Sverre PlankeHenrik Svensen, Ingrid Aarnes, Yuri Y. Podladchikov, Espen Jettestuen, Camilla H. Harstad and sedimentary basinssediment mobilization during solidification of magmatic sheet intrusions in Sandstone dikes in dolerite sills: Evidence for high-pressure gradients and
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Sandstone dikes in dolerite sills: Evidence for high-pressure gradients and sediment mobilization during solidifi cation of magmatic sheet intrusions
in sedimentary basins
Henrik SvensenIngrid AarnesYuri Y. PodladchikovPhysics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway
Espen JettestuenPhysics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway, and International Research Institute of Stavanger (IRIS), Prof. Olav Hanssensvei 15, 4068 Stavanger, Norway
Camilla H. HarstadAGR Group, Karenslyst allè 4, 0278 Oslo, Norway
Sverre PlankePhysics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway, and Volcanic Basin Petroleum Research (VBPR), Oslo, Norway
211
Geosphere; June 2010; v. 6; no. 3; p. 211–224; doi: 10.1130/GES00506.1; 10 fi gures; 1 table.
ABSTRACT
Sediment dikes are common within doler-ite sill intrusions in the Karoo Basin in South Africa. The dikes are subvertical and as much as 2 m wide, sometimes with abundant fragments of sedimentary rocks and dolerite. The matrix consists of contact-metamorphic sandstone. There is no petrographic evidence for melting within the sediment dikes. The maximum temperature during heating is restricted to the plagioclase and biotite stabil-ity fi eld, or above ~350 °C. Thermal model-ing of a sandstone dike in a dolerite sill shows that a temperature of 350–450 °C is reached in the dike after a few hundred years of sill cooling. The calculated pressure history of a cooling sill and its contact aureole shows that substantial fl uid pressure anomalies develop on a short time scale (1–15 yr) and are main-tained for more than 100 yr. Calculated pres-sure anomalies in the sill (−7 to −22 MPa) and the aureole (4–22 MPa) are signifi cant and may explain sill fracturing and sediment mobilization from the aureole into the sill. We conclude that sediment dikes represent common features of sedimentary basins with sill intrusions in which fl uid pressure gra-dients have been high. Sediment dikes thus signify that pore fl uids may escape from the
aureoles on a short time scale, representing an intermediate situation between fl uid loss during formation of microfractures and fl uid loss during violent vent formation.
INTRODUCTION
Subsurface sediment mobilization and fl u-idization have been recognized from many geological settings, ranging from overpressured clastic reservoirs (Jolly and Lonergan, 2002; Mazzini et al., 2003; Nichols et al., 1994) to contact metamorphism around magmatic sill intrusions (Jamtveit et al., 2004; Svensen et al., 2006). In sedimentary basins affected by mag-matic sill intrusions (i.e., volcanic basins), like the Karoo Basin in South Africa, sediment dikes are reported from within doleritic sills (Van Bil-jon and Smitter, 1956). It is interesting that these dikes comprise metamorphic sandstone, demon-strating that the sand intruded the dolerite while the sills were still hot. The importance of these observations is that they form direct evidence for high pore fl uid pressure during sill emplace-ment and subsequent contact metamorphism.
In a classic study by Walton and O’Sullivan (1950), it was suggested that pressure drop during sill cooling and fracturing (i.e., thermal contraction) led to boiling of aureole pore fl u-ids that ultimately led to sediment fl uidization.
That study was based on fi eld examples from a sill emplaced in sediments during formation of the Central Atlantic Magmatic Province. The role of pore fl uid boiling in causing high aureole pressures and subsequent fl uid movement was explored in more detail by Delaney (1982; and more recently, e.g., Jamtveit et al., 2004).
Understanding sediment mobilization from contact aureoles may put important constraints on pressure evolution of aureoles. The past decade has seen an increasing interest in degas-sing of volatiles from sedimentary basins with magmatic intrusions, where high pore fl uid pressure plays a key role (Ganino and Arndt, 2009; McElwain et al., 2005; Retallack and Jahren, 2008; Svensen et al., 2004, 2007, 2009). Gas venting triggered by overpressure in con-tact aureoles within shale has been proposed to have caused global climate changes in the end-Permian, Early Jurassic (Toarcian), and at the Paleocene-Eocene boundary (Svensen et al., 2004, 2007, 2009).
The aim of this study is to understand the formation of sandstone intrusions in dolerite sills. We present several case studies of sedi-ment dikes and sediment breccias within sills in the Karoo Basin. However, the results can be applied to other sedimentary basins where sedi-ments have been injected into magmatic sheet intrusions, including the Vøring Basin offshore
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Norway, the Tunguska Basin of east Siberia, and the Amazonas Basin in Brazil. The process of sediment injections is addressed by adopting a new theoretical model for sill pressure evolu-tion during cooling and crystallization (Aarnes et al., 2008).
GEOLOGICAL SETTING
The Karoo Basin (Fig. 1) covers more than half of South Africa. The basin is bounded by the Cape Fold Belt along its southern mar-gin and comprises as much as 6 km of clastic sedimentary strata capped by at least 1.4 km of basaltic lava (e.g., Johnson et al., 1997; Smith, 1990). The sediments were deposited from the late Carboniferous to the Middle Jurassic, in an environment ranging from dominantly marine (the Dwyka and Ecca Groups) to fl uvial (the Beaufort Group and parts of the Stormberg Group) and eolian (upper part of the Stormberg Group) (Catuneanu et al., 1998; Veevers et al., 1994). The Beaufort Group is a thick sequence of dominantly sandstones. The overlying Storm-berg Group includes the Molteno Formation (coarse sandstone, shale, and coal), the Elliot Formation (sandstone, shale; red beds), and the Clarens Formation (sandstone with occasional siltstone horizons).
Both southern Africa and Antarctica under-went extensive volcanic activity in early Juras-sic times, starting ca. 182.5 Ma. Dolerites and lavas of the Karoo-Ferrar large igneous province were emplaced within a relatively short time span. The main phase of fl ood volcanism lasted <1 m.y. (Duncan et al., 1997; Jourdan et al.,
2005), although volcanism in southern Africa continued for several million years (Jourdan et al., 2005). Sills and dikes are present throughout the sedimentary succession in the Karoo Basin (Fig. 1) (Chevallier and Woodford, 1999; Pol-teau et al., 2008b), where they locally compose as much as 70% of the stratigraphy (Rowsell and De Swardt, 1976).
