Heat flow on the southern Colorado Plateau

51

Tectonophysics, 200 (1991) 51-66 Elsevier Science Publishers B.V., Amsterdam

Heat flow on the southern Colorado Plateau Jeffrie Minier ’ and Marshall Reiter New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA

(Received May 30, 1989; revised version accepted May 8, 1991)

ABSTRACT Minier, J. and Reiter, M., 1991. Heat flow on the southern Colorado Plateau. Tectonophysics, 200: 51-66. Heat-flow data from the study area in the southern Colorado Plateau indicate a pattern of local anomalies having relatively high heat flow superimposed on a regional, intermediate heat-flow setting. While many of the conventional heat-flow data are relatively shallow and may be perturbed by groundwater circulation, bottom-hole temperature data from two relatively deep petroleum exploration drill holes near the southern plateau periphery yield intermediate heat-flow estimates. The mean heat flow within the volcanically active Jemez zone does not appear to be significantly greater than the mean heat flow for the remainder of the study area. This is due to the presence of high heat-flow values outside the Jemez zone. Sites with relatively high heat flow located towards the plateau interior and away from recent volcanic activity of the Jemez zone may reflect magma intrusion and/or groundwater movement along crustal zones of weakness associated with Laramide deformation (monoclines). The heat-flow data are consistent with coal maturation data, which suggest that any regional post-Cretaceous thermal events that may be associated with the southern Plateau boundary have been initiated relatively recently, or are occurring at relatively great depths, or are occurring south of the Jemez lineament.

Introduction The study area is located on the southern margin of the Colorado Plateau (CP, Fig. 1). Although the CP appears to be relatively quiescent when compared to its neighboring tectonic provinces, the occurrence of recent volcanism, seismicity, and relatively high heat flow in the margins of the plateau suggests that the Rio Grande rift and Basin and Range Province are growing at the expense of the stable plateau interior; e.g., tectonic boundaries along the western margin of the CP may be migrating toward the plateau interior as indicated by probable crustal or lithospheric thinning (Keller et al., 1979; Thompson and Zoback, 1979; Bode11 and Chapman, 1982; Eggleston and Reiter, 1984). Located in the study area is a portion of the Jemez lineament (JL), a northeast-trending chain

’ Present address: Roy F. Weston, Inc., 1350 Treat Blvd., Suite 200, Walnut Creek, CA 94596, U.S.A.

of late Tertiary-Quaternary volcanic fields which is also approximately coincident with a Precambrian age-province boundary (Chapin et al., 1978; Lipman and Mehnert, 1979; Van Schmus and Bickford, 1981; Chapin and Cather, 1983). Radiometric data in the study area along the JL suggest volcanic activity during latest Miocene and Pleistocene times (approximately 7-6 and 1 Ma, Minier et al., 1988). Volcanism may have occurred as late as 0.0229 Ma at Zuni Salt Lake (Bradbury, 1966). It has been proposed that the JL is a zone of crustal weakness where increased extension beginning about 7-4 Ma, has resulted in several areas of localized volcanism (Chapin et al., 1978; Baldridge, 1979; Zoback et al., 1981; Aldrich and Laughlin, 1984). It is interesting to note that no age-progression of volcanism is apparent along the JL (Lipman and Mehnert, 1979). It appears that the JL, acting as a zone of weakness, has allowed the upward migration of magmas through several pathways. For example, electrical conductivity and gravity data indicate the presence of shallow magma intrusions along the

0040-1951/91/%05.00 0 1991 - Elsevier Science Publishers B.V. All rights reserved

