ELSEVIER
Tectonophysics 312 (1999) 217–234
www.elsevier.com/locate/tecto
Normal fault thermal regimes: conductive and hydrothermal heat
transfer surrounding the Wasatch fault, Utah
Todd A. Ehlers Ł , David S. Chapman
University of Utah, Department of Geology and Geophysics, Salt Lake City, UT 84112, USA
Received 23 June 1998; accepted 18 June 1999
Abstract
The thermal regime across an active normal fault is affected by tectonic processes of exhumation and erosion of the
footwall and burial and sedimentation on the hanging wall. An enhanced thermal regime in the footwall is juxtaposed
against a thermally depressed regime in the hanging wall causing significant two-dimensional (2D) heat flow. These
thermal processes have been simulated with 2D numerical models and applied to the Wasatch fault of central Utah.
Simulations included variable fault angles of 90º, 60º, and 45º and a vertical displacement profile accounting for hanging
wall and footwall tilt. After 20 m.y. of fault movement, 60º fault, isotherms are displaced ¾1 km on the footwall and
hanging wall. Furthermore, model surface heat flow is enhanced by 25% above the footwall and depressed by 15% above
the hanging wall 10 km from the fault trace; the heat flow transition has a half width of 10 km. Present-day surface heat
flow was compiled for 23 sites along the Wasatch Front. Heat flow has a mean value of 92 mW=m2 (standard deviation
25 mW=m2 ) but, unlike model predictions, does not show a discernible variation in heat flow across the fault. Part of
the discrepancy between predicted and observed conductive heat flow may be caused by groundwater which recharges
in the uplifted footwall, is heated in the subsurface, and discharges as thermal water along the range bounding fault or
into the hanging wall valley. Flow rates and water temperatures from 29 tectonic hot springs along the Wasatch Front
indicate a minimum hydrothermal thermal power loss of 90 MW or 0.24 MW per kilometer of strike along the fault. This
thermal power is equivalent to a heat flow of 21 mW=m2 captured uniformly between the range crest and range front, and
discharged in hot springs.  1999 Elsevier Science B.V. All rights reserved.
Keywords: heat flow; conductive; hydrothermal; normal fault; Wasatch; numerical model; hot spring
1. Introduction
The thermal regime within and surrounding fault
zones influences the strength of rocks (Smith and
Bruhn, 1984), the brittle–ductile transition, and
chemical reactions that seal fractures (Rimstidt and
Ł Corresponding
author. Tel.: C1-801-585-3588; Fax: C1-801581-7065; E-mail: [email protected]
Barnes, 1980; Smith and Evans, 1984; Bruhn et al.,
1990). All of these factors play a key role in the
nucleation and propagation of earthquake ruptures
(Smith and Bruhn, 1984; Parry and Bruhn, 1990).
Furthermore, details of footwall exhumation (from
apatite fission track, K–Ar ages of sericite, and fluid
inclusions) (Roedder and Bodnar, 1980; Bowers and
Helgeson, 1983; Parry and Bruhn, 1987) are sensitive
to an assumed configuration of isotherms and the
thermal gradient around the fault.
0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 1 9 5 1 ( 9 9 ) 0 0 2 0 3 - 6
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T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
Previous work related to normal fault thermal
regimes is not as extensive as for strike-slip (Henyey,
1968; Brune et al., 1969; Lachenbruch and Sass,
1988; Koons, 1989; Shi et al., 1996), and thrust fault
thermal regimes (Barton and England, 1979; Molnar
et al., 1983; England and Thompson, 1984; Molnar and England, 1990). One-dimensional effects of
sedimentation and erosion on the hanging wall and
footwall geotherms, respectively, have been known
for quite some time (Benfield, 1949; Birch, 1950;
Carslaw and Jaeger, 1959). Rupple et al. (1988)
made an analysis of normal fault thermal regimes, in
two dimensions, with emphasis placed on the effect
of pure and simple shear on metamorphic pressure,
temperature, and time paths. Ketcham (1996) investigated thermal models for core-complex evolution
with insights into present-day heat flow and ancient
cooling paths. Other, more general, studies related
to thermal effects of lithospheric thinning during extension (Lachenbruch and Sass, 1978; Furlong and
Londe, 1986; Buck et al., 1988) focus on largerscale, regional, thermal processes rather than those
processes occurring near the fault plane.
This investigation complements previous studies
by providing conductive heat flow and hydrothermal
heat loss data for the Wasatch normal fault, western
U.S.A. (Fig. 1). These data, coupled with a 2D numerical model of the thermal regime, provide insight
Fig. 1. Location map for the Wasatch fault in north-central Utah. Symbols show locations of conductive (borehole) heat flow
determinations; circles indicate hot springs. Faults (thick lines include the Wasatch fault), composed of the Brigham City, Ogden, Salt
Lake City, Provo, and Nephi segments, and the Stansbury, Oquirrh, and Mercur faults, located 40–60 km west of the Wasatch fault. Of
all the faults shown in this figure, the Wasatch fault is the locus of the most sustained, and rapid displacement. Circles around hot springs
are scaled according to the magnitude of thermal power output from each spring. Quaternary scarps are shown with small dots. Thick
lines indicate fault scarps that are either historically seismogenic or are known to have displacement rates greater than about 0.2 mm=yr.
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
into the transient thermal processes surrounding normal faults.
