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Subduction Channel Units & Deformation (Pseudotachylite Veins (Found…
Subduction Channel Units & Deformation
Pseudotachylite Veins
Found throughout
Austroalpine crystalline basement
Upper plate ~ amphibolite facies ?
Substantial volumes of molten rock
Along fossil plate interface above base of upper plate
Chaotic
Textures
chaotic pseudotachylite = expression of molten rock being under extremely high hydraulic pressures
Dissipated fragments (puzzle like)
Fracture-like pattern
suggesting explosive melt intrusion & instant freezing
Host rock fragments floating in matrix
Inferring palaeoseismic moment
Backcalculate energy required to melt given volume of rock
required to form observed pseudotachylite volume
Lab experiments inform this
Melt given volume of rock with known energy input
Pseudotachylite Significance
Along fossil plate interface above base of upper plate
Delineate unstable slip region
within seismogenic coupling zone
Units in SZ
Upper plate: Austroalpine crystalline basement
Lower Plate: Penninic melange
Penninic Melange
Mess of blocks floating in soft mass
Matrix = flysch
= deep sea sediments deposited via gravitational events
Pseudotachylite Reactivation
After equake: molten rock cools very quickly
Resulting rock volume:
Cryptocrystalline - extremely fine crystals
Very weak - lacking dislocations
Oriented preferentially for shear activation
∴ Pseudoctachylite rock is prone to mylonitic overprint
Dominant deformation mechanism
Dominant deformation mechanism in subduction channel
= pressure solution
Undifferentiated melange
Limestone blocks arranged chaotically
Blocks dragged down subduction zone
Lotsa hydrofracturing due to
--> fluid rich envmts & floating in turbidite matrix
Lotsa different lithologies floating in matrix (chaotic/no system)
Limestone Block Features
Stylolites
Material dissolved by pressure solution into fluid
Fluid supersaturated & overpressurised (wrt CaCO3) migrates elsewhere
Leaves serrated surfaces within rock mass
--> where mineral removed by pressure solution
Veins
Precipitation of CaCO3 accrued in fluid from pressure solution
Stylolites & veins relate