CONVERGENT PLATE MARGINS
1) Intra-oceanic (ensimatic) subduction
2) Andean margins
3) Continent - continent collision zones
1)
2)
3)
REMEMBER, IN 3-D A CONVERGENT MARGIN MAY
HAVE DIFFERENT MATURITY ALONG STRIKE!
THE ANATOMY OF A SUBDUCTION COMPLEX
Outer
non-volcanic
island
Compression
sea level
Fore-arc
basin
alternating
compression
amd tension
Active
volcanic
arc
Back-arc
basin/spreading
Remnant-arcs
from
arc-splitting
Tension
Trench
Accreationary
prism
Please notice that Benioff zones frequently have an irregular shape in 3-D (ex.
Banda Arc). 80% of all seismic energy is released in Benioff zones.
The low geotherm in subductions zones makes them the prime example of high P low T regional metamorphic complexes. The high geotherm in the arc-region gives
contemporaneous high-T low P regional metamorphism, together these two regions
give rise to a feature known as”Paired Metamorphic Belts”
Blueshists normally
originate here!
Compression
sea level
alternating
compression
amd tension
Ophiolites normally
originate here!
Tension
Trench
Seismic quality factor (Q): The ability to transmitt seismic energy
without loosing the energy. Low Q in high-T regions.
Seismic quiet zones---NB potential build-up to very large quakes!
Arc-splitting - tensional regime above subductions zones. Subduction
roll-back.
High heat-flow in the supra-subductions zone regime give rise to
relatively low shallow sealevel above the back-arc basins. Most
ophiolite complexes have their origin is a supra-subduction environment
3 - D MORPHOLOGY
SEISMICITY
NB! NOTICE INTRA-SLAB
EARTHQUAKES
PAIRED METAMORPHIC BELTS
Formation of the
Southern
Uplands Accreationary
Prism, Scotland.
Age:
Late Ordovician to Late
Silurian ca 450-420 Ma
FROM INTRA-OCEANIC SUBDUCTION
TO OPHIOLITE OBDUCTION
Crust
Mantle
The internal ”pseudostratigraphy”
of an ophiolite complex.
Example from OMAN:
the Semail Ophiolite Complex
THE OBDUCTION CHRONOLOGY, OMAN OPHIOLITE
Metamorphism
below the
St. Antony Ophiolite
Complex,
Newfoundland.
Pressure apparently
increases downward
from the contact with
the mantle
peridotite.
How can we explain
the enigmatic
PT distribution????
CONSEPTUAL MODEL FOR
THE OBDUCTION OF THE
ST. ANTONY OPHIOLITE
COMPLEX, NEWFOUNDLAND,
AND THE DEVELOPMENT OF
THE OBDUCTION AUREOL
SCHEMATIC MODEL FOR
THE INTERNAL
STRUCTURE AND
FORMATION OF THE
LEKA OPHIOLITE COMPLEX
IN NORD-TRØNDELAG,
NORWEGIAN CALEDONIDES
SCHEMATIC EVOLUTION TO ANDEAN MARGIN
AN ANDEAN MARGIN IS COMMONLY FLOORED BY AN OPHIOLITE COMPLEX
--some ANDEAN MARGIN CHARACTERISTICS:
• Low-angle subduction zones, great distance from trench to active arc.
• Magmatic events produce large composite batholiths, with superunits
and units which individually show mafic to acid (primitive to mature)
compositional trends.
• Very large volumes of magma are emplaced into the crust, and can in
some cases (like the type area) produce crustal thicknesses comparable
to continental collision zones.
• The plutonics are dominated by tonalitic to granodioritic bodies, which
commonly are emplaced as ”permitted” intrusions, i.e. by stoping to
cauldron subsidence mechanisms.
• The volcanoes are dominated by build-up of large strato-volcanoes with
andesitic to rhyolitic compositions.
• Andean margins may be long-lived depending on the width of the ocean
which is subducting.