METHODS
Sampling and Petrography
Sediment dikes are common within thick (70–120 m) dolerite sills within the Beau-fort Group sediments. The depth of magma emplacement is estimated as 600–1000 m below the paleosurface, based on present-day stratigraphic levels. We have done detailed studies of three localities with sediment dikes in dolerites: (1) the Waterdown Dam area, (2) the Elandsberg roadcut (Nico Malan Pass), and (3) the Golden Valley (Fig. 1). Many more localities with sediment dikes have been dis-covered during our fi eld work in the Karoo Basin during the past decade (e.g., south of Cathcart), but the chosen localities are repre-sentative. One of the sediment dikes from the Waterdown Dam locality contains numerous fragments of sediments and dolerite. It was mapped in detail by covering it with transpar-ent A4 plastic sheets and tracing individual clasts by hand. This method was preferred over photo analysis due to better accuracy and the benefi t of doing on-site interpretations on clast type and clast outline. The resulting map repre-
sents a two-dimensional (2D) slice through the dike. We then used image analysis techniques and a MATLAB (http://www.mathworks.com/) code to quantify the clast content (i.e., area). Probability densities were calculated using a smoothing procedure, where data were binned in either 10 consecutive areas (for sedi-ment clasts) or 5 consecutive areas (for dolerite clasts). The aspect ratio between the long and short axes of the fragments was also calculated. Since our mapping analyses are done in 2D, and we only have one slice through the dike, the results should be regarded as approximate.
Thin sections of collected samples were stud-ied by optical and electron microscopes (scan-ning electron microscope, SEM) at the Depart-ment of Geology, University of Oslo. The SEM is a JEOL JSM 840, and was also utilized for cathodoluminescence (CL) imaging.
Phase Stability Calculations
We used Perple_X (Connolly, 2005) to com-pute phase diagrams for rocks with a pelite composition to predict the temperature stabil-ity of the mineral assemblages identifi ed in the sandstone dikes. The calculated phase diagram is projected from an average pelite composi-tion (Caddick and Thompson, 2008), with SiO
2 = 59.8, Al
2O
3 = 16.6, FeO = 5.8, MgO =
2.6, CaO = 1.1, Na2O = 1.7, K
2O = 3.53,
TiO2 = 0.75, H
2O = 5.0 (all in wt%). We calcu-
lated the reactions using quartz saturation, which means that the phase assemblages obtained are not dependent upon the bulk content of quartz. Hence the phase diagram is valid for sandstones
30
32
20 24 28 150 km
Cape Fold belt Indian OceanCape Town
Waterdown Dam
Basement
Dwyka Gr.Ecca Gr.
Beaufort Gr.
Stormberg Gr.Drakensberg Gr.
Sill intrusion The Whitehill Fm.
Farm
Road
(R67)
WaterdownDam
1
32
Golden Valley
A B
2 kmDolerite dike
Sediment dike
Cathcart
Waterdown Dam
Nico Malan Pass
Figure 1. (A) Simplifi ed geological map of the Karoo Basin in South Africa; black dots are sill intrusions and hydrothermal vent complexes. The three study localities are shown. One more locality with sediment dikes in sills is shown (Cathcart), but not included in this study. (B) Simplifi ed geological map of the Waterdown Dam area based on the 1:250 000 geological map of Council for Geo-science, South Africa. Note that there is one locality with dikes that have not been included in this study.
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as well as pelites, as long as the ratios of the other oxides do not change signifi cantly.
Numerical Modeling
We have developed a numerical model using the fi nite element method (FEM) in MATLAB. We couple standard heat conduction to pressure (or hydraulic) diffusion using the equation for thermal stress similar to that of Aarnes et al. (2008). We calculate the pressure anomalies arising from pore fl uid expansion of pure water in the contact aureole, and the pressure changes related to phase transitions (melt to crystal) in the sill. The pressure anomalies diffuse over time according to Darcy’s law. The equations are solved on a 2D square grid with a resolution of 25 × 200 elements. Initial conditions for the thermal solver is a host-rock temperature T
hr of
35 °C, and a sill temperature, Tm, of 1200 °C.
For temperature boundary conditions we fi xed both the upper and lower boundaries at initial host-rock temperature, as the geothermal gradi-ent is negligible on the scale of a few hundred meters. We assume a hydrostatic pressure gradi-ent with a fl uid density of 1000 kg/m3 as initial conditions for pressure. The upper and lower boundaries are fi xed according to initial hydro-static pressure. The boundaries do not infl uence the calculations.
Model AssumptionsWe have developed a numerical model to
quantify the fi rst-order effects associated with sill cooling and pressure evolution. The model is conceptual and does not attempt to describe the full system. We assume an instant emplace-ment model of the sill because sediment dikes are related to postemplacement processes
occurring at subsolidus conditions. The ther-mal diffusivities are equal for the sill and the sedimentary host rock, as differences in thermal properties are negligible (see Table 1). However, the hydraulic diffusion coeffi cients of melts and pore fl uids differ by approximately one order of magnitude in our model. We assume no heat advection by fl uids in either the sill or the con-tact aureole. This is justifi ed from studies show-ing that heat advection by fl uids is a second-order effect (Connolly, 1997; Podladchikov and Wickham, 1994). Apart from the sandstone dikes, there is little evidence of high fl uid circu-lation in the intruded sediments, which makes heat advection within the intrusion negligible (cf. Norton et al., 1984).