JL between Mount Taylor and the White Mountains (Ander and Heustis, 1982; Ander et al., 1984). Also, in the Zuni-Bandera volcanic field geochemical data indicate that some basalts did not reside in crustal holding chambers long enough for significant differentiation to occur, but rather migrated to the surface from the mantle rather rapidly (Renault, 1970). Bode11 and Chapman (1982) suggest that crustal thinning is occurring beneath the plateau and is most apparent at the periphery where lateral heating from the Basin and Range Province has occurred. This model is consistent with the present elevation and the general heatflow pattern of the CP (i.e. low to intermediate heat-flow interior with higher heat flow generally

along the periphery). Although much of the relatively recent tectonic activity appears to have occurred primarily around the margins of the CP, volcanism and structural deformation arc also present in the central plateau. For example, monoclines, perhaps the most characteristic structural feature of the CP, and Oligocene intrusive rocks are present in the central CP (Kelley, 1955; Naeser, 1971; Figs. 1 and 2). Davis (1978) has presented a model to describe the development of monoclines on the CP. The monoclines appear to be upper crustal expressions of near vertical reverse movement, during Laramide time, on reactivated high-angle faults in the Precambrian basement. During Laramide time (about 80 to 40 Ma) the CP experienced regional SW-NE com-

Fig. 1. Map showing distribution of late Cenozoic ( < 11 Ma, shaded areas) volcanic fields in Arizona, Colorado, New Mexico and Utah (after Aldrich and Laughlin, 1984). Volcanic field names: W.S. = White Mountains-Springerville; Z.B. = Zuni-Bandera; M.T. = Mount Taylor; J.M. = Jemez Mountains. D.M. and S.J. indicate the location and approximate areal extent of the Datil-Mogollon and San Juan volcanic fields. Asterisks indicate Oligocene volcanic rocks (Naeser, 1971). The Jemez lineament is located by the stippled pattern. The study area is located between the W.S. and M.T. volcanic fields (box, dashed line).

HEAT

FLOW

ON THE

SOUTHERN

COLORADO

53

PLATEAU

shall attempt to relate these inferred fractures to the observed heat-flow pattern in the study area. Data presentation

AZ

;

NM

Fig. 2. Map of the Four Corners area of the Colorado Plateau.

The dark, curved lines represent monoclines; the dotted lines represent the basement fracture system inferred from the monocline pattern (Davis, 1978). Large dots represent intrusive rocks (Naeser, 1971). The asterisks indicate the location of the Hopi Buttes volcanic field.

pression. Observations of monoclines in the Grand Canyon region, and laboratory models, support his hypothesis (Davis, 1978). Davis (1978) has also constructed a map of inferred basement fault zones for the CP by comparing relationships obtained from a laboratory model to the monocline fold pattern of the CP. These relationships consider aligned end-points of monocline segments, aligned monocline segments, abrupt bends in the traces of individual and branching-converging monomonoclines, clines. Figure 2 illustrates the spatial relationships between the monoclines (solid lines) and inferred basement fracture system (dotted lines) for much of the CP (Davis, 1978). Based on the relationships presented by Davis (1978), Anderson (1986) has inferred the presence of additional northeast-trending basement fractures in the study area that are approximately parallel to those fractures inferred by Davis (1978) to the north. In order to examine regional heat-flow values, we

New heat-flow data obtained in the study area are presented in Table 1 and Fig. 3. Thermal conductivity was determined from drill cutting samples using techniques discussed in Reiter and Hartman (1971) and Sass et al. (1971). At sites where lithologic samples were not available, estimates of thermal conductivity were used to calculate heat flow (Table 1; Reiter and Tovar, 1982). Temperature gradients were measured, and in a few cases estimated, as described by Roy et al. (1968), Reiter et al. (1975) and Reiter and Tovar (1982). Many of the temperature data are from quite shallow depths and therefore may be susceptible to numerous disturbances. The effects of possible errors attributable to near-surface temperature perturbations are discussed below. The method of calculation of the correction for vertical groundwater flow is also discussed. Two petroleum drillholes are present in the study area and have been used to estimate heat flow (Table 1). Estimates of terrestrial heat flow may be made by calculating the temperature gradient at a site based upon petroleum bottom-hole temperature data (Reiter and Tovar, 1982; Eggleston and Reiter, 1984). Reiter and Tovar (1982) discuss the uncertainties associated with temperature gradient and thermal conductivity estimates. Statistical analyses of heat-flow estimates and measurements indicate that the mean of estimates and the mean of deep measurements in the northern Colorado Plateau are in close agreement. This observation suggests that carefully calculated heat-flow estimates may provide reasonable heat-flow values (Reiter and others, 1985). Diurnal and annual variation of the groundsurface temperature affect temperatures below the surface. The annual variation may be approximated as a periodic temperature boundary condition for a semi-infinite half-space (Carslaw and Jaeger, 1959, p. 65). Annual surface temperature fluctuations will have a negligible effect on subsurface temperatures at depths greater than 1522 m (l.O-0.1% of the surface temperature varia-