2. Geologic background
The Wasatch fault (Fig. 1) is a major normal
fault in north-central Utah that marks the easternmost boundary of the Basin and Range physiographic
province (Gilbert, 1928; Cluff et al., 1975; Swan et
al., 1980). It extends 370 km from southern Idaho
to central Utah. This seismically active fault is capable of M 7.5–7.7 earthquakes (Arabasz et al., 1992)
and has an approximate 0.7–1.5 mm=yr relative vertical displacement rate with 11 km exhumation over the
last 11–18 Ma (Zoback, 1983; Parry and Bruhn, 1987,
1990; Parry et al., 1988; Arabasz et al., 1992; Bruhn
et al., 1990; Machette et al., 1991). The Wasatch fault
has an average surface dip of 60º to the west (Smith
and Bruhn, 1984). Abundant geologic and geochemical studies of the fault zone, including fault trenches
(Machette et al., 1991; Hecker, 1993), fission track
(Evans et al., 1985; Kowallis et al., 1990) and fluid inclusion analysis (Parry and Bruhn, 1987, 1990; Parry
et al., 1988) make this location suitable for an analysis of the thermal regime because the duration and
rate of faulting is reasonably constrained.
Sevier Orogeny thrust faulting thickened sedimentary strata in the footwall prior to movement on
the Wasatch fault, which was initiated about 11–18
Ma. The hanging wall of the fault is covered with
unconsolidated sediment ranging in thickness from
1 to 5 km with an average thickness of about 1.5
km (Zoback, 1983; Radkins, 1990), underlain by
Tertiary and older sedimentary rocks akin to strata
exposed in the footwall.
3. Numerical model
3.1. Mathematical model
The background thermal state of the upper crust is
controlled primarily by basal heat flow, and thermal
properties (conductivity, heat production) of crustal
rocks. This background thermal state, however, can
be significantly perturbed in regions of active tectonism. For the case of the Wasatch normal fault
219
a depressed (decreased heat flow) thermal regime
caused by sedimentation on the hanging wall is juxtaposed against an enhanced (increased heat flow)
thermal regime on the exhumed footwall. Lateral
heat flow may be significant. To address the problem
of 2D heat flow in the vicinity of an active normal
fault, one must formulate the advective heat equation
in two dimensions (Carslaw and Jaeger, 1959):
div.K Ð rT /
²Ðc
v Ð rT
@T
D
@t
A
²Ðc
(1)
where v is the velocity of the medium relative to the
boundary surface, A represents the radioactive heat
production rate per unit volume, K is the thermal
conductivity, ² is density, and c specific heat. Eq. 1
is appropriate to study 2D transient temperature distributions around an active normal fault. There is
no analytic solution for Eq. 1 in 2D so a numerical
solution was implemented.
3.2. Numerical model
The algorithm for simulating normal fault thermal regimes was constructed using a central difference approximation for spatial derivatives and a
forward difference in time (see Peacock (1989) and
Furlong et al. (1991) for a description of explicit
finite difference method). Mathematical stability was
maintained by constraining time step sizes and nodal
spacings according to the criteria described by Peacock (1989). For the parameters specified in Table 1,
time steps of 100 years assured numerical stability.
Numerical model accuracy was tested by comparing model results to simplified situations for which
analytic solutions exist. Model test runs were compared to solutions of the 1D steady-state heat conduction equation and the 1D analytic solution of the
advective heat equation (Carslaw and Jaeger, 1959).
For model tests involving material advection errors
were between 0 and 1.5% for simulations up to 10
m.y. of fault movement.
The finite difference model was constructed to
simulate the first-order processes affecting heat flow
in a normal fault regime and has several limitations.
First, the model does not allow for three-dimensional (3D) topographic evolution during exhumation. Rather, we assumed a constant topographic
profile, generalized to the dimensions of the Wasatch
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T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
Table 1
Numerical model input parameters
Material property
Model input value
Country rock thermal conductivity
Basin thermal conductivity
Density
Specific heat
Heat production
3 (W m-1 K-1 )
2.2 (W m-1 K-1 )
3000 (kg=m3 )
1000 (J kg-1 K-1 )
see Fig. 2b
Model parameters
Time step
Horizontal node spacing
Vertical node spacing
Valley surface temperature
Range surface temperature
Basal heat flow
Initial surface heat flow
Basin hingeline distance from fault
Footwall hingeline distance
from fault
Maximum topographic relief
Orogen width
a Used
100 (years)
130, 150 a (m)
225, 150 a (m)
15 (ºC)
15–7 (ºC=km) ð height (km)
0.06 (W=m2 )
0.09 (W=m2 )
20 (km)
40 (km)
2 (km)
37 (km)
in 45º and 90º fault simulations.
Range (relief of 2.0 km, range width of 40 km).
Neglecting 3D topographic variations limits the accuracy of near-surface temperature gradients, but
subsurface distributions below about 1–2 km are
unaffected. Second, since a constant thermal conductivity was assumed in the basin we neglect to
simulate the process of sediment compaction and
the formation of a variable conductivity sedimentary
basin. Our next stage of modeling will explore these
additional processes.
The specific 2D model simulated in this study is
shown schematically in Fig. 2. Simulations were made
with fault dips of 45º, 60º (the surface outcrop dip of
the Wasatch fault), and 90º. The modeling domain
used was a rectangular region with lateral and vertical
dimensions of 120 by 40 km. The model was constructed in an Eulerian framework, so the finite difference grid remained constant throughout the simulation while temperatures and material properties were
translated through the grid to simulate advection.
Material properties used are shown in Table 1
and Fig. 2. The hanging wall basin was assumed
to have a constant thermal conductivity of 2.2 W
m 1 K 1 and country rock a conductivity of 3.0 W
m 1 K 1 . Material advection in the model allowed
for the development of a sedimentary basin through
time. Radiogenic heat production was assumed to be
variable with depth. A heat production–depth profile
typical for Basin and Range province lithologies
(Lachenbruch and Sass, 1977; Pollack and Sass,
1988) was implemented and is shown in Fig. 2b.
Material advection allowed for the ‘stripping off’ or
erosion of heat-producing layers.
Fault displacement rates were chosen in a manner consistent with observations of the present-day
Salt Lake Basin geometry and the averaged Wasatch
Range exhumation rate. Footwall exhumation rates
shown in Fig. 2c were assumed to be 1 mm=yr adjacent to the fault with a linear decrease in velocity to
0 mm=yr at the footwall hinge 40 km from the fault.