The major assumption concerning the equa-tion of thermal stresses is that expansion of pore fl uids and contraction of melt due to crystal-lization are prevented either by the sediment matrix or the crystal network. This assumption is valid until the expanding fl uids break the sedi-ment matrix and reduce the overpressure, either by fl uidization or by pervasive fl ow along the overpressure gradient. We account for fractur-ing of the host rock by resetting pressures that exceed the tensile strength of the host rock to hydrostatic pressure. We assume the tensile strength of our model sandstone host rock to be on average 35 MPa (Ai and Ahrens, 2004). We expect a drop in overpressure gradients with time, depending on how freely the mobi-lized sediments can move and reequilibrate the overpressure anomalies. For the underpressure, we expect the assumption of prevented vol-ume change to be valid for the intrusion until the thermal contraction produces fracturing of the sill. Tensile strength of gabbroic rocks is >125 MPa (Ai and Ahrens, 2004). Such under-
TABLE 1. SYMBOLS AND VALUES USED IN THE NUMERICAL MODEL
Symbol Description Initial value Unit References z Vertical system size 500 m 1d Sill thickness 100 m 1Tm Initial temperature of melt 1200 + 273 K 1Thr Initial temperature of host rock 35 + 273 K 1TL Liquidus temperature of melt 1200 + 273 K 1TS Solidus temperature of melt 900 + 273 K 1KT Thermal diffusivity melt (K/CP/ρ) 10–6 m2s–1
P Pressure 1000 × g × z Pa 1g Standard gravity 9.81 ms–2
t Time s 1 Note: References: 1—this study; 2—Delaney (1982); 3—Hersum et al. (2005);
4—Turcotte and Schubert (2002).
pressure is not achieved in our model, which suggests that we are using conservative values. The main equations used for the modeling are shown in the Appendix.
RESULTS
Sediment Dikes in Dolerite Sills
Waterdown DamSeveral sediment dikes within dolerite sills
are located in roadcuts along the Waterdown Dam north of the Elandsberg area in South Africa (Fig. 1). The main sites are numbered 1–3 in Figure 1B, where thick sediment dikes are exposed close to the lower contact of a trans-gressive dolerite sill. The intruded sediments are mainly sandstones, all from the Permian and Triassic Beaufort Group. An overview of the locality is given in Figure 2A. At all sites, the fi eld evidence suggests upward movement of sediment, based on the presence of dolerite bridges. The maximum upward penetration is not known, but is estimated to 10%–15% of the sill thickness based on the exposed dike heights and sill thickness.
At site 1 (S32°18.2′, E26°52.6′), a sediment breccia dike can be traced for ~150 m westward from the main road, cutting vertically through at least 15 vertical meters of dolerite. The strike is 80° east, and it pinches out in both directions. The maximum thickness is 0.5 m and it splits in two branches toward the west. Sediment and dolerite fragments as much as 40 cm long are common, and bridge-like portions of dolerite are present locally (Fig. 2E). The latter suggests an eastward direction of emplacement.
Several thin sandstone dikes crop out at site 2 (Fig. 2B). The maximum width is 0.5 m, the strike is 84° east, and their vertical extension can be traced for 10–15 m in the roadcut. A few pieces of fresh dolerite are located within the dikes. These represent fragments of wall-rock dolerite broken off during dike emplacement, as also seen at site 1.
At site 3, a vertical dike as thick as 2.2 m crops out along the road (Fig. 2C), striking 78° east. This is, to our knowledge, the thick-est sediment dike ever found in a sill intrusion. The contact with the dolerite is sharp, although weathered, and it comprises a breccia with sedi-mentary fragments as long as 40 cm. Some of the fragments show sedimentary layering. The lateral extension of the dike is unknown due to poor exposures, but the dike is located ~10 m above the dolerite-sediment contact cropping out to the west. In addition to the sedimentary clasts, the dike contains numerous fragments of dolerite (Fig. 2D). The dike has been mapped in detail, and the results are presented in Figure
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3A. The area percentage occupied by clasts and their size distribution have been quantifi ed (Fig. 3B). The results show that the sandstone matrix (including clasts <0.5 cm) composes 86.4% of the area, sediment fragments occupy 10.6% (320 clasts), and dolerite fragments com-
pose 3.0% (70 clasts). There is a four order-of- magnitude variation in clast size for the sedi-mentary fragments, but a lesser variation for the dolerite fragments. Note that the probability versus size relationship is similar for both sedi-ment and dolerite clasts. The aspect ratio of the
clasts length and width is calculated and shown in Figure 3C. It is interesting that the aspect ratio is independent of the clast size. The sediment clasts are more elongated compared to the doler-ite clasts (aspect ratios of 2.51 and 1.95, respec-tively), which is also evident from Figure 3A.
Sed ment DDoleleeriritetet ss
Sediment dikes
Doleer tee s lSeSeS did mementtdd keke
DoDoDDolelerir tee sss
SeSS ddd me tnt dididdd kekes
DoDo eerirrrr tee sis ll
Meta-sandsstot ne
DoDoD eeriritee
Dolerr tee
DoDoDoleleeririrr tetetett
Sedimentdike
Sediment Dolerite sill
Sediment dikes
Dolerite sillSedimentdike
Dolerite sill
Sediment dikes
Dolerite sill
Meta-sandstone
Dolerite
Dolerite
Dolerite
Sedimentdike
AA
B C
D E
A
B C
D E
ssR67
Figure 2. The Waterdown Dam locality. (A) Overview of the locality, showing the transgressive dolerite sill and the roadcut along R67 with sediment dike localities. (B) Site 2, with sediment dikes that can be traced 10–15 vertical meters. (C) Site 3, with the >2-m-thick breccia dike within the dolerite. (D) Close-up of the dike at site 3, showing a dolerite fragment within the baked sandstone. Note the irregular fragment in the lower right, possibly representing altered magmatic material. Coin for scale. (E) The sediment dike at site 1. Note the sediment fragments and the dolerite bridge extending from the walls and into the dike. Hammer for scale.
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Elandsberg Roadcut (Nico Malan Pass)The locality is located in the great escarp-
ment defi ned by thick sill intrusions in the Beaufort Group sediments. A sediment dike was found intruding into the lower contact of the upper sill encountered when driving north toward the Nico Malan Pass along the R67 (S32°30.2′, E26°50.2′). The dike has pen-etrated 2.3 m into the inclined dolerite sill, and has a slightly curved and irregular shape (Fig. 4A). The maximum width is ~20 cm, and the dike pinches out upward. No doler-ite fragments were found in the dike, and the sandstone texture was markedly different at the tip of the dike compared to the surrounding contact-aureole sandstone, becoming increas-ingly recrystallized. No fl ow structures were observed in the sediment beds below the dike or within the dike.
