin =

Answer 13

sedimentary basin between mountain chain and adjacent carton

- form along continental interior flanks of continental margin orogenic belts

Question 14

Processes in the foreland basin

Answer 14

1. STRUCTURAL THICKENING
   = thrust stacks and tectonic subsidence
   e.g. Zagros, Iraq
2. FLEXURE of foreland basin
   = marine/non-marine

Question 15

Types of collision boundaries

Answer 15

1. Continental-continental collision

2. Andean type active continental margin

Question 16

Types of folds formed by thrusts

Answer 16

Fault-bend-fold

Fault propagation fold

Detachment fold

Trishear folds

Question 17

Detachment fold =

Answer 17

layers above deform more than substrate

Question 18

Piggy-back formation

Answer 18

When younger horses thrust over older horses

Question 19

What is the tip of a fault/fold?

Answer 19

Where displacement = 0

The edge of the fault plane

Question 20

Where do back thrusts form?

Answer 20

Common at FRONT of foreland thrust belts in ‘triangle zone’

= thrusts in opposite direction to general thrusting direction

Question 21

Soft vs hard linkage

Answer 21

Hard linkage = tear/transfer faults

Displacement along single faults decreases along its strike to 0, but overall shortening constant due to TRANSFER ZONES

Question 22

Sedimentation in thrust belts

Answer 22

Foreland = oldest deposits = fine-grained, turbiditic

1. Mountain belt not established = low sediment supply
2. 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

Question 23

Molasses =

Answer 23

conglomerates/sandstones deposited as alluvial fans/lacustrine deposits

e.g. post-tectonic flysch = interbreeds of marine shales/greywacke if submerged under water

Question 24

Example of petroleum in thrust belts

Answer 24

Oligocene-Pliocene E Venezuela, Columbia

Canadian Rockies

Question 25

Source rock

Answer 25

(for petroleum)

from pre-compressional rift successions

Question 26

Reservoir

Answer 26

Foreland basin clastics

Question 27

Trap

Answer 27

Compressional fold

Question 28

What can compromise petroleum in thrust belts?

Answer 28

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

Question 29

Ophiolite =

Answer 29

oceanic lithosphere fragment emplaced/obducted on land

Commonly pre-dates initial continental collision

Question 30

Accretionary prism =

Answer 30

thrust belt of scraped oceanic sediment/basement at subduction zones = new continental crust (!)

• accreted onto NON-subducted plate
• • underplating

Question 31

Underplating =

Answer 31

Emplacement beneath prism = accretionary prism

Question 32

What kind of metamorphism takes place in the Hinterland?

Answer 32

Classic Barrovian metamorphism

• major ductile fold nappes and contractional shear zones

Question 33

Leucogranites =

Answer 33

generated by crustal melting “anatexis” due to thickening etc
- distinct from mantle-derived melting products

Question 34

Andes

Answer 34

examples of continental thrusting during oceanic subduction

Question 35

Principles/assumptions in line length restoration

Answer 35

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

Question 36

% shortening, e =

Answer 36

(l-lo)/lo

Question 37

Crossed fault planes in focal mechanism =

Answer 37

oblique

Question 38

Normal vs inverse progression

Answer 38

Normal = ramps younger in transport direction

Inverse = ramps younger opposite to transport direction