The decrease in velocity with increased distance from
the fault simulates a magnitude of footwall tilt consistent with observations from structural reconstructions
(Parry and Bruhn, 1987) for model simulation durations of ¾11–15 m.y. Hanging wall burial rates in
Fig. 2 were assumed to be 0.5 mm=yr adjacent to the
fault with a linear decrease in velocity to 0 mm=yr
at the hanging wall hinge 20 km from the fault. The
assumed hanging wall burial rates produce basin geometries and thicknesses consistent with seismic and
gravity observations (Smith and Bruhn, 1984; Radkins, 1990) for model simulation durations of ¾11–
15 m.y. Eroded material was not entirely conserved
by deposition in the adjacent basin because some sediment from the Wasatch Range is believed to be deposited in thick localized depocenters to the west of
the fault, and out of plane of the model profile.
Boundary conditions for the model included a
constant surface temperature in the valley with a
variable, elevation-dependent, surface temperature in
the range, a constant basal heat flux at the bottom,
and no-flux boundaries on the sides (Fig. 2a). Initial
temperatures were specified at each node replicating
a steady-state thermal regime consistent with the
input basal heat flow.
4. Model results
4.1. Effects of exhumation and fault angle on
subsurface temperatures
Fig. 3 shows transient subsurface isotherms and
temperature residuals for 5 and 15 m.y. of dis-
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
221
Fig. 2. Schematic normal fault thermal model. (a) Three different fault angles of 90º, 60º, and 45º were simulated. Generalized
topography of the Wasatch Range is shown at the top of the model. Table 1 summarizes model input parameters. (b) Typical Basin and
Range province heat production profile (Lachenbruch and Sass, 1977; Pollack and Sass, 1988) used in model. Heat-producing layers
were stripped off (footwall) or buried (hanging wall) as material advected. (c) Vertical-component velocity profile, or exhumation rate,
assumed in model to account for footwall and hanging wall tilt (see text).
placement on a 60º fault with an actively forming
sedimentary basin. Cross-section geometries coincident with each time step are shown in Fig. 3c,d. The
isotherms for other fault angles differ only slightly in
geometry, and these differences are discussed in later
sections. Fig. 3a,b show the general trend in hanging
wall and footwall isotherm configuration. Far from
the fault where the velocity field is zero, isotherms
maintain their original positions. Close to the fault
footwall isotherms are swept upward by the advecting material. Hanging wall isotherms approaching
the fault would normally be depressed by downward
advecting material. In this case, three thermal processes offset the advection effect: (1) a low thermal
conductivity basin forming which causes increased
temperatures within and below the basin; (2) the
crust below the basin is being heated additionally
by advection in the footwall below the 60º fault; (3)
there is a 2D lateral flow of heat from footwall to
hanging wall.
The isotherm displacement is most pronounced in
the first 5 m.y. and slowly increases with sustained
faulting. For example, the initial depth (2D steadystate initial condition) of the 150ºC isotherm is at
4.5 km depth below the valley floor on the hanging
wall and 3.7 km depth below the valley floor on the
footwall 10 km from the fault. After 5 m.y. of fault
movement (Fig. 3a) the 150ºC isotherm is at 4.25
km on the hanging wall and 2.9 km on the footwall
(10 km from the fault). After 15 m.y. of tectonism
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T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
Fig. 3. Transient isotherms, cross-section geometries, and residual temperatures after 5 (a, c and e) and 15 (b, d and f) m.y. of
displacement on a 60º fault. Isotherms (a and b) are swept up on the hanging wall and footwall. Model cross-section geometries (c and d)
after 5 and 15 m.y. of faulting, respectively. 2D residual temperatures (e and f) (defined as 2D advective model minus 2D static model).
Contour values in ºC.
(Fig. 3b), the same isotherm is located at 3.7 km
on the hanging wall, and 2.7 km on the footwall.
The 2D advective model with an evolving basin
used in this study provides improved subsurface temperatures compared to 2D and 1D static (steady-state)
models. Residual temperatures, defined as the full 2D
advective temperature field a minus 2D static temperature field without thermal conductivity contrasts,
are shown in Fig. 3e,f. Hanging wall and footwall
temperatures in particular are under-predicted, or too
low, when advection is neglected. Positive residual
temperatures also occur in the hanging wall close to
the fault resulting from the fault dip and the presence
of a ‘warm’ footwall underlying the hanging wall and
the low thermal conductivity basin. Residual temperatures in Fig. 3e,f vary from 0ºC at the surface to 40ºC
at 6 km depth below the valley floor.
One-dimensional residual temperatures (defined
as the full 2D advective temperature field minus a 1D
static model) yield a similar result in under-predicting hanging wall and footwall temperatures. Onedimensional models have an additional drawback of
not accounting for the thermal topographic effect of
isotherm depression below topographic highs.
The imperative lesson learned from comparing
2D advective with 2D and 1D static thermal models
is that errors in isotherm location of about a kilometer occur when using static models. For example,
after 15 m.y. of faulting the 150ºC isotherm is located between about 4.0 to 3.1 km in the hanging
wall and footwall 10 km from the fault, respectively
(Fig. 3b). Inspection of the residual temperature plot
for 15 m.y. (Fig. 3f) at a depth of 4 km in the
hanging wall (10 km from the fault) suggests that
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
the static models differ from the advective model
by about ¾25ºC. Residual temperatures in the footwall at 3.1 km (10 km from the fault) are also
¾25ºC. Using an average Basin and Range thermal
gradient of 30ºC=km these residual temperature variations translate into an approximate 0.8 km error in
the predicted depth of an isotherm. Residual temperature variations at a depth are largest near the
footwall and close to fault. These high residual temperatures are a result of footwall tilt which causes
material in the footwall, close to the fault, to move
the fastest (Fig. 2c). The magnitude of residual temperature variations discussed here has significant implications when calculating exhumation rates from
thermochronometers where the depth to a closure
temperature is needed (Stuwe et al., 1994).
