Title: Thermo-rheological response of halokinetic numerical models to
temperature-dependent thermal transport properties of sedimentary materials and
implications for basin maturation
Rock salt deforms in a ductile fashion at temperatures and pressures much
lower than those of most geologic materials. This phenomena gives rise to diapiric
displacement of salt, and consequent brittle faulting of adjacent sedimentary rocks
creating structural “traps” for fluids such as hydrocarbons. The growth of such
structures has been the focus of much work in both the fields of hydrocarbon
exploration and structural geology. However, questions still remain about the
development of these structures and, namely, their role in the maturation of
hydrocarbons.
Recent advancements in the science of thermal transport properties of
geologic materials at elevated temperatures has lead to the reexamination and
refinement of many geodynamic phenomena, but to date, no measurements of the
transport property thermal diffusivity have been made on halite, rock salt, or
common sedimentary rocks. Here I propose the measurement of thermal diffusivity
using the laser flash method (LFA) of the afore mentioned geologic materials to
explore the hypothesis that thermal transport properties of salt are unusually
temperature-dependent due to high structural disorder at elevated temperatures,
and that this process plays a strong role in the evolution and maturation of basins
along passive margins.
This project will include the measurement of thermal transport properties of
common sedimentary rocks including thermal diffusivity (LFA), heat capacity
(isobaric calorimetry), and density (multipicnometry), and the application of these
results in combination with existing rheological data to model thermal maturation
of hydrocarbons, the triggering of salt-dome growth, and the sequence of potential
maturation between halokinetic events using 2D finite-difference numerical
modeling.
References:
van Keken, P.E.; Spiers, C.J.; van den Berg, A.P., and Muyzert, E.J., 1993. The effective
viscosity of rocksalt; implementation of steady-state creep laws in numerical
methods of salt diapirism. Tectonophysics, v. 225, i. 4, pp. 457-476.
Abstract
A steady-state creep law for rocksalt, describing the two parallel mechanisms of
dislocation creep and fluid-enhanced grain-boundary diffusion creep, has been used
in numerical models of salt diapirism, to study the effective viscosity of rocksalt.
Typical models included a 3 km thick sedimentary layer on top of 1 km of rocksalt.
The grain size of the salt has been varied between 0.5-3 cm and the geothermal
−12
−15 −1
gradient between 25-35 K/km. For strain rates of 10
− 10
s , typical of salt
diapirism driven by buoyancy alone, the diffusion creep mechanism dominates at
the fine grain sizes, with dislocation creep becoming important in coarsely grained
17
salt. The effective viscosity ranges from 10 Pa ⋅ s for small grain size and high
20
temperature salt to 10 Pa ⋅ s for large grain size and low temperature salt. The
viscosity is strongly dependent on grain size and moderately dependent on
temperature. For the larger grain sizes, the dislocation creep mechanism is most
effective during the diapiric stage, but the non-Newtonian effects in the salt are not
important in determining the growth rate and geometry of the diapirs. The
estimates for the Newtonian viscosity of salt that have traditionally been used in
modelling of salt dynamics are at the lower end of the range that we find from these
numerical experiments.
Ter Heege, J.H., De Bresser, J.H.P., and Spiers, C.J., 2005. Rheological behavior of
synthetic rocksalt: the interplay between water, dynamic recrystallization and
deformation mechanism. Journal of Structural Geology, 27, pp. 948-963.
Abstract
The ductile deformation of rocks in nature can be greatly enhanced by the presence
of water. Part of the water-induced weakening of rocks at depth may come from
fluid-assisted deformation or recrystallization mechanisms that are absent in dry
rocks. In this study, we investigate the effect of water on the rheological behaviour
of rocksalt. We focus on quantification of the contribution of individual deformation
and recrystallization mechanisms to deformation. We also aim to calibrate a flow
law that incorporates the effect of all the relevant microphysical processes and
hence more accurately describes the flow of rocksalt in nature. For this purpose, the
mechanical behaviour and microstructural evolution of synthetic rocksalt samples
that are similar, except for differences in water content (determined using FTIR
analysis), are investigated. The samples are deformed to natural strains of 0.07–0.46
at 50 MPa confining pressure, strain rates of 5!10K7–1!10K4 sK1 and temperatures
of 75–240 8C, resulting in flow stresses of 7–22 MPa. The flow stress of samples
with a water content below w5 ppm (‘dry’) is higher than that of samples with a
water content of w9–46 ppm (‘wet’) at all strains under the investigated conditions.
The difference in flow stress can be explained as due to the operation of only work
hardening dislocation creep without dynamic recrystallization in the dry material
versus combined dislocation and solution-precipitation creep plus fluid-assisted
grain boundary migration in the wet material. The results allow us to calibrate a
flow law for wet rocksalt that incorporates the effects of solution-precipitation
creep and fluid- assisted grain boundary migration. The results also suggest that
strain localization in natural rocksalt is more likely to be localized due to fluid
infiltration and associated rheological weakening, than due to progressive removal
of strain hardening substructure by grain boundary migration.
Whittington, A.G., Hofmeister, A.M., and Nabelek, P.I., 2009. Temperaturedependent thermal diffusivity of the Earth’s crust and implications for magmatism.
Nature, vol. 458, pp. 319-321.
Abstract
The thermal evolution of planetary crust and lithosphere is largely governed by the
rate of heat transfer by conduction The governing physical properties are thermal
diffusivity (k) and conductivity (k=5 kρCP), where ρ denotes density and CP denotes
specific heat capacity at constant pressure. Although for crustal rocks both k and k
decrease above ambient temperature, most thermal models of the Earth’s
lithosphere assume constant values for k (~1mm2s-1) and/or k (3 to 5 W m-1 K-1)
owing to the large experimental uncertainties associated with conventional contact
methods at high temperatures. Recent advances in laser-flash analysis permit
accurate (+/- 2 per cent) measurements on minerals and rocks to geologically
relevant temperature. Here we provide data from laser-flash analysis for three
different crustal rock types, showing that k strongly decreases from 1.5–2.5 mm2 s-
1 at ambient conditions, approaching 0.5 mm2 s-1 at mid-crustal temperatures. The
latter value is approximately half that commonly assumed, and hot middle to lower
crust is therefore a much more effective thermal insulator than previously thought.
Above the quartz a–b phase transition, crustal k is nearly independent of
temperature, and similar to that of mantle materials. Calculated values of k indicate
that its negative dependence on temperature is smaller than that of k, owing to the
increase of CP with increasing temperature, but k also diminishes by 50 per cent
from the surface to the quartz a–b transition. We present models of lithospheric
thermal evolution during continental collision and demonstrate that the
temperature dependence of k and CP leads to positive feedback between strain
heating in shear zones and more efficient thermal insulation, removing the
requirement for unusually high radiogenic heat production to achieve crustal
melting temperatures. Positive feedback between heating, increased thermal
insulation and partial melting is predicted to occur in many tectonic settings, and in
both the crust and the mantle, facilitating crustal reworking and planetary
differentiation.