Golden ValleyThe Golden Valley sill complex (Galerne
et al., 2008; Polteau et al., 2008a, 2008b) is characterized by a fl at inner sill that is partly exposed along a small river in the southern end. Here (S31°58.4′, E26°16.4′) a well-exposed part of the sill-roof hosts several small (<30 cm wide) sandstone intrusions (Fig. 5A). Note that the sediment source is located above the sill contact, demonstrating downward sediment movement. Note that in general, downward sediment movement is not unique for this loca-tion (e.g., Harms, 1965; Peterson, 1968; Vita-nage, 1954). The dikes are irregularly shaped, and are characterized by a network-like pattern. Brownish alteration haloes are common around the dikes. The intruded sediments are Beaufort Group sandstones and shales, where the sand-stones contain abundant nodules with radial
fracture patterns (Fig. 5B). These nodules were originally composed of carbonate, but were modifi ed during metamorphism.
Sediment Petrography and Petrology
We studied thin sections of sediment dikes from the Waterdown Dam (site 3), the Nico Malan Pass, and Golden Valley. The main aims were to identify metamorphic minerals, char-acterize the texture, and characterize the meta-morphic conditions. The diageneses of non-metamorphic sandstones located far from sill intrusions in the Karoo Basin are characterized by authigenic minerals stable at relatively shal-low burial (clay minerals, K-feldspar, calcite, albite, and quartz) (e.g., Rowsell and De Swardt, 1976; Svensen et al., 2008; Turner, 1972). Typi-cally, detrital grains (like quartz and K-feldspar)
0.5 meter
Cover
Metamorphicsandstone
Sediment
Dolerite
Cover
Cover
DoleriteSedim
ent
A B
C
Figure 3. (A) Graphical representation of the rock fragments in the sediment breccia dike at site 3. The dike content was drawn on transparent plastic sheets (1:1 scale), scanned, and redrawn. Note the abundant dolerite fragments and sedi-mentary fragments with preserved sedimentary layering. (B) Image analysis shows that the rock fragments constitute 13.6% of the dike area. (C) The fi gure shows a scatter plot of the minor and major half-axis for ellipses fi tted to the frag-ments, with dolerite in red and sediment in gray. The plot suggests a linear trend between the minor and major half-axis, hence the aspect ratio is independent of the fragment size. The mean aspect ratios for dolerite and sediment fragments are 1.95 and 2.51, respectively. Very small fragments (<0.1 cm2 [i.e., 10 pixels]) are disregarded.
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Figure 4. The Elandsberg roadcut (Nico Malan Pass). (A) Sandstone dike extending ~2.5 vertical meters from the base of an ~100-m-thick dolerite sill. (B) Scanning electron microscope photograph showing metamorphic biotite, chlorite, and feldspar in the meta-sandstone from 2.3 m into the dike.
DoleriteDolerite
SedimentSedimentdikedike
QuartzQuartz
PlagPlag
ApAp
ChlChl
IlmIlm
PlagPlag
QuartzQuartz
QuartzQuartz
PlagPlag
ChlChl
ZeoliteZeolite
b
QuartzQuartz
GarnetGarnet
100100 µmµm
Dolerite
Sedimentdike
A B
C D
A
Quartz
Plag
Ap
Chl
Ilm
Plag
50 µm
Quartz
Quartz
Plag
Chl
Zeolite
b
Quartz
Garnet
100 µm
B
C D
AlAlAl
Figure 5. The Golden Valley locality. (A) Network of sandstone dikes within the upper 1 m of a sill in the fl oor of the Golden Valley saucer. (B) Reaction nodule in sandstone from 1 to 2 m above the contact with the sill. (C) Scanning electron microscope (SEM) photograph showing metamorphic plagioclase (plag), chlorite (Chl), and apatite (Ap) in a sample from the thickest dike shown in A. Ilm—ilmenite. (D) SEM photograph showing authigenic garnet, zeolite, quartz, and chlorite in the nodule in B. Alb—albite.
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get coated and overgrown by authigenic miner-als during burial without affecting the composi-tion or texture of the grain interiors. At 1 km of burial in the Karoo Basin, the original sandstone porosity could have been ~10%–25%, presum-ably fi lled with low-salinity pore fl uids. After contact metamorphism of sandstone within the sediment dikes, detrital components of the quartz grains are still easily recognized, whereas feldspar grains (plagioclase and K-feldspar) were recrystallized in mosaic patterns. More-over, the rock porosity is negligible, and chlo-rite and biotite are commonly present. Further details of the effects of contact metamorphism of sandstone injections from the examined localities are given in the following.
The sediment dike at Waterdown Dam con-tains metamorphic sandstone. Former grains and grain boundaries, representing the original sedimentary components, are easily recognized (Fig. 6A). This is confi rmed by CL imaging of quartz (Fig. 6B). In addition to quartz, the dom-inant minerals that recrystallized in the dike are K-feldspar, plagioclase, and chlorite. Detrital feldspar grains are recrystallized and contain a mosaic of K-feldspar and plagioclase. Based on the gray-scale variations on SEM backscat-ter images, the plagioclase is characterized by several different compositions, apparently in textural equilibrium (Fig. 6C), and thus recrys-tallized during high-temperature metamorphic conditions. Biotite was not identifi ed in the studied sample, but abundant chlorite could possibly be a product of biotite retrogression. The altered dolerite fragments in the dike are dominated by chlorite.
The Elandsberg sandstone dike contains identifi able detrital sand grains with quartz overgrowths. A sample from 2.3 m into the dike was studied using SEM, where CL imaging revealed detrital quartz cores. The presence of metamorphic epidote and biotite is important. The biotite is partly altered to chlorite, although the dominating mode of chlorite occurrence is in fresh patches unrelated to alteration. Feldspar grains are recrystallized and comprise mixtures of K-feldspar, albite, and plagioclase. Generally, the textures within the dike sample are tight and typical hornfels-like.