4.2. Effect of exhumation and fault angle on surface
heat flow
Fig. 4a shows the isolated components of model
advective and topographic surface heat flow. The advective component is for a 60º fault active for 10 m.y.
223
with an evolving sedimentary basin. Topographic effects on heat flow have been removed from the
advective component curve. The topographic component is for a static model with no sedimentary
basin. Both the advective and topographic components have the basal and radiogenic background heat
flow removed to isolate the heat flow components of
advection and topography. The 2D, static effect of
topography on surface heat flow is visible in Fig. 4a
(dashed line). Heat flow is increased by about 10%
above the background heat flow at the topographic
toe of the slope or nick point (0 km distance from the
fault trace) as isotherms are pulled up towards the
elevated mountain range. Heat flow is depressed in
regions of high elevation, with a minimum coinciding with the range crest. The advective component
shows a 8% increase above background heat flow 20
km from the fault in the hanging wall and depressed
heat flow over the sedimentary basin. The 8% increase is due to a conductivity contrast between the
country rock and the sedimentary basin (Table 1). A
40% increase in advective heat flow is present immediately adjacent to the fault in the footwall and then
Fig. 4. Model advective and topographic components of surface heat flow after 10 m.y. of displacement on a 60º fault. (a) Calculated
model advective (solid line) and static topographic (dashed line) components of surface heat flow. Advective curve includes the heat flow
effect of a sedimentary basin, but not topography. Topographic heat flow curve does not include material advection and the effect of a
sedimentary basin. Model cross-section (b) showing the sedimentary basin and topographic geometry for the results shown in (a).
224
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
Fig. 5. Model transient surface heat flow shown in Fig. 4 (at 5 m.y. intervals) versus distance from fault. Heat flow was normalized using
the 2D static model heat flow shown in (a). Curves in each panel represent models with fault dips of 90º, 60º, and 45º.
tapers of to 0% 40 km from the fault. The decrease
in advective heat flow with increased distance from
the fault is a direct result of footwall tilt (Fig. 2c).
Fig. 5 shows a series of heat flow patterns illustrating heat flow anomalies resulting from material
advection. All curves had the topographic components of heat flow removed and were normalized by
the background heat flow.
Normal faulting accompanied by erosion and sedimentation, in contrast to the static topographic effect, depresses heat flow on the hanging wall and
enhances heat flow on the footwall (Fig. 5). A verti-
cal fault produces a heat flow pattern sharply defined
at the fault by depressed heat flow on the hanging
wall adjacent to the fault and enhanced heat flow
in the footwall. Lessening the fault dip creates a
heat flow profile that deviates from that of a vertical fault. Footwall heat flow is maintained close to
background at distances beyond the footwall hinge
at 40 km, but gradually increases towards the fault
as a result of the increased material exhumation rate.
The crossover point where surface heat flow equals
basal heat flow is shifted over 5 to 7 km into the
hanging wall basinward from the fault trace at low
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
dip angles. Hanging wall heat flow is lowest near the
fault and gradually increases towards the 2D static
heat flow near the basin hingeline at 20 km. Both
the magnitude of the heat flow perturbation and the
crossover point distance from the fault increase with
increased duration of faulting.
The Wasatch fault has an approximate surface dip
of 60º and has been active for between 10 and 15
m.y. Fig. 5 indicates that normalized surface heat
flow across the Wasatch fault attributed to material
advection might vary from about 1.3 on the footwall,
to about 0.8 on the hanging wall, at a distance of 10
km from the fault. For a background heat flow of 90
mW=m2 , the expected heat flow pattern would vary
from ¾110 mW=m2 in the footwall to ¾80 mW=m2
in the hanging wall.
5. Surface heat flow on the Wasatch Front
Surface heat flow observations constitute a prime
constraint for evaluating crustal thermal regimes. A
225
compilation of 23 surface heat flow determinations
along the Wasatch Front made by previous investigators (see compilation in Pollack et al., 1993) is given
in Table 2; the sites are plotted in Fig. 1. The density
of sites is highly variable within the north–south
corridor with the majority of the sites located west of
the Wasatch fault.
The distribution of 23 Wasatch Front heat flow
values is shown in Fig. 6. These values range from
58 to 154 mW=m2 with a mean of 92 mW=m2 (s.d.
25 mW=m2 ). Previous work in the Basin and Range
province (Sass et al., 1994) found a heat flow of 84
mW=m2 (s.d. 25 mW=m2 ). The heat flow average
for the periphery of the Colorado Plateau province,
located within the study area, is 80–90 mW=m2 (s.d.
