Making and deforming oceanic crust
Key observations:
(1) The oceans have a crust 5-7 km thick
(2) All basaltic, former melts pooled at mid-ocean ridges
(3) Older away from ridges, as old as 200 Ma.
(4) Get subducted away; remnants scrapped off are called
ophiolites
(5) Get metamorphosed and interacts strongly with seawater
(6) Overall the OC is formed in an extreme extensional
environment; the structural look is one of low angle normal
faults- they all form at the mid-ocean ridge
(7) The structures get complicated by the presence of
transform boundaries segmenting the oceanic crust
More oceanic crust facts:
(8) Spreading is typically normal to the ridge axis;
(9) Oceans form passive margins (no subduction, e.g. Atlantic) or
active margins (subduction present, e.g. Pacific) at continental
margins;
(10) Away from the ridge, the oceanic crust cools and becomes
topographically more subdued than at ridges;
(11) Oceanic crust is loaded with a number of seamounts oceanic
plateaus, hot spots, and other non-ridge material; these are too
buoyant to get subducted and are prone to be accreted to continental
margins
More oceanic crust facts:
(8) Spreading is typically normal to the ridge axis;
(9) Oceans form passive margins (no subduction, e.g. Atlantic) or
active margins (subduction present, e.g. Pacific) at continental
margins;
(10) Away from the ridge, the oceanic crust cools and becomes
topographically more subdued than at ridges;
(11) Oceanic crust is loaded with a number of seamounts oceanic
plateaus, hot spots, and other non-ridge material; these are too
buoyant to get subducted and are prone to be accreted to continental
margins
Young!
Topography of ocean basins
Oceanic basins form
via rifting old
continental margins
Tectonic style is similar to
continental extension,
except the extension is
much higher, > 500%
Example: the Red Sea
Development of passive vs active margins
The anatomy of the passive North American margin
The ocean floor is full of seamounts
What happens to them at subduction margins?
E.g. the Pacific
All the “unsubductable”
They get accreted to the
continental margin and
become terranes, “exotic
terranes”; I.e. blocks of
unrelated origin that were
once far apart but got
assembled by accretion onto
a continental margin
Hot spots, seamounts in the oceanic crust
(1) Crust can be thickened to 25 km; looks more like continental than oceanic;
however it is basaltic in composition;
(2) Some hot spots are located on the mid-ocean ridge (e.g. Island) and could have
been responsible for opening that oceanic segment
(3) Off-ridge magmatic products require an additional source of heat ( plumes); we
understand that well when we see a long lived plume like Hawaii; trouble is that
the oceans are loaded with seamounts most of which can not be explained that
easily;
(4) These seamounts, etc. can be accreted to continents and overall add to the
continental mass;
Know the following re: ocean tectonics
1. General rules of oceanic crust formation
2. How does the oceanic crust deform
3. The influence of seamounts and hot spots on oceanic
tectonics
4. Examples of newly formed ocean basins