Please enable JavaScript.
Coggle requires JavaScript to display documents.
Serpentinite Geodynamic Implications (Deep equakes & subduction…
Serpentinite Geodynamic Implications
Subduction zone earthquakes
Mostly in seismogenic zone
mid-lower crust downdip of trench (~20 - 40 km)
on plate interface
Some equakes
@ greater depth
i.e. mantle zs > seismogenic zone z
Deep equakes
& subduction
Earthquakes occur within subducting slab
Dehydration reactions responsible ?
Cause
1. Eclogitisation weakens crust
Pore presure increases
due to fluid released by serpentinite dehydration
Equakes @ depths ~ 500 - 300 km (> seismogenic zone)
Earthquakes
at depth cause
1. Dehydration of subducting slab
i.e. water released from alteration metaseds / metabasalts
Mantle wedge = hydrated by fluid released
--> forms serpentinite & brucite
Sediments ∆ to eclogite
which weakens crust
Crust breaks with continued subduction & bending
due to eclogite weakening it
Decoupling Upper & Lower Crust
Decoupling can occur between:
1. Oceanic crust
which is weathered into serpentinite
2. Eclogitised sediment on slab
i.e. weaker & contrasting behaviour vs oceanic crust
∴ separation of upper crust & lower subducting crust
Ladakh Juxtaposition Serp & Eclogite
Eclogite & serpentinite juxtaposed in Ladakj
Zildat serpentinites = mantle-derived
Tso Morari eclogites = slab-derived
Units metamorphosed in similar conditions
T~600°C, P~20 kb
Similar deformation & shear sense
Yet differing origins
--> mantle vs subducting slab
Ladakh Juxtaposition
Eclogites & serpentinites found together today
Similar deformation, shear sense & metamorphosed in similar conditions
Yet: differing origins
Exhumation of deep rocks
Eclogite
(Tso Morari)
Eclogitisation of upper crust at Z > 50 km
Eclogite = brittle ∴decouples from lower crust
Serpentinite
(Zildat)
Forms by mantle wedge hydration
(by fluids released from slab eclogitisation)
Serpentinite: Low density & ductile behaviour
∴ lubricates exhumation of serpentinite & eclogite blocks plucked from slab
Serpentinite
= lubricant
Serpentinite = ductile (weak) & low density (~2.7)
Serpentinite rises buoyantly
(less dense vs surrounding mantle)
Serp plucks upper crustal eclogite with it
∴ Acts as lubricant facilitating exhumation of subducted rocks
Further evidence serp aids exhumation
Serpentinite often found associated with
HP & UHP metamorphic belts = result of subduction
∴ Serpentinite aids exhumation
bringing these v deep rocks to surface after subduction