18 mW=m2 ) whereas the ‘thermally cool’ interior
of the plateau averages 60 mW=m2 (s.d. 9 mW=m2 )
(Bodell and Chapman, 1982). Thus the Wasatch
Front, which forms the transition between the Basin
and Range and Colorado Plateau provinces, has a
mean heat flow not significantly different from the
Basin and Range average. The high variability of
Table 2
Conductive heat flow determinations along the Wasatch Front
Site name
Latitude
Longitude
q
(mW=m2 )
QC Value
No. sites
Reference
GUNNISON
GORDON C
DESRT MT
SHEEPRCK
GC-1
ET-5
FIFTH WATER
BINGHAM
D-142
ALTADH16
1-2-C5
JORDANELLE
BROTHERS
STATE LD
HAMBLINI
OLSEN
UTE ALLOTE
JORDAN V
UTE
PINEVIEW
FRANKLIN
ANSTZR-E
ANSTZ-CC
39º190 N
39º340 N
39º440 N
39º490 N
39º520 N
39º570 N
40º5.80 N
40º310 N
40º320 N
40º33.420 N
40º340 N
40º25.70 N
40º420 N
40º440 N
40º440 N
40º440 N
40º450 N
40º470 N
40º490 N
40º560 N
41º010 N
41º040 N
41º040 N
111º520 W
111º010 W
112º360 W
112º24.50 W
112º030 W
112º030 W
111º18.60 W
112º090 W
112º090 W
111º40.670 W
112º240 W
111º25.50 W
112º180 W
111º570 W
112º050 W
112º180 W
112º150 W
112º04.30 W
112º10 W
111º090 W
111º320 W
111º040 W
111º070 W
95 š 20
88 š 17
92 š 5
100 š 9
82 š 20
154 š 4
153 š 10
70 š 2
96 š 13
72 š 10
100 š 20
89 š 10
113 š 22
89 š 18
103 š 20
100 š 19
86 š 17
75 š 25
102 š 20
63 š 8
64 š 13
58 š 8
60 š 8
2
2
1
1
2
1
1
1
2
2
2
2
2
2
2
2
2
3
2
2
2
2
2
1
1
1
1
1
1
2
2
3
1
1
7
1
1
1
1
1
1
1
2
1
2
2
Eggleston and Reiter, 1984
Eggleston and Reiter, 1984
Chapman et al., 1981
Chapman et al., 1981
Roy et al., 1968
Roy et al., 1968
Powell and Chapman, 1990
Roy et al., 1968
Constain and Wright, 1973
Blackwell et al., 1991
Eggleston and Reiter, 1984
Moran, 1991
Eggleston and Reiter, 1984
Eggleston and Reiter, 1984
Eggleston and Reiter, 1984
Eggleston and Reiter, 1984
Eggleston and Reiter, 1984
Constain and Wright, 1973
Eggleston and Reiter, 1984
Deming and Chapman, 1988
Eggleston and Reiter, 1984
Deming and Chapman, 1988
Deming and Chapman, 1988
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T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
Fig. 6. Distribution of heat flow determinations in the Wasatch
Front corridor.
heat flow is typical of young tectonic provinces with
magmatic or hydrologic perturbations.
The quality of a heat flow value depends on
several factors. Analysis of heat flow at a site generally includes a correction for the local topography
surrounding the borehole, which affects the measured temperature gradient (Sass et al., 1971). Furthermore, the availability of thermal conductivity
measurements and the presence of vertical groundwater flow, above a threshold rate, can introduce errors into surface heat flow determination (Smith and
Chapman, 1983; Forster and Smith, 1989). Original
sources for the heat flow determinations in Table 2
were consulted to make a quality assessment. An error range was assigned to determine a quality control
(QC) value for each heat flow determination. Quality
control values are based on the method described by
Sass et al. (1971) and consider both the number
of thermal conductivity measurements available, the
likelihood of vertical groundwater flow, and the topographic correction. A QC value from 1 to 3 was
assigned to each determination, where a value of 1
represents the highest quality determination and a 3
represents the lowest quality. Out of twenty-three heat
flow values shown in Fig. 1, six have a QC value of 1,
sixteen have a value of 2, and one has a value of 3.
A profile of heat flow perpendicular to the
Wasatch fault is shown in Fig. 7 including 19 heat
flow determinations within 60 km of the Wasatch
fault. Although this profile extends across other
faults beside the Wasatch fault, such as the Oquirrh
fault, the exhumation rate on these faults is one tenth
to half as much as the Wasatch fault, and therefore
not likely to perturb surface heat flow significantly.
Heat flow to the west of the Wasatch fault is moderately well constrained between 80 and 115 mW=m2 ;
heat flow east of the Wasatch fault is highly scattered. There remains a lack of data in the most
critical region within 10 km of the fault trace.
6. Hydrothermal heat loss
Hydrothermal heat loss near the Wasatch fault occurs through low-temperature groundwater systems.
The Wasatch Range rises 1000 to 2000 m above
the Salt Lake Valley and receives an average annual precipitation about 14 cm greater than the valley. Rainwater and snow pack in the Wasatch range
drain not only from the range as surface water,
but also through topographically driven groundwater
Fig. 7. Profile of heat flow values across the Wasatch fault.
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
Fig. 8. Distributions of hot spring flow rates (a), and temperatures
(b) from 45 springs in the study area.
flow systems. Some of the groundwater is focussed
and discharges locally as warm and hot springs in
fault and fracture zones near topographic breaks in
slope (Rybach, 1981); other groundwater participates
in a distributed flow system into the valley. Both localized and distributed groundwater flow can redistribute significant amount of heat (Smith and Chapman, 1983).
A thorough inventory of hot springs, including
the temperature and flow rate, for the state of Utah
was completed recently by Blackett (1994). Of these,
107 hot springs are located along the Wasatch Front
within the map area shown in Fig. 1. For the initial
107 springs and seeps, 45 locations had measurable
water flow rates (Figs. 1 and 8, Table 3). The remaining 62 hot springs, which are not plotted in Fig. 1,
had no measurable flow at the time they were visited
but did have temperatures above the ambient surface temperature, which implies that they must have
had some flow in the recent past. The abundance of
hot springs along the Wasatch Front suggests that
227
hydrothermal heat loss is present through topographically driven groundwater flow.
There is some uncertainty in the quality of data
presented in Table 3. The data presented in these
tables were collected over tens of years and by different investigators (Blackett, 1994), so there was
a lack of continuity in the method of temperature
and flow rate measurement. Furthermore, temperature and flow rate measurements were generally
measured only once at each location, and whether or
not these values are representative of the hot springs
temperature and flow over longer periods of time
is uncertain. Currently, there is no good estimate of
the quantitative error that the above issues introduce
into the temperature and flow rate measurements (R.
Blackett, pers. commun., 1995).
Flow rates range from slightly above 0 to 568
l=s with a calculated mean of 39 l=s (Fig. 8a). The
distribution of flow rates is a function of several
parameters, including the driving head gradient, the
permeability of the rocks through which the water
has traveled, and the water supply available. Hot
spring flow rates most likely vary seasonally depending on rainfall and the depth of winter snow pack.