The sandstone dikes from Golden Valley have the same mineral content as the one at Water-down Dam. Quartz, plagioclase, and chlorite are the main minerals. One difference, however, is the plagioclase textures. In Golden Valley, the detrital plagioclase is apparently completely recrystallized and zoned, present as tabular crystals (Fig. 5C). Ilmenite and apatite are minor minerals. The chlorite is locally present as tabular crystals, possibly suggesting biotite replacement. We have compared this mineral
200200 µmµm
100100 µmµm
QuartzQuartz
QuartzQuartz
ChloriteChlorite
K-fsK-fsp
PlagPlag
ChloriteChlorite
QuartzQuartz
QuartzQuartz
FspFsp
IlmIlm
PlagPlag
QuartzQuartz
200200 µmµmChloriteChlorite
QuartzQuartz
FspFsp
FspFsp
FspFsp
200 µm
100 µm
Quartz
Quartz
Chlorite
K-fsp
Plag
Chlorite
Quartz
Quartz
Fsp
Ilm
Plag
Quartz
200 µmChlorite
Quartz
Fsp
Fsp
Fsp
A
C
B
A
C
B
Figure 6. Scanning electron microscope (SEM) petrogra-phy from the sediment dike at Waterdown Dam, site 3. (A) The sandstone matrix shows well-defi ned quartz grains that have survived metamorphism. Fsp—feldspar; Ilm—ilmenite. (B) The cathodoluminescence imaging demonstrates that melt-ing never took place, as the quartz grains have retained their detrital core. (C) The feldspar grains are, however, pervasively recrystallized, present as mosaics of K-feldspar (K-fsp) and plagioclase (Plag) with various compositions.
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assemblage with the assemblage within a for-mer carbonate nodule with sandstone from the same locality. The sample (Fig. 5B) is located ~2–3 m above the contact with the dolerite sill, and is characterized by radial fractures extend-ing out from a zoned nodule. In thin section, the main minerals are quartz (with detrital cores and overgrowths), feldspar, chlorite, and zeo-lite (Fig. 5D). The boundaries between detrital cores and metamorphic quartz are marked by rims of metamorphic garnet. The plagioclase is partly dissolved, and the pores fi lled by zeolite. Chlorite is common within the zeolite.
The studied textures from the examined local-ities show that the sandstone dikes underwent medium-temperature metamorphism following injection. Original quartz grain boundaries and grain cores are still preserved and document that the sediment dikes did not undergo partial melting after emplacement. This is consistent with the absence of macroscopic melt patches in the dikes. Diagnostic peak metamorphic minerals are sparse in metasandstones due to the low iron and magnesium content. Recrys-tallization of quartz and feldspar grains is the dominant mode of mineralogical transforma-tion. However, the occurrence of minerals like chlorite, biotite, plagioclase, and epidote is typical for greenschist facies conditions. Based on the general presence of these phases in the sediment dikes, we can use phase petrology to constrain the peak metamorphic conditions. We have made a phase diagram projected from a pelite composition, and compared the calcu-lated phase assemblages with those identifi ed in the rocks in order to determine the temperature during dike emplacement (Fig. 7).
Fluid Pressure Evolution During Sill Cooling
We have developed a numerical model in order to calculate the pressure gradients devel-oping between an igneous sill and the sur-rounding sedimentary rocks as a function of temperature. Here we present snapshots of the temperature and pressure state during sill cool-ing. The modeling is based on the parameters listed in Table 1.
15 yr After EmplacementAt the time of instantaneous emplacement,
the 100-m-thick sill is hot (1200 °C) with a sharp thermal boundary to the cold host rock (35 °C) (Fig. 8A). Note that the gradient will be similar even if the sill is emplaced by continu-ous infi lling and infl ation. After 15 yr the tem-perature increases rapidly in the host rock, caus-ing thermal expansion of the pore fl uids, which results in overpressure of ~22 MPa relative to
Figure 7. Phase diagram calculated using Perplex for a rock with a pelite composition. The mineral assemblages are shown between the black lines (dehydration reactions), and the positions of the sediment dike samples are indicated by stars (W—Waterdown Dam; G—Golden Val-ley; E— Elandsberg). Chl—chlorite; Ms—muscovite; Ab—albite; Ksp—K- feldspar; Zeo— zeolite; Ep—epidote; Bio—biotite; Plag—plagioclase; San—sanidine; Opx—orthopyroxene; Ol—olivine; Qtz—quartz; hCrd—–high-cordierite.
the hydrostatic pressure gradient (~7–8 MPa) (Figs. 8A, 8B). A fracture pressure of 35 MPa is indicated by a red dashed line in Figure 8B. The sill is in a state of underpressure due to cooling and crystallization of interstitial melt in a solid crystal network. The major mechanism of under-pressure within the sill (−22 MPa relative to the hydrostatic pressure gradient) is due to a den-sity change when interstitial melt (2600 kg/m3) is crystallizing (2900 kg/m3) within a confi ning crystal network during cooling. Note that the tensile strength of a gabbroic rock is >125 MPa (Ai and Ahrens, 2004).
100 yr After EmplacementAfter 100 yr, the sill has solidifi ed and the
temperature gradients become less steep (Fig. 8C). Correspondingly, the pressure gradient anomalies are reduced through diffusive fl uid fl ow. The internal gradient within the sill is
dispersed, and the main gradient is now from the host rock into the sill (Fig. 8D). The dif-ference between the maximum overpressure (~4 MPa) in the host rock and underpressure in the sill (~–7 MPa) is ~10 MPa, relative to hydrostatic pressure.
Fluid Pressure Evolution During Sediment Heating
The fl uid pressure in the aureole is increasing after sill emplacement due to the density change associated with heating of the H
2O pore fl uid.
Assuming a pore fl uid pressure of 25 MPa at ~1 km depth, H
2O undergoes a density reduc-
tion from 1004 kg/m3 to 162 kg/m3 when heated from 35 °C to 400 °C (Wagner and Pruss, 2002). The thermal expansion of the sediment matrix for the same temperature interval is negligi-ble relative to the phase transition in the fl uid
P (
MP
a)
200 400 600 800
60
120
180
240
300
T (°C)
E
G
W
ChlChlMsMsAbAbKspKspZeoZeo
ChlMsAbKspZeo
ChlMsAbKspZeo
ChlMsAbKspZeo
ChlMsAbKspEp
BioChlMsPlagEp
BioBiohCrdhCrdMsMsPlagPlag
BiohCrdMsPlag
BiohCrdSanPlag
hCrdPlagSanOpxOl
+H2O+Qtz
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Figure 8. (A) Pressure gradients developed after 15 yr of sill cooling. The sill is still close to 100% molten (see vertical dashed line). (B) There is a strong pressure gradient between the sill margins and the aureole, where the aureole pressure is generated by thermal expansion of pore water. The arrows indicate pressure gradients along which melt and fl uids are expected fl ow. The tensile strength of dolerite is indicated. (C) The sill is solidi-fi ed after 100 yr of cooling. (D) Note that there is still a strong gradient from the underpressure at the margins to the overpressure in the aureole, but most of the overpressure that was generated previously has diffused away.