TABLE Heat-flow

I data

108-115

w

1644-3745

w

HEAT

FLOW

ON THE

SOUTHERN

COLORADO

55

PLATEAU

with a still smaller maximum correction probable. Beck (19771, however, indicates that larger corrections may be required for shallow ( < 400 m> data. The study by Lachenbruch and Marshall c1986) is unique in that the climatic effect was investigated using temperature measurements that were made in permafrost where heat transport occurs by conduction alone, i.e. is not affected by groundwater circulation. It is unlikely that reliable corrections for the effects of climatic changes can be estimated given the uncertainty in local climate for the study area and the probable profound influence of groundwater movement on subsurface temperatures. Topographic variations may perturb subsurface temperatures via the effects of a three-dimensional thermal boundary (the ground surface) and the decrease of air temperature with increased elevation. Analyses of the effect of topography on subsurface temperatures are presented by Lees (19101, Jeffreys (1938) and Birch (1950). The topography in the study area is typically

tion; see Lachenbruch and Sass, 1977). The subsurface temperature disturbance due to diurnal temperature variations decreases much more rapidly with depth than does the annual disturbance since the diurnal variation has a much higher frequency. Most of the temperature measurements have been made at depths greater than 25 m and thus should not be significantly affected by diurnal or annual ground-surface temperature variations. Several investigators have discussed the effects of climatic changes on subsurface temperatures (Birch, 1948; Beck, 1977; Clauser, 1984; Lachenbruch and Marshall, 1986). The magnitude of the climatic effects will depend on latitude, thermal conductivity, temperature measurement depth, and local climate variations (Beck, 1977). Birch (1948) estimates the effects of Pleistocene climatic variations upon geothermal gradients using schematic paleoclimatic histories. His analyses suggest that the climatic correction to the geothermal gradient may never exceed 3 o C/km

Notes to Table I: NOTE:

For wells

numbered

conductivity

was estimated

M - medium

in which

- range

Range

NC - number NT

End-Point Interval

-

of thermal

GW

- estimate

Corr

- interval

Bad

- estimated

f

heat flow

conductivity

of measurements

the heat-flow

within

determined

by harmonic

deviation

gradients

were

obtained

from Levitte and Gambill

(19601.

Thermal

(19601.

measurement

depth interval.

measurement

depth interval

the heat-flow

measurement

depth interval.

by the first and last temperature mean thermal

conductivity

correction

measurements

and the end-point

for heat flow

for that depth interval.

temperature

(after Mansure

gradient.

and Reiter.

1979;

Reiter et al., 1969).

correction

for that site.

calculated

from measurements

from lithologic

on representative

and geophysical

for each rock type (Reiter and Tovar,

of measured

the temperature

and Gambill

the heat-flow

of ground-water

for grOUndwater

heat flow

rock type as determined from average

gradient

prefix LANU,

by Levine

made; a = air/ w - water.

layers within

measurements

determined standard

logs presented

values within

conductivity

temperature

26 (well names with

were

conductivity

of temperature

- heat flow

1 Thermal

the temperature

of thermal

- number

10 through

from lithologic

19621 represented

values from the study area.

rock samples.

logs, and drill cuttings. Conductivities

Mean conductivity

Whole rock conductivities

obtained for wells

in the lithologic log. Numbers in parentheses for wells

8 and 9 estimated

by considering

percent

10. 11, and 13-26

are thermal conductivity

from measurements

of interval for each

estimated

from average

estimates

of samples from well

7.

obtained

Condwtivities

for well 12 estimated from measurement of samples from a nearby well. In-situ saturation estimated to be 0.5 IStone, 1984) where temperature measurements were made in air. 1 .O otherwise. Porosities generally estimated from density logs. Porosity of coal determined from density bottle measurements. 2 Not estimated correction, water 3 NE

for one of the following

2) magnitude

of correction

in the liquid state is unlikely

- not

estimated

4 Temperature

corrected

I

1 standard

deviation

logs were

for ground-water

correction

due to small number of temperature

in the vadose zone. vapor transport

since temperature

gradient

reasons:

not well determined

made shortly

after Kehle (given in Bebout

is not considered.

is greater

measurements

than about 0.5 of tha value of the ground-water in that depth intervil,

See text for discussion

after well drilling and completion

(20-46

of ground-water

or 31 upward

movement

of

correction.

hours).

et al., 1976).