Thus, the timing of the measurement could have
bearing on the flow rates. Some of the hot springs
with no flow are located within a hundred meters of
a spring with flow and have a similar, or identical,
temperature as the flowing spring. In such a case, the
spring with no flow is likely part of the same hydrologic system as the flowing spring and either has
intermittent flow, allowing for water above ambient
surface temperature, or is a ‘seep’ or warm puddle
which could exist if the ground is saturated with hot
water from the adjacent flowing spring (Elder, 1965).
Another possibility is that water circulates within a
fracture system, advecting heat upward, but does not
discharge water.
For the flowing hot springs, the discharge temperatures range from 19º to 84ºC with a mean of
32ºC and a standard deviation of 14ºC (Fig. 8b). The
temperature of spring water depends on the depth
to which it penetrates and how fast the water flows
to the surface. Previous studies of alpine hydrologic
settings suggest that water recharge entering near the
top of a range can penetrate several kilometers depth
and then exit at the base of the range (Forster and
Smith, 1988a,b, 1989).
228
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
Table 3
Advective heat loss through hot springs along the Wasatch Front
Location name
Latitude
(ºN)
Longitude
(ºW)
Elevation
(m)
Calculated ground
temp. (ºC)
Temp.
(ºC)
Flow
(l=s)
Thermal power
(kW)
U.S. Forest Service
Abraham Hot Spr.
Bureau of Land Mgmt.
R. Lunt a
Castilla Hot Spr.
Diamond Fork Warm Spr.
Unnamed a
Wood Spr.
Bird Is. Warm Spr.
Unnamed a
Saratoga Hot Spr.
Unnamed a
Russels Warm Spr.
Morgans Warm Spr.
Warm Spr.
E. Payne a
Unnamed a
Coleman Hot Spr.
Deseret Livestock So. Spr.
Deseret Livestock So. Spr.
Grantsville Wrn Spr.
Redlum
Unnamed
Unnamed
Utah Fish and Game
Big Warm Spr.
Wasatch Hot Spr. a
Becks Hot Spr. a
Como Warm Spr. a
Ogden Hot Spr.
Compton
Wolf Cr. Resort – Patio S
Unnamed
Utah Hot Spr.
V. Poulsen
Stinking Spr.
Spring near Little Mounta
Unnamed
Unnamed
Garland Springs
Crystal Hot Spr. (Madsen)
Blue Creek Spring
Cutler Warm Spr.
Uddy Hot Spr. (Belmont)
D. Gancheff a
39.346
39.613
39.774
39.788
40.038
40.117
40.137
40.163
40.176
40.177
40.349
40.351
40.390
40.397
40.450
40.522
40.532
40.535
40.556
40.565
40.647
40.656
40.697
40.733
40.743
40.745
40.790
40.816
41.039
41.236
41.238
41.327
41.332
41.339
41.451
41.577
41.674
41.728
41.729
41.730
41.748
41.833
41.834
41.855
41.914
111.219
112.728
112.087
111.881
111.533
111.337
111.829
111.621
111.802
111.801
111.905
111.901
112.424
112.403
110.817
111.471
111.481
111.483
112.739
112.742
112.524
112.907
111.493
112.623
111.638
112.665
111.900
111.918
111.654
111.924
112.413
111.826
112.405
112.031
112.439
112.233
112.260
112.286
112.540
112.106
112.087
112.454
112.056
112.157
111.955
2768
1448
1600
1600
1752
2210
1372
1752
1360
1365
1448
1366
1600
1600
2210
1752
1752
1783
1600
1600
1448
1448
1900
1295
2514
1295
1295
1295
1905
1448
1448
1603
1448
1600
1448
1295
1295
1295
1600
1295
1448
1448
1448
1295
1295
4.7
12.7
11.7
11.7
10.5
7.6
12.9
10.5
12.9
12.9
12.2
12.7
11.3
11.2
7.4
10.2
10.2
10.0
11.1
11.1
12.0
12.0
9.2
12.9
5.3
12.9
12.9
12.9
8.9
11.6
11.6
10.6
11.6
10.6
11.5
12.4
12.3
12.3
10.4
12.3
11.3
11.2
11.2
12.2
12.1
22.5
84.0
20.5
20.0
42.0
20.0
36.5
22.7
32.0
32.0
44.0
44.5
21.7
24.0
26.0
39.0
39.8
45.3
21.0
22.7
30.0
21.1
21.0
22.2
22.2
19.0
42.0
56.0
25.0
56.0
21.0
24.0
25.0
58.0
22.0
47.0
24.5
22.0
20.5
27.0
54.0
27.0
25.0
53.0
30.1
0.0
20.0
0.3
0.1
1.3
28.3
8.5
103.5
21.6
20.9
12.0
1.6
28.4
63.1
12.6
3.2
9.5
11.3
200.0
113.6
25.2
0.1
3.2
1.6
1.9
190.0
4.0
14.5
567.8
0.3
2.7
21.5
19.6
2.0
13.9
1.6
1.0
0.6
0.1
30.8
60.0
30.0
0.6
100.8
3.8
3
5967
11
3
176
1468
837
5303
1731
1673
1594
210
1241
3367
982
380
1173
1673
8253
5497
1898
5
156
61
134
4834
487
2618
38239
62
104
1203
1099
400
611
229
49
26
3
1902
10726
1980
36
17230
284
a ‘Tectonic’
hot spring associated with the Wasatch fault (see text).
In general, the hot springs with higher temperatures have low flow rates and low temperature springs
have high flow rates. This relationship is important
to note because it implies that the majority of heat
released from springs is from low-temperature (about
25ºC) springs.
