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 Trench Accreationary prism Fore-arc basin Active volcanic arc Back-arc basin/spreading Tension alternating compression amd tension sea level Remnant-arcs from arc-splitting Outer non-volcanic island Compression Low geotherm High geotherm 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”

Trench Tension alternating compression amd tension sea level Compression Low geotherm High geotherm 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 Ophiolites normally originate here! Blueshists normally originate here!

SEISMICITY 3 - D MORPHOLOGY NB! NOTICE INTRA-SLAB EARTHQUAKES

PAIRED METAMORPHIC BELTS

Formation of the Southern Uplands Accreationary Prism, Scotland. Age: Late Ordovician to Late Silurian ca Ma

FROM INTRA-OCEANIC SUBDUCTION TO OPHIOLITE OBDUCTION

The internal ”pseudostratigraphy” of an ophiolite complex. Example from OMAN: the S Ophiolite Complex Crust Mantle

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 --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.