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9.2 Arakapas Transform Zone (Arakapas Bonninitic Pillows (Why cracked ?…
9.2
Arakapas Transform Zone
Topog high nr transform
Lageia
Large, laterally continuous faults ∴ signif fault enscarpments
Height differential
= driver for energy changes & high energy reworking
Poss cyclicity: debris flow - background sed - lava flow ?
All volcaniclastic deposits
but deposited via mechanisms of varying energy
but delivered in variable energy envmt
Debris flows
Form on huge enscarpments at sealevel
material input from many levels
Changes in sorting/grain size
reflect changes in source material available
Why fines toward lava?
Coarse material all reworked already/source = depleted ?
Poss cyclicity: debris flow - background sed - lava flow ?
Lava
Flat, very mafic lava w spheroidal weathering
Baked margin w underlying sediments sticking to it
Lava flow ?
suggests movement of lava over partically consolidated seds
Drag structures in soft sed below
Lageia Sequence
Debris flow event
Background sed or block (hiatus - pinch out)
Debris flow event 2 (turbidity current?)
Lava flow (or intrusion?)
Coarse debris flow
Basalt/gabbro clasts, some seds
Broken up sequence from top ophiolite / cover
Some reworked blocks
High energy envmt
lotsa
Arakapas Bonninitic Pillows
Bread-crust texture
pillow edges not smooth & fresh
suggests V HIGH TEMP
Why cracked ?
Erupted @ much higher temps
∴ lavas are of bonninitic composition (~150°C on eruption)
Thick glass crust shatters as erupts
Crust cracks in polygonal nature
Upper pillow lavas
contrast lower, andesitic pillows seen previously
Subduction signature
High Mg (primitive?) but High SiO2 (evolved?)
V. depleted in heavy elements (that melt is enriched in i.e. Ti, Zr)
Enriched in water transported elements
--> remelting driven by water from subducting slab
supports 'dehydration melting'
Hyaloclastite & flows in between
Clastic glassy fragmental material
Int: Records mvmt of lava unit
Why Troodos = depleted ?
Dont think its from subduction initiation
From area where SZ existed previously ?
Why bonninites here?
Transform provides
Open conduit for melt travel from Z
Acts as conduit for water being fluxed into the mantle
Given we see same signature in Troodos
suggests active conduit allows sample deeper parts
(high temp, depleted melt signature)
Dyke Def
Nr Ora
Dykes = v discontinuous (lenses etc)
Interp
As approach transform fault,
--> pockets of o.c. = rotated around
Block injection / dyke injection
Many dykes here show
fault block rotation / reinjection / dyke def
Sheeted dykes
Dhierona
Dykes = anastomosing, braided / crosscut
Background dykes = v sheared & jointed
∴ lotsa internal deformation
Primitive dykes injected into v sheared bckgrnd
Different colours
--> suggest ↑ evolved magma
Faint yellow dyke - anastomosing
Why dykes sinuous?
Dyke follows planes of weakness
May follow fault planes then resumes path to surface
Magma injects in direction of easiest passage
Depth vs
nr surface
Depth - Heavy overburden ∴ magma follows straight path
Nr surface - dykes less constrained ∴
anastomose
Dykes not in-situ
= tectonic slither between shear zones
Strong tectonic fabric in fault belt
(E-W lineament)
Serpentinite bodies act as shear zones
--> separating slivers of curstal material
Juxtapositioning crustal fragments from various levels
Highly sheared serp body
originally ultramafic rock, caught up in shearing of transtensional fault zone
Moving downseq
Pillows w Perapedhi cover
Pillows w hyaloclasite / lava flows etc
Sheeted dykes
Peridotite- Wehrlite Outcrop
Units
Picrite basal dykes
Wehrlite dyke shaped blocks
Tectonised harzburgite host
Heavily serpentinised series of ultramafic intrusions
Infer: signif vertical block mvmt
Given juxtaposition of deep & shallow components of conventional sequence
Picrite
Basalt
Primitive basalt - olivine-rich mush in chamber then intruded
Characteristic of shallower levels
Wehrlite
Heavily serpentinised/altered; olivine & cpx
Deep mantle rock