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
7. Discussion
7.1. Comparison between 2D thermal models and
observed surface heat flow
Observed heat flow across the Wasatch fault
(Fig. 7) shows little discernible trend of elevated
heat flow close to the fault on the footwall and
depressed heat flow on the hanging wall basin, as
predicted by modeling (Figs. 5 and 9). The predicted
surface heat flow 10 km from the Wasatch fault,
beyond the severe topographic effect, after 10 m.y.
of exhumation along a 60º fault is 107 mW=m2 on
the footwall and 85 mW=m2 on the hanging wall
(Fig. 9). Therefore, a minimum 22 mW=m2 variation
in heat flow is predicted over 20 km distance; this
variation could be as much as 30 mW=m2 if the
Wasatch fault has been active for 15 m.y.
Although too few data are available for a detailed
analysis, the observed heat flow on either side of
the Wasatch fault exhibits only some of the predicted perturbations from the last 11–18 m.y. of
displacement. Predicted and observed heat flow in
the hanging wall generally are of the same magnitude although uncertainties in the hanging wall
observed data preclude a more detailed analysis.
The lack of an observed heat flow anomaly in the
footwall suggests that other thermal processes have
affected present-day heat flow and are not accounted
for in the numerical model. For example, the basal
heat flow may vary laterally and=or the thermal
regime around the fault is not purely conductive.
229
However, the numerical model results do provide
us with an estimate for how the surface heat flow
is expected to vary around the fault in a purely
conductive regime, and how future heat-flow studies should be designed to discern more accurately
the hydrothermal=conductive heat flow partition and
delineate the possible presence of other tectonic processes.
7.2. Hydrothermal heat transfer along the Wasatch
Front
The presence of 29 tectonic hot springs along
the Wasatch Front suggests a hydrothermal redistribution of heat through active groundwater flow
systems. The term ‘tectonic hot spring’ is appropriate for such systems because they are characterized
(1) by high topographic relief to drive groundwater
flow, (2) by high heat flow to heat groundwater, and
(3) by permeable fault zones to bring heated water
to the surface. A tectonic hot spring system is shown
schematically in Fig. 10a, whereby groundwater percolates downward from the range crest, capturing
heat along its descent, and then exits through the
permeable fault zone as a hot spring at the surface
(Lachenbruch and Sass, 1977). The expected effect
on surface heat flow is schematically shown at the
top of Fig. 10a, and has been modeled in previous
work by Forster and Smith (1988a,b, 1989). A heat
flow low is expected throughout the vertical recharge
zone where heat is advected downward by flowing
groundwater, whereas elevated heat flow is expected
Fig. 9. Profile of model and observed heat flow across the Wasatch fault.
230
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
Fig. 10. Redistribution of heat by tectonic hot springs. (a) Active
groundwater flow depresses surface heat flow in the recharge
area and enhances surface heat flow in the discharge region. (b)
Hot spring thermal power inventory for the Wasatch Front.
near the hot spring where hot water exits the system.
Conservation of energy in this system requires that
the integrated heat flow deficit in the recharge area
is balanced by the excess heat flow in the discharge
region. The principal uncertainty in constructing an
energy balance for such a system is partitioning the
discharge into localized (hot spring) and distributed
systems, in order to estimate how much heat is transported past the fault to the hanging wall sedimentary
basin.
7.3. Thermal power calculations
In spite of uncertainties, it is useful to consider
the energetics of tectonic hot spring systems and to
quantify their possible effects on heat flow determinations. The thermal power P delivered advectively
to the surface by one or more hot springs is given by:
@m
c.Tw Tg /
(2)
PD
@t
where @m=@t is the mass flow (flow rate), c is
specific heat of the fluid, and .Tw Tg / is the temperature difference between the hot spring water and
the ambient ground temperature.
Ground temperatures at each hot spring were estimated as follows: (a) mean annual air temperatures
over the last 80 years at 8 meteorological stations
(Karl et al., 1990) along the Wasatch Front between
38º and 39ºN latitude were used to determine latitudinal and elevation temperature gradients; (b) mean
annual air temperatures at each hot spring location
(latitude, elevation) were estimated by extrapolation
from the nearest weather station; (c) an adjustment of
C3ºC was made to account for excess ground surface
heating by solar insolation (Putnam and Chapman,
1995).
Water temperatures, flow rates, and estimated
ground temperatures were used for each hot spring
(Table 3) to calculate the thermal power of the
45 flowing hot springs. The thermal power output
ranges from 0 to 38 MW with a mean of 2.8 MW
(Table 3, Fig. 10b). The high and low thermal power
values are primarily a function of the large variation
in flow rates. Hot spring temperatures range between
19º and 84ºC, but flow rates vary from 0 to 568 l=s,
and therefore have a large influence on the thermal
power.
The magnitude of thermal power output from
each hot spring in Fig. 1 is represented by the circles
surrounding the springs. The area A within each
circle was calculated by:
P
(3)
AD
q
where P is the thermal power output from the spring
and q is the background heat flow (assumed to
be 90 mW=m2 for this calculation). Schematically,
each circle represents the area over which all of the
background heat flow would be collected to provide
an equivalent amount of heat discharging at a spring
(Lachenbruch and Sass, 1977). The larger the circle
around the hot spring the greater the thermal power
output from the spring.
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
7.4. Heat flow anomaly from hot springs
Twenty-nine hot springs with measurable flow
rates are located along the Wasatch Front. The cumulative thermal power loss from these 29 hot springs
is 90 MW. Dividing the cumulative power loss by
the strike length of the fault gives a 0.24 MW=km
power loss per unit length of the fault. The drainage
basin area feeding the hot springs along the Wasatch
Front was calculated to be 4.3 ð 109 m2 (370 km
fault trace times 11.5 km average half width of the
Wasatch Range). Capture of 21 mW=m2 of the background heat flux across this region would provide
the calculated 0.24 MW=km or 90 MW total hot
spring power loss. This constitutes a hydrothermal
heat flow redistribution of 21 mW=m2 that would
not be observed at the surface of the footwall block.
As present-day surface heat flow determined from
23 boreholes along the Wasatch Front has a mean
value of 92 mW=m2 , hydrothermal heat redistribution could constitute a minimum of 23% of the total
heat flow budget along the footwall side of the fault.
