Transcript SEISMIC ACTIVITY (mainly shallow earthquakes)

Ocean floor:
Technological progress has enabled a better mapping of the
seafloor: direct observations and sampling, bathymetry, internal
architecture, structure and geophysical properties
The discovery made ca 40-50 years ago showing that oceanic
lithosphere is generated by the magmatic and tectonic activity
along the Mid-Ocean-Ridges is one of the most central elements
of Plate-Tectonics.
The Mid-Ocean-Ridges have a length of ca 60 000 km; they are a
formidable system with major importance for the driving force of
the plates and in the enery-budget of the plate-tectonics.
Mid-Ocean-Ridges
Have these characteristics:
1. TOPOGRAPHY (≈1000 km) broad ridges with narrow
central rifts.
2. BASALT VOLCANISITY mostly tholeitic composition
3. HIGH HEAT-FLOW
4. NEGATIVE GRAVITY ANOMALIES (melts)
5. SEISMIC ACTIVITY (mainly shallow earthquakes)
6. MAGNETIC ANOMALIES oriented parallel with the
ridges
Observations show that the width of a spreading ridge is proportional
with the spreading velocity, illustrated below (NB scale: h/v = 1/60)
A spreading centre comprises
a rift-valley between normal
faults. The rift is often sharply
defined as a narrow (10-30km)
zone. The lithosphere is at its
thinnest above such a rift
over en slik rift, and in many
models, the astenosphere is
considered to reach the
seafloor.
The crust and lithosphere
thicken away from the rift.
This is compensated by
Isostasy and the crust uplifted
in the rift-shoulders.
TOPOGRAPHY and gravity along mid-ocean-ridges provide
important information about processes which are responsible
for their formation. The high topography is a result of thermal
expansion and lower density (). i.e. a mass-deficiency/volume
which is isostatically compensated by the topography
(mid-ocean-ridges ”float” high).
Curve-fitting or empirical measurements show
that the ocean depth (D) is:
D = a√t + d0
t: ocean floor age in million years;
d0: water depth at the spreading ridge (≈ 2.5 km)
a: constant = 0.336
NB This equation is purely empirical and says nothing
about the processes.
DEPTH:
D = a √t + d 0
t - age; depth- d0≈ 2.5 km); constant- a = 0.336
What is the water depth above 16 myr old ocean floor?
D = 0.336√16 + 2.5km = 3.8 km
The graph shows depth of the ocean as a function of age out from a ridge.
Empirical studies show that the subsidence follows another curve:
D = 6.4 - 3,2e-t/62,8 when the ocean floor gets older than 70 myr.
Again this is pure curve-fitting and not quantified from any process. So how
Can this be done quantified using the physical laws?
For t < 70 mill år -->
D = a√t + d0
For t > 70 mill år -->
D = 6.4 - 3,2e-t/62,8
Havdyp
Alder
T    m 1  T1  T 
Depth of oceans:
(z), coeff. of thermal expansion= 3 x 10-5K-1
z
A
2) Column A at compensation
   w gw   zgdz
w- water depth
0
- density or mantle(m) (3.2) water (w)
 B   m gw   m gz
3) Column B at compensation
t- time
z
o
T- temperature, T1=1280 C, Ts=0oC
 m gz  w(  m   w )   zdz 4) (2), (3) and isostasy, see Stüwe p157
0
- thermal diffusivity
w(  m   w ) 
z
     dz
m
z
0
w(  m   w ) 
1) Density as function of temperature
5) (4) first term after =, finds derivative with respect to z and wrights into integral and it gets form
which says that water depth depends on the density sturcture as a function of depth
z
   T  T(z)dz
m
1
0
w(  m   w ) 

z

z

 z 
dz
 4t 
m (T1  Ts )erfc
0
w
n
 z 
dz
 4t 
 mT1  T  (T1  Ts )erfc
0
w(  m   w ) 
6) Inserting (1) into (5) where T(z) is the unknown (determined from heat conduction
equation see Stüwe p 96)
 z 
 m T1  T  z
erfc
dz

m  w 0
 4t 
z
4t

erfcn dn 
9) After taking constanst out of the integrall. If we introduce the constant n in (10)
11) Integral of the errorfunction is not know the 0 and z but is know for integration with
Limit infinity, it is:
1
0
w
8) and to :
10) We can take all the constants out to the integral and get:
 m T1  T  z
w  4t
 erfcndn
( m  w ) 0

