Geoscience Reference
In-Depth Information
phase changes can be thirty times smaller than needed in an intrusion model and
still give rise to the same load. Many other similar metamorphic changes can
occur, each with particular rates of change. Such metamorphic changes occur
slowly, at rates controlled by the extent to which volatiles are present and by the
temperature-time history of the rock. The rate of nucleation of a new phase can
be expressed as
rate
=
K
e
−
G
∗
/
(
RT
)
e
−
H
a
/
(
RT
)
e
−
P
V
/
(
RT
)
(10.7)
G
∗
, the free energy of activation, is proportional to
1
(
T
equilibrium
−
T
)
2
where
H
a
is the enthalpy of activation,
P
the pressure,
V
the change in volume,
R
the gas constant,
T
the temperature and
K
a constant. At temperatures close to
equilibrium,
G
∗
is large, so the transformation rate is slow. At intermediate
temperatures or at temperatures much greater than the equilibrium temperature,
the transformation rate is faster. At low temperatures, however,
T
is small, so
(Eq. (10.7)) the transformation rate is slow. The reaction rates are not known
with any accuracy; but, for example, a gabbro layer at depth 50 km - such as
would be produced by underplating in a major flood-basalt event (comparable
to the Karoo event in southern Africa) or by emplacing a slab of mafic material
under the Williston area during the last stages of the Hudsonian plate collision -
would cool and transform to eclogite over a time of the order of 10
9
or fewer years.
Initially, massive uplift and erosion would occur, but then cooling, contraction
and transformation to eclogite would take over and progressively load the litho-
sphere. This would produce nearly steady subsidence, as may have occurred in the
Williston basin. Either of the models discussed could allow for a mid-Proterozoic
event to produce Phanerozoic subsidence. Such models are supported by indepen-
dent seismological evidence: a COCORP deep-reflection profile over the basin
shows that the lowermost crust is characterized by relatively high-amplitude
reflections. Detailed seismic-refraction surveys have shown that, beneath the
basin, where the crust is some 45 km thick, there is a high-velocity lower-crustal
layer and some indication of a high P-wave velocity of 8.4 km s
−
1
for the upper
mantle. This high-velocity reflective material can most simply be explained as
alayer of eclogite. These are simply models, and other models have also been
proposed. The origin of the basin is as yet unknown. The inversion of thermal
and subsidence data is extremely non-unique.
10.3.6 Extensional basins
Sediments deposited on the passive continental margins record the sedimentary
history of the rifting apart of old continents. Passive margins are economically
important because the thick sedimentary basins that develop along their length are