This redistribution is a minimum estimate of hydrothermal heat loss since another 62 hot springs
were identified in the study area, but did not have
a measurable flow at the time they were visited.
Furthermore, an unknown amount of topographically driven groundwater moves beyond the fault and
transports heat to the hanging wall, elevating temperatures there. Conductive heat flow measurements on
the hanging wall side of the fault are typically not
in the vicinity of geothermal systems and therefore
are less disturbed by topographically driven groundwater flow systems, unless a significant amount of
warm water passes from the footwall to the hanging
wall.
Given the magnitude of footwall hydrothermal
heat redistribution (23%) the surface heat flow measurements in Figs. 7 and 9 must be increased about
21 mW=m2 in the footwall within 20 km between the
fault and the drainage divide. Therefore, simple calculations of heat loss due to meteoric water recharge
in the footwall and warm spring discharge in the
hanging wall can account for a significant part of
the discrepancy between the modeled and measured
surface heat flow (Fig. 9).
The ¾30 mW=m2 discrepancy between predicted
and observed heat flow 30–60 km from the fault in
231
the footwall is unlikely to be the result of a thinned
radiogenic heat-producing layer. A 30 mW=m2 decrease in heat flow would require a 15 to 30 km
thinning of a 1 to 2 µW=m3 heat-producing layer.
There is no geological or geophysical evidence suggesting that this magnitude of thinning occurred in
this area. The remaining discrepancy between modeled and observed heat flow in the footwall could be
explained if there were a lateral decrease in mantle
heat flow at distances greater than about 30 km (approximately 1 crustal thickness) from the fault. This
is an attractive possibility because the fault coincides
with a major tectonic boundary; however, a previous
study 100 to 200 km south (Bodell and Chapman,
1982) suggests that Basin and Range heat flow extends about 80 km east of the Wasatch Front into the
Colorado Plateau.
7.5. Limitations
In searching for first-order thermal effects on normal faults we have made several simplifications that
need to be addressed in further studies. There are
several additional processes which may affect the
thermal regime surrounding a normal fault. First, 3D
thermal effects of topography and topographic evolution may have an impact on subsurface isotherms
if the topographic relief is large (e.g. more than 1
km offset between range crest and canyon floor) on
the footwall. Second, sediments deposited on the
hanging wall generally have high porosity at the
time of deposition that decreases with burial and
compaction. The compaction of sediments on the
hanging wall will result in thermal conductivity variations with depth that could affect the cross-fault
heat flow. Third, we have introduced simple, planar fault geometries into our modeling assumptions.
More complicated fault plane geometries, including
listric faulting may also produce 2D thermal perturbations. Fourth, predicted heat flow was compared to
surface heat flow determinations along all segments
of the Wasatch fault (Figs. 1 and 9). Implicit in this
comparison is the assumption that every segment of
the Wasatch fault has an identical displacement rate.
If along-strike variations in the displacement rate
exist then differences in predicted and observed heat
flow would occur. In spite of these limitations this
simple thermal numerical analysis combined with
232
T.A. Ehlers, D.S. Chapman / Tectonophysics 312 (1999) 217–234
heat flow studies, and a quantitative estimate of heat
distribution by tectonic hot springs, have revealed
the critical 2D interplay of tectonics and fluids for
interpreting the thermal regime adjacent to normal
faults.
8. Conclusions
Numerical simulations of the thermal effects of
normal faulting, combined with a synthesis of heat
flow and hot spring data for the Wasatch Front of
central Utah, lead to the following observations and
conclusions.
(1) 2D numerical models of the Wasatch fault
thermal regime predict an enhanced thermal regime
on the footwall and the hanging wall. For displacement rates which account for footwall tilt and sedimentary basin formation (Fig. 2) on a fault dipping
60º, surface heat flow would be between 107 and
110 mW=m2 on the footwall and between 80 and
85 mW=m2 on the hanging wall. The values of predicted surface heat flow are sensitive to the dip angle
of the fault.
(2) Thermal model results indicate warmer temperatures at shallower depths on the footwall and
hanging wall. At 4 km depth, isotherms are predicted to be about 0.5 km shallower on the footwall,
and 1 km shallower on the hanging wall than if the
temperatures were calculated using the steady-state,
one-dimension heat-conduction equation.
(3) Twenty-two conductive heat flow determinations were compiled for the Wasatch Front,
which covers the transition zone between the Basin
and Range and Colorado Plateau physiographic
provinces. The mean heat flow for this transition
zone is 92 mW=m2 (standard deviation 25 mW=m2 ).
Heat flow determinations across the Wasatch fault
show no discernible trend of enhanced and depressed
heat flow on the footwall and hanging wall, respectively, as predicted by the numerical models. A much
higher spatial density of heat flow determinations
and lower uncertainties are needed to document such
a thermal effect.
(4) Water temperature and flow rates were compiled for 45 hot springs located within the study
area. These springs have a calculated cumulative
thermal power output of 90 MW. Hot spring thermal
power loss along-strike of the Wasatch fault is 0.24
MW=km and can be provided by groundwater intercepting 21 mW=m2 , or 23% of the background heat
flow in the Wasatch Mountains, and focusing it into
the springs.
(5) Predicted and observed footwall heat flow determinations agree better with each other when the
observed values are corrected for hydrothermal heat
loss through groundwater flow systems. Hanging
wall predicted and observed heat flow are in reasonable agreement. Remaining discrepancies between
predicted and observed footwall heat flow may be
explained at distances greater than 40 km from the
fault by a lateral contrast in heat flow from Basin and
Range to Colorado Plateau at mantle and lithospheric
depths.
Acknowledgements
Thoughtful discussions with Phil Armstrong,
Kevin Furlong, Sean Willet, and Steve Shaeffer
contributed to this work. Giorgio Ranalli and Kip
Hodges are acknowledged for their constructive reviews
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