7) Inserting heat conduction equation in (6), which simplifies to:
12) Substutuing this integral into (11) we get an expression for the water depth :

2  (T1  Ts ) t
(m  w )

w  5.91105 t
13) Which after inserting standard values for all the constanst give :
14) The water depth in oceans is proportional to the square root of the age and 5.91 times 10 -5
NB! +
w at the
spreading
ridge
Mid-Ocean-Ridges
Have these characteristics:
1. TOPOGRAPHY (≈1000 km) broad ridges with narrow
central rifts.
2. BASALT VOLCANISITY mostly tholeitic composition
3. HIGH HEAT-FLOW
4. NEGATIVE GRAVITY ANOMALIES (melts)
5. SEISMIC ACTIVITY (mainly shallow earthquakes)
6. MAGNETIC ANOMALIES oriented parallel with the
ridges
ELEVATED HEAT-FLOW
It is very hard to obtain
accurate heat-flow
measurements!
Heat-flow is given in
heat-flow-units defined as
milliwatt/m2.
A simplified empirical
formula for heat flow (Q)
is:
Q = 473 t -1/2
t - age, t > 120 Ma
Q = 33.5 + 67e -t/62.8
Also a purely empirical expression, how can we quantify ?
Q= k(T1-Ts)[x]1/2
k-conductivity; T-temperatures, u rifting rate;  diffusivity; x-distance
HEAT-FLOW:
Q = -k[(T+dT-T)/dz]
= -k dT/dz
(rate of flow pr unit area up through plate)
where k - thermal conductivity (Wm-1 oC-1)
T - temp (T (z + dz) > Tz
z - thickness of plate
z + dz
TdT
a
z
Flow of heat
k - thermal conductivity (Wm-1 oC-1)
T
Consider a small volume of height dz and cross-section “a”
Change in temperature dT in time dt depends
1) Flow og heat across the surface (net heat-flow in or out)
2) Heat generated in the volume
3) Thermal capasity (spesific heat) of the material
2
d
T
k
d
T
A
One dimensional heat conduction equation:


2
- density, cP - spesific heat,
dt c P dz
c P
A - heat production pr unit time
Temp is assumed to be function of time and depth only, can be expanded to 3-d
3-dimentional heat conduction equation:
2
2
2 

dT
k d Td Td T
A

 2 2 2 
dt c P dx dy dz  c P
or [using differential operator notation (Laplacianoperator)]
dT
k
A
2

