Term 2: Thrust and Orogenic belts

Question 1

Orogeny basics:

Answer 1

• Orogeny = structural processes at convergent plate boundaries
- most continental basement is comprised of old orogenic belts
• Reflects crustal accretion process that forms continental crust and lithosphere
• Crust thickens; therefore, creates high elevations via isostasy, and topographic relief via erosion

Question 2

Wilson Cycle

Answer 2

The main regions of shortening in all orogenic belts are fold-and-thrust belts.
 One rock on top of the other

Some thrust belts accommodate 100s to 1000s of km shortening.

They may represent re-stacked, previously extended margins of continents that collide

Plate tectonic theory, which involves rigid plates moving on a sphere, was not intended to explain structures like those of the Alps. We need another perspective

Weakened zones of continental crust allow for deformation

Question 3

What is a thrust?

Answer 3

• A thrust is ‘An originally low-angle contractional fault in which one body of rock is moved via the fault surface over another body achieving a net shortening of datum surfaces’
 - in simple English, thrusts are faults that produce shortening and thickening of the crust
• Brittle kinematic indicators in thrust systems – should mainly be in the dip slip direction

Question 4

Terminology in thrust and fold belts

Answer 4

• Displaced rocks are ‘allochthonous’ - thrust sheet or nappe
• Undisplaced rocks are ‘autochthonous’
• Flat-lying thrust sheets when eroded may form ‘klippen’ or ‘windows (fensters)’

Question 5

Thrust diagnostic properties

Answer 5

• Cut up stratigraphic section
• Place older on top of younger rocks
• Thicken or duplicate section
• Ramps/flats in both HW and FW - (HW flat on FW flat, HW ramp on FW flat HW flat on FW ramp)
• Ramps lead to formation of ‘passive folds’ – fault bend folds

Question 6

Tip-line folds

Answer 6

• Active folding occurs at and beyond thrust tips – fault propagation folds
Shortening accomplished by folding
Thrust loses displacement upwards
Tip = edge of fault plane where displacement becomes zero

Question 7

Thrust systems

Answer 7

• Branch points/lines
• Blind thrusts
• Imbricate fan
• Duplex containing fault bounded ‘horses’
• ‘roof’ thrust

Question 8

Thrust propagation

Answer 8

‘foreland propagating’ thrust sequence – is normal – and produces ‘Piggy- back’ thrusting
• Older thrusts are folded by younger, lower thrusts

• Amount of slip versus length of horse controls the type of duplex
- small slip - ‘Hinterland’ dipping - normal
- large slip - ‘foreland’ dipping
- intermediate - ‘Antiformal’ stack
• ‘hinterland propagation’ sequence leads to an ‘overstep’ sequence

• Common to find ‘out of sequence’ thrusts formed by a process of ‘breaching’
o Also ‘backthrusts’ - these are common at front of foreland thrust belts -triangle zone’

Question 9

Transfer Zones

Answer 9

•	Displacement along single faults may decrease along strike to zero, but, overall regional shortening remains constant
o	How?
o	Concept of transfer zones
	‘soft’ linkage
	‘hard’ linkage - ‘transfer’
	or ‘tear’ faults

Question 10

Foreland Basins – Sedimentary Basins between the front of a mountain chain and the adjacent craton

Answer 10

• Form along the continental interior flanks of continental-margin orogenic belts
• Structural thickening (e.g. by stacks of thrusts) drives tectonic subsidence
• e.g. Modern day Zagros foreland (Iraq)

Question 11

Lateral transfer of upper crustal mass triggers a series of responses

Answer 11

• Produces topographic high and adjacent basins (foreland basins)
• Sediments eroded from the high and fill the basins. Erosion
– causes denudation of mountain belt
• Sedimentation causes loading and more subsidence
• Thrust propagation takes place further and further onto the foreland
• Foreland basin stratigraphy is cannibalised by next thrust
• wedge

Question 12

Sedimentation and Petroleum

Answer 12

• Oldest deposits found in foreland basins are dominated by fine grained, turbiditic sediments.
• Topography and sediment supply is relatively low – mountain belt not extablished.
• Later deposits of foreland basins are dominantly shallow water or continental - abundant sediment supply. It should be pointed out, however, The very front of the thrust belt may act as barrier to sedimentation.
• For example, in the southern Pyrenees a lot of material has been deposited from fluvial channels running parallel to the thrust belt front. These river systems cut through the frontal thrust structures at localised positions, carrying with them sedimentary material derived from within the orogenic belt.
• Sedimentary fill often termed ‘molasse’ conglomerates and sandstones deposited as alluvial fans and lacustrine deposits – post tectonic (flysch – interbeds of marine shales and greywacke sandstone during syn-tectonically during thrust and fold growth)
• Source rocks from pre-compressional rift successions, Clastics of foreland basin provide reservoirs, Compressional Fold Traps
• Uplift may impact maturation, Fault reactivation may impact seal integrity