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(i.e., boiling). The overpressure is released through diffusive fl ow, with rates depending on the permeability of the host rock. Similarly, the underpressure within the sill is created during cooling and crystallization. The thermal contrac-tion associated with the melt to crystal transition is several magnitudes larger than the thermal con-traction of the surrounding network for the same temperature interval. Hence, an underpressure will develop as a response to the density change of interstitial melt (>55% crystals; Marsh, 1996; Philpotts and Carroll, 1996). This underpressure can be relaxed through internal melt fl ow from the molten to the crystallizing regions of the sill. When the sill is 100% crystallized, the thermal stresses will continue to develop as long as the thermal contraction is larger than what can be accommodated by volume change. The stresses can be released through brittle fracturing of the rocks, which in turn can be fi lled in by, for exam-ple, fl uids, interior melt, or fl uidized sediments (e.g., Norton et al., 1984).
When estimating the thermal expansion of pore fl uids in the aureole, we use a conservative coeffi cient value of 4 × 10–4 (Delaney, 1982), resulting in pressure anomalies to ~25 MPa. Using the defi nition of thermal expansion coef-fi cient α,
1
P
v
v Tα ∂⎛ ⎞= ⎜ ⎟∂⎝ ⎠
, (1)
where v is specifi c volume (per unit mass; 1/ρ; ρ is density), the expansion coeffi cient for pore fl uid is 2.3 × 10–2 K–1, where boiling occurs, and for melt-to-crystal transition it is 3.5 × 10–4 K–1. The maximum pressure anomaly by boiling and expansion of pore fl uids may thus be as much as two magnitudes larger than our estimates.
Thermal Modeling of Sediment Dikes
We have made a thermal model with a real-istic sediment dike geometry to estimate the maximum temperature attained within the dike at a given sill temperature. We emplace a 20-m-tall and 2-m-thick sandstone dike with an initial temperature of 35 °C into a 100-m-thick sill with sill temperatures between 1100 and 1200 °C (Fig. 9). As expected, the dike rapidly reaches peak temperature (i.e., within 1 yr). Hence, the initial temperature of the sandstone dike is not important for the fi nal maximum temperature recorded in the sill. If the dike is injected 15 yr after sill emplacement, the sedi-ment dike reaches a temperature of ~850 °C. Injection at the time of sill solidifi cation (i.e., at ~100 yr), the peak temperature in the dike is ~650–675 °C. For the sandstone dike to be heated to a maximum of ~450 °C (cf. Elands-
100 200 300 400 500 600 700 800 900 10000
100
200
300
400
500
600
700
800
900
1000
Year
s af
ter
sill
empl
acem
ent
Max temperature in sandstone dike [°C]
1100 °C1200 °C
1100 °C1200 °C
100 meter sill50 meter sill
Walton and O’Sullivan (1950)
This study (E)
This study (W+G)
melt
100% solid
Figure 9. Calculated maximum (max) temperature of a 2 × 20 m sediment dike injected into a 100 m sill (solid lines) and a 1 × 10 m sediment dike injected into a 50 m sill (dashed lines) for intrusion temperatures of 1100–1200 °C as a function of injection time after sill emplacement. The gray area indicates when the sill is still molten. By knowing the maximum temperature of the sandstone dike, we can infer that the injection time was 200–300 yr after sill emplacement for the Elandsberg dike (E) and ~600 yr for the Golden Valley (G) and Waterdown Dam (W) dikes. For comparison, the calculated injection time of the dikes from Walton and O’Sullivan (1950a) occurred closely after sill solidifi cation (~150 yr).
berg), injection after 300 yr of sill cooling is indicated. After 600 yr the sill has cooled to such an extent that the temperature in the sedi-ment dike never exceeds 350 °C (cf. Golden Valley and Waterdown Dam).
DISCUSSION
Contact Metamorphism in Sedimentary Basins
In contrast to the 30–70 m.y. time scale of fl uid production and pressure buildup during regional metamorphism and orogenesis (e.g., Connolly and Thompson, 1989; Walther and Orville, 1982), contact metamorphism around igneous sill intrusions in sedimentary basins have dramatic and short-term effects on fl uid fl ow. This is particularly important in basins with rapidly cooling sill intrusions compared to settings with >100 k.y. of contact meta-morphism around plutons (e.g., Hanson, 1992, 1995). When sedimentary host rocks
are heated around sills, pore fl uid expansion and boiling occur on a time scale of years, dominating the fl uid production compared to devolatilization reactions (e.g., Delaney, 1982; Hanson, 1995; Jamtveit et al., 2004). Overpressure related to boiling and pore fl uid expansion may ultimately lead to hydrofrac-turing and the formation of hydrothermal vent complexes in the upper 1 km in the basin (e.g., Jamtveit et al., 2004). In the Karoo Basin, the hydrothermal vent complexes commonly crop out in the Stormberg Group sediments. In addition, numerous breccia pipes are rooted in contact aureoles of black shale, demonstrating that high pore fl uid pressures developed dur-ing rapid cracking of organic matter to meth-ane (Svensen et al., 2007). Thus contact meta-morphism around sill intrusions is a process that causes rapid pressure buildup and drives fl uid fl ow on a very short time scale. In this setting, sediment dikes represent direct evi-dence for the rapid release of aureole pressure and fl uids.
