6 - Convergent Boundaries

three types of convergent boundaries

O-O

O-C

C-C

what are convergent boundaries driven by

heat/temp differences, motion in mantle (slab/pull, ridge/push)

why is earth a recycling medium

ocean plates are made, reworked and subducted

how long has subduction been functioning for

since the archean

what type of material gets recycled most

oceanic materials

two types of subduction

accretionary and erosive

what is accretionary subduction and what are some features of it

Adding material to continent - slower, shallower angle, large accretionary wedges

what is erosive subduction and what are the features of it

removal of material — steeper, faster subduction, thin sediments, trench rollback

which type of subduction is slower

accretionary

what kind of subduction is steeper

erosive

trench rollback is due to… s______

suction

which type of subduction creates trenched that get filled?

non accretionary (erosive)

forearc basins are thicker in which type of subduction zones

accretionary

accretionary prisms are present in what type of subduction

accretionary lol

what kind of subduction zone are we in

accretionary

what kind of crust is created from subduction and how

oceanic crust, from partial melting of the mantle

what rocks are formed from partial melting of the mantle

tholeiitic basalts

what are tholeiitic basalts made of

more Fe and les Mg and Al

what happens to the newly created oceanic crust as the plate develops

develops sediments, and becomes saturated with water, O2 and CO2

how many oceans have been carried down with subduction, where do they get trapped

like 11, trapped in mantle in hydrous reservoirs

two end members of subduction

1. Andean (continental)

2. Marianan (oceanic)

features in a O-O island arc

• mafics-intermediates

• from mantle

• forearc, magmatic arc, back arc

• accretionary wedge

• trench—filled in

features in a O-C plate boundary

• felsic-intermediates

• produced at surface

• forearc (fold/thrust), back arc (orogenic plateau)

• no trench, no accretionary wedge

what 5 things do typical magmatic arcs have

1. volcanic front

2. forearc basin

3. deep sea trench

4. back arc basin (with or without a remnant arc)

   1. accretionary wedge

what is the volcanic front

where the volcanoes are

what is the forearc basin

head of arc, has sediments shredding off mountains produced by the arc

what is a remnant arc

from extension from trench suction

whats an example of a backarc basin

sea of japan

what is slab rollback

slab subducts, pulls down, and rolls back

what does trench suction cause and create

causes extension, creates rifting with a backarc spreading region

what are the phase changes that subducting slabs go through

1. “wet” mafic crust to anhydrous eclogite

2. “Wet” serpentinized mantle dehydrates

3. released fluids = hydraulic fracturing and other failures

   1. Triggers earthquakes = Benioff zone

what is the benioff zone

pattern of earthquakes we see in subducting zones

what causes shallow and deep earthquakes on cold, old, deeply hydrated subducting slabs

• phase changes

• adding or taking away water

• volume changes

• fracturing

what inferences can you make about why this diagram of arc volcanoes has two different sets of point

deep vs shallow earthquakes, due to a “double layer” of subducting slabs

what is slab bending

the degree angle to which a slab subducts, which impacts geology

do strongly bent slabs experience compression or extension, and what happens as a result of the steep angle

extension, creates normal faulting and fractures, which brings in more water

do steep or shallow angled slabs bring in more water

steep

what does the presence of alot of water mean for the subducting slab

the water has to exit somewhere from the slab, more intermediate depth seismicity

why does a steeper subduction angle mean its more active

more water needs to escape the slab, quicker phase changes

can a single zone see different degrees of subduction angles

yuppers

is buoyancy positive or negative as slabs subduct

negative, cuz density increases

do slabs continue to subduct forever

no they stall

at what depth does the mantle undergo a fundamental change

660 km

why do some slabs bend or even break

due to subduction angles

4 types of fractures that occur in subducting slabs

1. propagating tear - just a simple tear

2. fracture in between a steep slab and a shallow slab

3. slab window in between a steep slab and a shallow slab

4. slab window creating a spreading ridge in between a steep slab and a shallow slab

what are the A and B types of subduction

A: ampferer/alpine

B: benioff/pacifc

what is ampferer/alpine subduction

continental crust or fragments being forced into the zone

stalling things out, slow subduction, reorganizing forces

what is benioff/pacific subduction

classic oceanic subduction,

plate goes down, nothing gets caught in it

what does the mafic crust turn into in high P and low T regimes, mention hydration

first into blueschist (hydrated) — then eclogite (less hydrated)

how often are eclogites exhumed at the surface, and how

rarely, theyre underplated then exhumed

if we see eclogite then theres likely lots of _____

garnet

why is eclogite a “messenger”

we can use thermobarometry to calculate geotherms (temp/depth) = history/time data of subduction

in eclogite, when the mineral assemblage changes, does the whole crystal crystallize

no, it produces zoning

do andean or marianan collisions cause jamming of continents, therefore stopping the subduction

andean causes jamming, stops subduction

how does more continent form at margins

during C-C collisions, crust being stuffed up onto side, major reorientation of forces

which plates collided to form himalayas

india and asia

effects of slab pull during C-C collisions

gets weaker as continent subducts slightly, ultimately breaks off = transitions to collision margin from subduction margin

what is the hinterland block and what are some features of it

it is lifted as lower density material is thrust under (underthrusting)

• high elevations, uplift + erosion

what did the hinterland block used to be

the back arc/magmatic arc, was uplifted and eroded away

are there volcanic rocks in the himalayas

only the roots of them because they have been uplifted and eroded

what is the suture zone and what types of rock are found there

contact between hinterland and foreland blocks, represents a closed ocean basin

scraps of ocean crust or accretionary complex

what is the foreland block and what does it form as collision progresses

it is pushed down by the overthrust block, forms a foreland basin and a large clastic wedge

why does a foreland basin form

cuz theres so much erosion and uplist on the hinterland block that the sediment has to go somewhere

what is an arc-continent

where subduction slows but doesnt stop, forms volcanic island arcs or continental fragments

what are “cookie bites”

when the slab takes a part of the continent down with subduction

example of a large fold and thrust belt

the rockies