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Mud Volcanoes Mariana Trench (Mariana Seamount Model (1. Accretionary…
Mud Volcanoes Mariana Trench
Forearc mud volcanoes
serpentinite mud volcanoes
--> in forearc region on overriding plates
1 km relief seamounts
forearc = region between oceanic trench & volcanic arc
Serpentinite Mud Composition
Sampled via drilling into serp seamounts (Savou et al 2005)
Exotic clasts from subducting plate = > 6% vol
Clasts include: talc schist, sodic/chlorite amphibole micaschist,
Serpentinite Seamounts
Contain metamorphosed oceanic crust fragments
∴ Subducted - metamorphosed - exhumed with serpentinite
Mariana Seamount
Model
1.
Accretionary wedge fractured
as seamounts subduct on slab
Faulting accommodates displacement
of blocks formed on overriding plate by fracture
Mantle wedge serpentinised
when slab sediments dehydrated at ~ 10 km depth
Buoyant serpentinite upwells to surface
through faults / fractures forming mud volcanoes above
Fragments from subducting slab
= plucked by rising serpentinite mud
Serpentinite
Mud Volcanoes
Signif structures of erupted serpentinite
Height ~ 2.5 km
Width ~ few tens km !
Deformation
Normal faulting at top of volcanoes --> collapse under its own weight
Thrusting at edges
--> shows lateral spreading following collapse
Décollement Thrusts
Thrusts at volcano base
e.g. Turqoise Seamount, Celestial Seamount
Basement substrate may be incorporated into volcano base
under weight of volcano via basal thrusts / décollement faults
or not - Big Blue seamount
Mariana Mud Volcanoes Summary
Mud volcanoes form atop faults = buoyancy driven
Episodic eruptions
from central conduit
--> suggested by internal layering
Lateral spreading
via basal décollements
--> gravitational collapse?
Substrate sediment may be incorporated
in sliding / thrusting @ base
Incorporated - celestial seamt or not - big blue seamt
Substrate
Type Varies
Undeformable
Slip along basal décollement possible
since
low friction coefficient @ base
(μbase ~ 0.09)
Deep seated normal faulting inside seamount
(μinternal = 0.49)
∴ summit subsidence & lateral spreading
Deformable
Normal faults extend into substrate
∴ incorporate substrate into edifice via thrust wedges
Still low μbase, but μinternal = lower
Seamount weaker & lacks cohesion ∴
deformable
How sliding possible @ base?
Water from mud / fluid upwelling
--> from underlying sediment compaction
∴
Decoupling possible
since
shear strength & basal friction lowered by :
High pore fluid pressures
@ contact of impermeable layer
Underlying sediment not easily deformed
Why Pf
High?
Possible if impermeable layer
@ base of volcanic edifice
∴ Upwelling fluid accumulates @ mud edifice-substrate contact
How substrate incorporated into edifice?
Sliding along décollement with:
deformable underlying sediment
little fluid input to volcano
∴ Substrate incorporated via thrust faulting
Alternate reason substrate incorporated into some seamts & not others
Two different stages in seamount formation
Active vs Inactive seamounts
Active: no substrate incorporated since basal decoupling
Inactive: substrate incorporated @ edifice base