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It has been shown that sill cooling and crys-tallization result in an underpressure within the sill (Aarnes et al., 2008). Underpressure gener-ation is caused by the following. At the earlier stages of the sill cooling, a solid crystal net-work (>55% crystals) with interstitial melt will form (Marsh, 1988, 1996; Philpotts and Car-roll, 1996). With further cooling the interstitial melt undergoes a signifi cant density change due to the melt-to-crystal transition. However, a strong crystal network prevents a volume change and causes a large underpressure to develop. Experiments have shown that a crys-tal network have considerable strength already at 35% crystals, and effectively behaves as a solid even with large amounts of interstitial melts (Philpotts and Carroll, 1996). Such an underpressure may induce melt fl ow, have con-sequences for the chemistry of the magmatic system, and induce sediment injections into the sill (Aarnes et al., 2008).
During the initial stages of sill cooling, the pore water in the aureole sediments will expand and fl ow either away from the sill or into the sill, depending on the pressure gradients. Melt may also fl ow within the sills along the pressure gradient toward the cooling margins (cf. Fig. 8). The fl uid fl ow is a result of the developed pressure anomalies and will act to even out the pressure anomalies with time. After 100 yr the pressure gradient within the sill is reversed, going from the margins to the center (Fig. 8B). However, the melt is now unable to fl ow as solidifi cation is complete. At this time, the pres-sure in the sedimentary host rock has effectively
been diffused by fl uid fl ow. Thus, the main pres-sure gradient is now from the host rock toward the sill, both above and below the intrusion. At this stage, heated pore fl uids will fl ow into the sill if permeability allows the fl uids to enter, i.e., if fractures develop.
Aureole Overpressure and Sediment Injections Into Sills
Fluidization due to heating of water-rich sed-imentary rocks is most likely to occur at depths where pressure is less than the pressure corre-sponding to the critical point of water (Jamtveit et al., 2004; Kokelaar, 1982). The paleodepth of the study areas with sediment dikes in dolerites is ~600–900 m, thus shallower than the critical depth. In some geological settings overpressure can cause horizontal fracturing through fl uids seeping away from the overpressurized source (e.g., Cobbold and Rodrigues, 2007; Mourgues and Cobbold, 2003), while in the case of boil-ing and very high overpressures, modeling has demonstrated that the gas release may localize vertically and eventually reach the atmosphere (e.g., Jamtveit et al., 2004; Rozhko et al., 2007). The key requirements for pressure-induced sediment mobilization in the aureole are low permeabilities, high porosities, and high ther-mal diffusivities (Delaney, 1982; Jamtveit et al., 2004). In the case of high permeabilities in shallow sandstones, the rate of heating must exceed the rate of pressure loss by fl uid fl ow in order to build up signifi cant overpressure. When the sedimentary host rock undergoes extensive pressure buildup, it may ultimately lose all cohesive strength and become fl uidized
and result in substantial sediment displacement (e.g., Harms, 1965; Kokelaar, 1982; Ross and White, 2005; Vitanage, 1954). The sandstones of the Beaufort Group in the Karoo Basin were still in the early to intermediate stages of dia-genesis (i.e., reached quartz cementation) at the time of sill emplacement. Thus the condi-tions were right for fl uidization to occur, at least where clay minerals limit relaxation of pres-sures through fl uid fl ow (Jolly and Lonergan, 2002), or as mentioned, if heating was rapid compared to pressure drop by fl uid fl ow (Jamt-veit et al., 2004).
Heat-induced overpressure and subsequent fl uidization of sediments in the contact aure-ole is here suggested to be the main formation mechanism of sandstone dikes in magmatic intrusions. We show that there is an additional strong gradient from the aureole into the intru-sion, and that this gradient makes sediment mobilization more likely to happen compared to injections driven by pore fl uid boiling and frac-turing during thermal contraction. However, we argue that fracturing during thermal contrac-tion is of lesser importance, as sediment dikes are not present in a hexagonal network even in areas with abundant fractures developed dur-ing thermal contraction (e.g., the Golden Valley locality). The overpressure scenario is sche-matically presented in Figure 10. Our results show pressure anomalies of as much as 108 Pa after solidifi cation of the sill, in agreement with the magnitude 107 Pa overpressure commonly found for several rock types due to expansion of pore fl uids from magmatic intrusion (Del-aney, 1982). It is important that the high pres-sure is suffi cient to break the tensile strength of
t = emplacement t = 1-10 years t = 250-600 years
Sill intrusion(100% melt)
Sandstone
Sandstone
Melt+crystals 100% crystals
100
met
ers
TP
A B C
Figure 10. Schematic evolution of a sill-aureole system with sediment injections. (A) Initial sill emplace-ment into cold sedimentary host rocks. (B) Contact metamorphism around the molten sill, and expansion of sedimentary pore fl uids (symbolized by circles). (C) Crystallization of the sill followed by fracturing due to the huge pressure (P) difference between sill and aureole. The sill is still hot enough to cause high-temperature (T) metamorphism of the injected sediments.
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sandstones above ~1 km depth (e.g., Kokelaar, 1982), thus fl uids can potentially fl ow from the aureole and into the sill.
We therefore argue that sandstone dikes form as a result of the difference in pressure between the sill and the aureole (~10 MPa) that develops during sill cooling and sediment contact meta-morphism. The pressure gradient is suffi cient for fracturing the sill (pressures beyond the lithostatic) and to act as a suction force on the sediments from the moment the chilled margin of the sill fractures. Once initiated, the fracture will propagate as a result of the injected pore fl uids and sediments; this also may lead to fur-ther tensile failure (cf. Rubin, 1993). The frac-turing process may be violent, as indicated by the high proportion of both sedimentary and doleritic rock fragments in the dikes at Water-down Dam. Sediment fragments compose 86% of the dike surface at site 3, and the size distri-bution between sedimentary and dolerite clasts suggests that the same process was responsible for brecciation of both dolerite sill and aureole sediments. The four orders of magnitude varia-tion in clast size (Fig. 3B) demonstrate that the brecciation was rapid and that the bulk of the breccia was injected into the sill.
