Term 2: Strike Slip systems

Question 1

2 main classes of strike-slip fault:

Answer 1

• Transform faults (whole lithosphere)– cut whole lithosphere, are conservative plate boundaries
• Transcurrent faults (continental crust) – Intraplate features -often only in crust – although may still be related to a plate boundary

Question 2

6 - main sub-types of strike-slip fault:

Answer 2

• Ridge transforms
• Boundary transforms
• Trench-linked transform or SS fault
• Indenter-linked SS fault
• Tear faults
• Intracrustal

Question 3

Oceanic transforms

Answer 3

• Mid-ocean ridges are offset by transform faults

Question 4

Continental Strike-slip faults

Answer 4

• ‘Andersonian’ wrench faults - only form in isotropic material with vertical σ2 and in conjugate sets

Question 5

Trench-linked strike-slip faults

Answer 5

Occur perpendicular to subducting slab

Tear fault

Question 6

Offset of strike-slip faults: “piercing points”

Answer 6

• Offset geological markers (e.g. dykes, other faults, dipping beds etc) can be used to constrain the offset of a strike-slip fault – not always as easy as it sounds.
• Offset typically varies along the length of a strike-slip fault.

Question 7

Structural complexity in strike slip

Answer 7

• Few strike-slip faults are straight - and faults have bends or step-overs/offsets
 Not planar but bend
• These cause an additonal local 3D component - forming ‘releasing’ or ‘restraining’ bends – pull-apart basins & push-up structures

Question 8

Simple shear

Answer 8

• In simple shear - a 2D deformation
o Start off with a circle, after 1st increment have a strain ellipse with long/short axes at 45º to the boundary
o then axes of ellipse rotate to shear plane, become more elliptical
o Material lines rotate slowly if near shear plane and fast if a long way off

Question 9

Structures in simple shear zones (overview)

Answer 9

• Form in strike-slip in map view
• (but equally can form in thrust or extensional shear zones in section view)
o Folds, fabric and thrusts form normal to the Z axis and parallel to X
o tensile fractures form parallel to Z and normal to X
o Conjugate strike-slip faults form bisected by Z - 1 set are synthetic, other set are antithetic (Riedel R & R’ shears)

Question 10

Structures in simple shear zones (List)

Answer 10

Folds - prograde in angle at initiation
Foliation - continue to initiate at initiation angle
Tensile fractures - prograde in angle at initiation
Strike-Slip shear fractures - Reidel shear - R1 (antithetic) and R synthetic - at 90-material internal friction angle to each other - conjugate pairs

Question 11

Characteristic structures

Answer 11

In map view
• En echelon - offset sub-parallel features oblique to PDZ
• relay structure - if no consistent offset
• anastomosing/braided structures
• have ‘tulip’ or ‘palm tree’ structure in 3D

Question 12

Obliquity and shear sense

Answer 12

• Structures are commonly oblique to the shear zone boundary or principal deformation zone (PDZ) boundaries - sense of obliquity = sense of shear
• If shear in zone builds up, structures rotate toward the PDZ, if zone broadens then get sigmoidal patterns

• Bend geometries
• If sense of shear is same as sense offset/bend then get ‘releasing structure’- if opposite then ‘restraining structures’
 Restraining bends for ‘push-up’ or ‘pressure ridges’ with folds and thrusts

Question 13

Pull-apart basins

Answer 13

• Releasing bends form ‘pull-apart basins’ which can form deep basins in short periods of time
• In general, basin length = displacement on fault, migration of depo centre with time. Get rapid subsidence - complex local stratigraphy. Basin may invert to become uplifted
• Example: Dead Sea

What happens at the limits of strike-slip faults? - Strain may transfer onto thrusts