Question 13

Ophiolites

Answer 13

fragments of oceanic lithosphere emplaced on land

Question 14

Accretionary prisms

Answer 14

thrust belts of scraped-off oceanic sediment & basement at subduction zones
• They are sometimes known as Accretionary complexes, or subduction-accretion complexes (also used to include the adjacent arc).
• They are an excellent way of creating new continental crust

Question 15

Thin vs Thick skinned locations

Answer 15

Foreland - thin skinned

Hinterland - thick skinned

Question 16

Thick skinned deformation

Answer 16

Thick-skinned deformation - shortening, deformation & metamorphic grade generally increase towards the suture

Question 17

PT-t paths in orogenic belts

Answer 17

Most orogenic belts show a clockwise path, burial to heating to uplift to unroofing
• Some however show anticlockwise paths
• Crustal thickening (& other factors) may lead to generation of leucogranites by crustal melting
• This type of melting of crustal rocks (often termed ‘anatexis’) is typical of collisional orogens and quite distinct from mantle derived melting products (Basalts, Andesites, Diorites, Granites)

Question 18

Andes

Answer 18

Orogeny without collision
• The Alps and Himalayas formed by continent-continent collision.
• Continental thrusting can occur during oceanic subduction
• “compressional convergent boundaries” – e.g. Andes, Mexico
• Therefore, there is no need for continental collision for orogeny – slab controls architecture

Question 19

The “mechanical paradox of thrusting”

Answer 19

• Forces applied to make large scale thrust sheets move must exceed the rock strength. So why do they move?
• Role for fluid pressure:
σc = σo + tanf(σN – Pf)

σc = critical shear stress for fault slip
σo = cohesive strength (0 if the rock is pre- faulted)
Tanf = coefficient of internal friction
σN = normal stress
Pf = pore fluid pressure

Compressed rock = built up of fluid pressure = acts as cushion ‘hovercraft effect’ so thrust sheet moves large distance over lubricated surface

Question 20

Thrust mechanics

Answer 20

• Critically tapered wedges:
o The idea that fold and thrust belts deform to keep the critical angle, a + b, constant
• Note the assumption that the thrusts all root into a basal “sole” thrust, at the base of the wedge.

Question 21

Examples of critical taper model fold-and-thrust belts:

Answer 21

• Canadian Rockies
• Western Taiwan

• The Himalayas may fit the critical taper model.
o Not only do cross-sections based on fieldwork and seismic sections suggest a northward-thickening wedge, but the earthquakes are thrusts, dipping gently north.
o Note – Thrust, extensional (normal) and strike-slip eqs all present

“Escape” or “extrusion” tectonics: moving lithospheric blocks laterally out of the collision zone
• Lateral extrusion: movement of india, pushes blocks towards china and Indochina

Question 22

Predicted characteristics of critical taper fold-and-thrust belts:

Answer 22

• The “toe” of the fold-and-thrust belt maintains a critical angle.
• The angle of critical taper is a function of the rock properties in the deforming wedge, pore fluid pressure and the strength of the basal detachment thrust (“decollement” horizon).
• Important parameters are the shear stress along the basal thrust and the gravitational force related to the height of the wedge – these govern angles, a + b

Angle a may vary across a wedge, and over time, leading to different behaviour, including thrusting and extensional faulting at the same time

Wedge theories can explain high pressure rocks brought to the surface of the earth

Question 23

Lithosphere buoyancy:

Answer 23

• Elevated crust is associated with a buoyancy force, because of the excess gravitational potential energy of the high area.
• This explains why plateaux don’t undergo shortening and uplift beyond a limit. What sets the limit is another story.
• If Tibet’s isostasy is only related to the crust, it will have elevations as it has (~5 km)
• But, Molnar et al argued that a cold, dense mantle lithosphere root should drag the surface down to ~ 3 km.
• So, why is it at 5 km after all?
The idea: Replacing cold, dense mantle lithosphere by hot, less dense asthenosphere raises the surface of Tibet up to 5 km