Transcript Orogenies as records of plate collisions

Plate Boundaries
colllisional
How collision builds mtns: processes are labeled on diagram.
The topography of a mtn is build from a combination of physical
(faulting and folding pushing the surface UP) and isostatic
(the crust rises because it gets thicker and can float higher in
the mantle)
Generalization of the internal structure of mtns. The region
colored mdm-grey-brown (labeled continental crust) will be
metamorphosed from the regional heat and pressure generated
from the collision. Thrust faults flank the metamorphic belt. The
diagram does not show the presence of volcanic rocks within
the mtn
the sequence of steps
associated with
accretion of terranes,
or microplates.
The terrane basically
“plugs up” the
subduction zone,
forcing it to move
backward.
Another way in which
terranes accrete to larger
plates. Again, the
terrane, made of more
buoyant material, won’t
subduct, so the site of
subduction moves. This
sequence also
illustrates an example
of “which plate
subducts?” At first, it’s
the ocean crust-bearing
plate of on the right,
but later it is the ocean-bearing
plate on the left. The
controlling factor? The more
dense plate will subduct.
Basins where thick piles of sediment can accumulate are also
characteristic of mtns. Immediate adjacent to the Canadian
Rockies, for example, is a basin that has tens of km of sediment
in it, accumulated from erosion of the mtn as it is uplifted.
How does a basin subside, or sink, in a regime of uplift?……
Basins are sites of sediment accumulation because they are
depressions that form from fault movement: the crust sinks
as a result of the weight of the extra pile of crust pushed on to
it. This basin then accumulates sediment, whose weight causes
further sinking, or subsidence.
Geologic record of orogeny
• Instrusive (granite plutons, dikes) and
extrusive (ash and flows) volcanic rocks
• Regional metamorphism
• Abundant faulting, especially thrust faults
• Basins with thick accumulations of sediment;
sandstones are lithic arenites and record
deposition in non-marine environments, such
as rivers, lakes, deltas.
• Abundant unconformities reflecting the times
of vertical movement
What does the geologic record of orogeny look like?
1. There will be mtns, or if erosion has been very severe, the
remains of the “innards” of mtns: folded, metamorphic and
volcanic rocks.
2. Abundant folds and thrust faults
Pluton
3. Stratigraphy that records the
intrusion
types of basins and sediments
that infilled them
4. Lots of unconformities!
Closure of
ocean basin
The stratigraphic record
of the Taconic Orogeny
in eVt & wMA
New ocean seds
& volcanics
Isostacy: why portions of the Earth’s
crust are at different elevations
A block of ice floats in water.
Why does part of it stick above
Water level?
Why not?
The pressure along the dashed line
Is everywhere the same. This is called
The compensation depth.
The pressure at PB
must = pressure at
PA. The block will
rise to equilibrate
pressures
Isostacy, continued
We can determine the pressure that is
pushing the block up…in other words,
how much does it have to rise to come
into equilibrium?
The block of ice will be at equilibrium when
PA = PB
PA is a function of the density of the block, the
gravitational acceleration downward, and its
thickness, or PA= (rho ice)(g)(thick)
PB is a function of: density of the water, the
gravitational accel downward, and its thickness,
or PB = (rho water)(g)(thick)
Let ZT = total thickness.
Let ZU = the upper part (above water level).
Let ZL = the lower part (below water level).
Then -ZT = ZU + Z L
if ZT= 100m, Zu= x ZL=100-x
PA =ice g ZT =
PB = water g ZL =
If the density of ice =
.917 g/cm3
The density of water
= 1g/cm3
grav const - 9.81 m/sec2
and the thickness Lt=100m
PA = PB
(.917)(9.81)(100))
(1) (9.81)(100-x)
(.917)(9.81)(100) = (1)(9.81)(100-x)
x = 8.3m
So ZL / ZT = ice / water
and ZU / ZT = (water – ice ) / water
In other words, ~92% of the
ice floats below water level
This is a preview of Airy isostasy:
Which says, for example, that mountains are higher than plains
because they have roots.
Or, continents are higher than ocean basins because continental crust
is thicker than oceanic crust.
Sample problem: if continental crust is 40km thick, how high
would the Earth’s surface extend above ground (=how high a
mountain would be)
This is solved the same way you approach the iceberg.
What is different is the density of the iceberg, which becomes
the density of cont. crust; the density of water becomes the
density of the mantle