 T 

dt c P
c P

Also considering movement of small volume of material with velocity uz

dT
k
A
2

 T 
 uT

dt c P
c P
conduction term; production term, advection term
NEGATIV GRAVITY ANOMALI (melts)
The free air anomaly: gf = gobs- glat - gh
where:
glat = g(l)= geq(1 + sin2l +bsin4l)
gh ≈ g0(1 - 2h/R)
l = latitude
The Bouger anomaly: gb = gf - dgb + dgt
= gobs- glat + gh- dgb + dgter
Where:
dgb = 2Gh (bouger correction)
G - gravitational constant (6,673 x 10-11 m3kg-1s-2)
dgt - terrain correction (deviations from horizontal)
h - height
 - density
NEGATIV GRAVITY ANOMALI (melts)
Mid-Ocean-Ridges
Have these characteristics:
1. TOPOGRAPHY (≈1000 km) broad ridges with narrow
central rifts.
2. BASALT VOLCANISITY mostly tholeitic composition
3. HIGH HEAT-FLOW
4. NEGATIVE GRAVITY ANOMALIES (melts)
5. SEISMIC ACTIVITY (mainly shallow earthquakes)
6. MAGNETIC ANOMALIES oriented parallel with the
ridges
SEISMIC ACTIVITY (mainly shallow earthquakes)
Earthquakes last week Jan- first week Feb, 2004
Fast spreading East-Pasific Rise
Intermediate spreading rate
Mid-Atlantic Ridge
and Southeast Indian Ridge
Earthquakes along
the Mid-Atlantic
Ridge near the Azores
Mid-Ocean-Ridges
Have these characteristics:
1. TOPOGRAPHY (≈1000 km) broad ridges with narrow
central rifts.
2. BASALT VOLCANISITY mostly tholeitic composition
3. HIGH HEAT-FLOW
4. NEGATIVE GRAVITY ANOMALIES (melts)
5. SEISMIC ACTIVITY (mainly shallow earthquakes)
6. MAGNETIC ANOMALIES oriented parallel with the
ridges
MAGNETIC ANOMALIES
We know that the earth is a magnetic dipol, with magnetic north
and south. The magnetic field varies in both intensity and orientation,
but over time (105yr) the magnetic poles coinside with the rotation poles
i.e. geographical north and south poles. Consequently the magnetic
Field is vertical near the poles and horisontal near equator!
Reversals of the magentic field leads to periods of
normal (present) and reverse magnetisation
During seafloorSpreading, the newly
formed crust will
function as a magnet
tape-recorder where
the alternating
Normal and reverse
Magnetizations will be
preserved as intensity
variations
The reversals produces periods of normal and (present) and
reverse magnetization, which is preserved in the geo-record
Reversals may also be studied on land
in volcanic or sedimentary rocks.
The reversals may be calibrated against mot
stratigraphy and radiometric age-determinations;
and magnetostratigraphy, is a dating method
if the anomaly-sequence may be identified.
The figure shows the theoretical distribution
of anomalies in a spreading ridge where the
introduction of new magnetic material occur
in a zone with width from 0 til 10 km.
Even in a relatively broad volcanic zone there
is an identifiable magnetic anomaly-pattern
The magnetic anomalies are among the best
evidence for seafloor spreading. It is hard
to explain this pattern in other ways, and
there is no other physio-chemical process
than reversal that can explain the change in
polarity.
The tectono-magmatic processes along spreading ridges gives a relatively
uniform architecture of the oceanic lithosphere in time and space.
Supra-custals,
(basalts and sediments)
Sheeted dyke complex
Isotropic
varied textured gabbros
Layered gabbros
Ultramafic cumulates
Ultramafic mantle
tectonites
Magma composition:
Tholteitic MORB
(Mid-Ocean-Ridge-Basalt)
Formed by relatively
high degree of partial
melting at relatively
shallow level in the
astenosphere
Intra-oceanic suspect/exotic terranes:
Modern oceans contains large
“anomalies” which have a
different origin than spreading at
ridges. These include:
1) Pieces of continents
2) Oceanic islands
3) Hot-spot traces and islands
4) Arc and back-arc compexes
5) Transform complexes
Such terranes may end up inside
suture zones of orogenic belts, in
which case they represent suspect
and/or exotic elements of the
mountain belt.
SUSPECT TERRANES:
TECTONOSTRATIGRAPHIC TERRANES THAT HAVE
UNSETTLED AFFINITY/ ORIGIN WITH RESPECT TO THE
CONTINENT WHERE IT ENDS UP AFTER AN OROGENY
EXOTIC TERRANES:
TECTONOSTRATIGRAPHIC TERRANES THAT HAVE
OUTBOARD ORIGIN WITH RESPECT TO THE
CONTINENT WHERE IT ENDS UP AFTER AN OROGENY.
EXAMPLES OPHIOLITES AND ISLAND ARC COMPLEXES,
CONTINETAL FRAGMENTS WITH AN ORIGIN IN ANOTHER
CONTINENT
Reconstruction of former plate motions.
Based on magnetic anomaly-patterns in the oceans it is possible to reconstruct the
face of the earth back in time for the period we have oceanic lithospere preserved
180
155 130
We can see that the oldest ocean floor is ca ≈180Ma, how
Can we reconstruct plate-motions before mid-Jurassic time?
Age of ocean floor,with
plate-reconstructions for
the past 130 Ma.
Notice that the
reconstruction also
shows oceanic
lithosphere that was
destroyed during this
time-span
(from:
www.geodynamic.no)