Page 176 - The Petroleum System From Source to Trap

168 Deming

density (-3300 kg/m3) mantle material moves below the
depth of compensation. Simultaneously, relatively low
density air, water, or sediment fills the void created at the
top of the column. This process is termed isostatic compen­
sation, and it continues until equilibrium is restored.
Isostatic subsidence may be inhibited by the bending
rigidity of the lithosphere. Flexural rigidity is determined
by temperature and composition; the lithosphere tends
to be weaker in areas of higher heat flow, and the quartz­
rich crust tends to be weaker than the olivine-rich
mantle. The degree to which the rigidity of the lithos­
phere can successfully resist isostatic subsidence also
depends on the width of the load. For typical continental
lithosphere, the critical value is about 80 km (Turcotte,
1980). Sedimentary basins with lateral dimensions much
larger than this number subside isostatically; the subsi­
�
--
�
dence of smaller basins is resisted by the flexural strength o � � -- -7 � -- � -- � 60 -- � 80 -- 100 �
of the lithosphere. If the lithosphere is loaded by hori­
6
zontal mass transport (e.g., a migrating fold-thrust belt), Time (10 yr)
the tendency of the lithosphere to flex elastically in
response to the imposed load can itself lead to the Figure 9.3. Surface heat flow as function of time and
formation of a sediment-filled depression, a foreland stretching factor �- (After McKenzie, 9 78.)
1
basin (Beaumont, 1981).
margin basin is well understood in the framework of
plate tectonic theory. An initial thermal event leads to
TYPES OF SEDIMENTARY BASINS
cooling, thermal contraction, and tectonic subsidence.
Rift and Passive Margin Basins The tectonic subsidence is then increased by loading
from erosional products washed off adjacent continents.
The largest set of sedimentary basins on earth are the In its final stage, an oceanic basin is destroyed through
oceanic basins, covering approximately two-thirds of the subduction or continental collision. The entire process
earth's surface area. The formation of these basins is well from formation to eventual destruction typically takes a
understood in the context of plate tectonic theory. New few hundred million years.
oceanic crust is formed by the upwelling of mantle The success of the plate tectonic model in explaining
material at mid-oceanic spreading ridges where the the thermal and structural evolution of ocean basins
effective lithospheric thickness is essentially zero. As the suggests that similar mechanisms may be involved in the
newly formed lithosphere moves away from the ridge formation of rift basins on continents. McKenzie (1978)
through the process of seafloor spreading, it cools and derived a simple and elegant model of continental
thickens, becomes more dense, and subsides through a extension as a mode of rift basin formation. In
process of isostatic compensation (Figure 9.1). McKenzie's model, the lithosphere is instantaneously
A relatively simple thermal model of a cooling half­ stretched by a factor � and the thickness of the lithos­
space can be used to derive expressions for surface heat phere decreased by a factor 1 / �- In the context of the
flow and subsidence of the oceanic lithosphere as McKenzie model, ocean basins are rift basins for which
functions of time. The theoretical model predicts both � = oo and continental rift basins are basins in which the
heat flow and tectonic subsidence to be proportional to rifting process stopped before completely eroding and
the square root of time elapsed since basin formation. splitting the lithosphere.
With few exceptions, the agreement of these theoretical The formation of a rift basin is characterized by two
predictions with heat flow and bathymetry data is phases of subsidence. During the initial extensional
extremely good. There is little doubt that the thermal event, relatively low density crustal material (-2800
model of cooling and subsidence is an essentially kg/m3) is thinned and replaced by higher density mantle
accurate description of reality (Sclater and Francheteau, (-3200 kg/m3) upwelling from below and isostatic subsi­
1970; Sclater and Parsons, 1981). dence occurs. The hot mantle material then cools, and its
At passive continental margins (e.g., the Atlantic density increases through thermal contraction, leading to
margin of North America), the ocean basin (basement a second phase of slower tectonic subsidence. A time
rock) is heavily loaded with sedimentary rock (basin fill), constant for thermal events in the lithosphere of about 50
leading to further subsidence as the lithosphere isostati­ m.y. governs the rate of cooling and tectonic subsidence.
cally compensates for the added load. The total sediment Relatively simple expressions for surface heat flow
thickness in this setting can reach 20 km (Turcotte, 1980). (Figure 9.3) and tectonic subsidence as functions of
Thus, the thermal and structural evolution of oceanic elapsed time and stretching factor � can be derived
basins from the initial rift basin to the final passive (McKenzie, 1978).