Sediment Dike Metamorphism and Injection Timing
The sediment dikes described in this study are all affected by contact metamorphism. Thus they were heated while the sill intrusions were still hot, either in situ in the contact aure-ole prior to injection, or within the sediment dike. Metamorphism of the injected sediments is a common observation from all sediment dikes in magmatic sill intrusions (Van Biljon and Smitter, 1956; Walton and O’Sullivan, 1950). Based on the metamorphic minerals in the dikes and aureoles from the Karoo Basin (chlorite, biotite, plagioclase, epidote, and gar-net), the metamorphic conditions were equiva-lent to those of the greenschist facies. Based on these minerals and the phase diagram (Fig. 7), a maximum temperature of ~450 °C is sug-gested. There are no accurate thermometers that can be applied to the identifi ed mineral assemblages, so the temperature is approxi-mate. Comparing with active hydrothermal metamorphism of sandstone, biotite appears at ~320 °C (e.g., Schiffman et al., 1985), so our estimate is reasonable. The absence of minerals like cordierite, clinopyroxene, and muscovite furthermore suggests temperatures <~450 °C, although the potential for generating some of these minerals depends on the bulk rock composition. As the temperature of heated sedimentary rocks around a sill intrusion will
never exceed about half the sill temperature, a doleritic sill (~1200 °C) will commonly not be able to melt the host sediments, and maximum temperatures should be close to 600–700 °C, depending upon the host-rock temperature at the time of emplacement. However, this situ-ation may be different in other geological sys-tems (e.g., Hersum et al., 2007).
The temperature estimates from the mineral-ogy are of importance when assessing the tim-ing of sediment dike emplacement. As we have shown, an early emplacement into a hot sill will result in high-temperature metamorphism in the dike. Based on our thermal modeling, injec-tion after 250–600 yr of sill solidifi cation will give 325–450 °C in the dike. Note that reaction kinetics or signifi cant latent heat of vaporization may contribute to discrepancies between mod-eled heat from conduction and that of a natural system. Earlier timing of sediment injection is therefore possible.
To summarize, our data suggest that the emplacement of the sediment dikes occurred after the sill was 100% crystallized, which puts a lower boundary to the timing of injec-tion of ~100 yr.
This means that sediment injection into sills has only limited potential for contaminating the magma, since the sill is 100% crystallized at the time of sandstone injection. For contamina-tion to happen, the sediments would have to be injected into a partly molten sill, for which we have no supporting observations.
Field evidence shows that sediment dikes can propagate tens of meters into dolerite sills from the lower contact. The vertical termination of dikes has, however, not been found in the fi eld. However, since the metamorphic recrystalliza-tion led to very low permeabilities, the sediment dikes were prevented from becoming long- lasting fl uid fl ow pathways.
The basin settings in which sediment dikes within igneous sills are not likely to form are (1) when the overpressure difference between sill and aureole is small, as when the sill intru-sion is thin, or (2) the aureole has limited poten-tial for generating overpressure during heating, e.g., when the porosity is very low or the content of organic matter is negligible. Thus the pres-ence of sediment injections in igneous systems may provide important constraints on the pres-sure evolution and fl uid fl ow history in sedimen-tary basins with sill intrusions.
CONCLUSIONS
Sediment dikes have been discovered within dolerite sill intrusions at several locali-ties in the Karoo Basin in South Africa. The sediment dikes contain metamorphic sand-
stone and clasts of sediments and dolerite. Field, petrographic, and numerical evidence suggests the following.
Both upward and downward movement of sediments into sill intrusions is common.
The sediments intruded while the sills were hot, producing mineral assemblages typical for >300 °C metamorphism.
Thermal modeling, to account for the dike metamorphism, shows that the sediment dikes were injected more than 100 yr after sill emplacement, depending on sill thickness and the initial sill temperature.
The presence of sediment dikes in sills is a result of the coupled pressure evolution of dol-erite sills and contact aureoles. Negative pres-sure anomalies in the sill form due to cooling, whereas high pressure develops in the aureole due to thermal expansion.
The pressure generated is of the correct order of magnitude required to explain fracturing of the solidifi ed sill. The sediments were accord-ingly drawn into the sill.
APPENDIX
Equations
The cooling of the sill and heating of the host rock follow the heat conduction equation:
2 2
2 2effT
T T TK
t x z
⎧ ⎫∂ ∂ ∂= +⎨ ⎬∂ ∂ ∂⎩ ⎭, (A1)
where T is the temperature, x is lateral direction, z is vertical direction, and eff
TK is the effective thermal diffusivity coeffi cient (λ / C
P / ρ), and λ is the thermal
conductivity, CP is heat capacity and ρ is density. The
effective thermal diffusivity accounts for the latent heat of fusion:
( )
(1 )
( )
eff TT S L
effT T S
KK for T T T
Ste
K K for T T
= < <+
= > (A2)
The nondimensional ratio quantifying the effect of the latent heat is the Stefan number, Ste, given by:
( )L S P
LSte
T T C=
−, (A3)
where CP is heat capacity and L is the latent heat of
fusion per unit mass.Equations A1–A3 are coupled with pressure
through thermal stresses,
dP dTαβ
= , (A4)
as described by, e.g., Turcotte and Schubert (2002), assuming isochoric conditions for crystallization.
.
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Taking the partial derivative of equation A4 with respect to time, the hydraulic equation becomes:
2 2
2 2H
P P P TK
t x z tαβ
⎧ ⎫∂ ∂ ∂ ∂= + +⎨ ⎬∂ ∂ ∂ ∂⎩ ⎭, (A5)
where P is pressure, α is the volumetric coeffi-cient of thermal expansion and β is the isothermal compressibility.
HK
χμβ
= , (A6)
where KH is the hydraulic diffusivity, χ is matrix
permeability, and µ is viscosity of fl uid. This modi-fi ed hydraulic diffusion equation is similar to that of Delaney (1982). The fi rst part on the right side of equation A5 describes the pressure diffusion (simi-lar to heat conduction equation A1); the second part describes the development of pressure anomalies due to changes in temperature. The initial overpressure is zero, because the fl ow only depends on the evolving pressure anomalies.
ACKNOWLEDGMENTS
This study was supported by a PetroMaks grant from the Norwegian Research Council to Svensen. We thank Goonie Marsh and Luc Chevallier for dis-cussions during our fi eld trips to South Africa, in par-ticular Goonie for showing us the Waterdown Dam locality, and Dirk Liss for the company and assistance during sampling of the sediment dikes. Else-Ragnhild Neumann and the Golden Valley Study Group at PGP (Physics of Geological Processes, University of Oslo) contributed with valuable input to the project. We also thank Joe Cartwright and an anonymous referee for critical comments.
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