5 The temperature data for well number 29 (Sun No. 1, San Agustinl using thermal conductivities corrected for 29% porosity.

was obtained

by Aeiter et al. (1976)

who estimated

the heat flow

at this site at 73 mW/m*

1 MIP.Il,K

109”W

I ‘\

I

I

1

/

ItI.1 I l.I<

;

I

67. .7’

I

LI

IOB’W

I

360

ANI)

t

0

20

IO

I

30

40

36-N

I

50 I

\

Stole.km

’

-5-5

\

/

3S’N

-35-N

/ /

34*N169-W

! 1&3-w

Pq

-34-N

Fig. 3. Location of heat-flow data, volcanic ages, monoclines, faults and inferred basement fractures in the study area (Decker and Birch, 1974; Reiter et al., 1975; Reiter and Shearer, 1979; Levitte and Gambill, 1980, Sass et al., 1982; Minier et al., 1988; this study). Volcanic age data are located by solid dots (Ma). Heat-flow data are located by triangles (mW/m’I. The letters G, S, Q and Z indicate the locations of Gallup, Springerville, Quemado and Zuni. Inferred basement fractures labeled WD, T and N are from Davis (1978); inferred basement fractures labeled A, G, J, and 0 are from Anderson (1986). M indicates the location of another inferred basement fracture zone based upon the relatively high heat-flow sites along the trend. Monocline and fault pattern is from Wilson et al. (1969) and Baldridge et al. (19831. The star east of Zuni indicates the location of an intrusion (Anderson, pers. commun., 1988). Mid-Tertiary dikes are indicated by dash-dot lines (Laughlin et al., 1983). Map after Wilson et al. (19691, Callender et al. (1983) and Aldrich and Laughlin (1984).

HEAT FLOW ON THE SOUTHERN COLORADO PLATEAU

gentle, however, some mesas and the Zuni Mountains are present. Lees’ (1910) two-dimensional model has been used to estimate the topographic effect on temperature gradients at measurement sites in the southern part of tbe study area (see Minier, 1987). Due to the relatively gentle topography and the lack of data in proximity to the topographic features, the disturbance of temperature gradients measured in this study by topography is considered to be negligible ( < 1%). The temperature disturbance induced by the circulation of fluids in the borehole during drilling may be quite profound, and is a function of rock and fluid properties, duration of circulation, and time elapsed between cessation of circulation and temperature measurements. Experimental and theoretical studies suggest that the temperature disturbance due to drilling decays exponentially with time and is neghgible after about 10 to 20 times the total drilling time (Lachenbruch and Brewer, 1959; Jaeger, 1961; Reiter and Tovar, 1982; Drury, 1984). Temperature gradients at some of the sites were measured after lo-20 times the total drilling time (sites 1-9, Table I>; however, the total drilling time, drilling date and temperature measurement date for many of the sites are not available (see Levitte and Gambill, 1980). The processes of sedimentation and erosion can disturb the near-surface temperatures in the earth’s crust. The sedimentary history of the study area since late Cretaceous time consists of periods of deposition, weathering and erosion (Campbell, 1981; Chamberlin, 1981; Anderson and Frost, 1982; Guilinger, 1982; Cather and Johnson, 1984). Although the sedimentation/ erosion history is complex, the magnitude of the temperature disturbance caused by sedimentation/erosion may be estimated by using an analytical solution of the one-dimensional, transient diffusion equation with a moving boundary (Carslaw and Jaeger, 1959, p. 388). Using estimates of sedimentation and erosion rates (Reiter et al., 1986 and Leopold et al., 1964, respectively), the disturbance to the temperature gradient by sedimentation and erosion is calculated to be a few percent of the regional temperature gradient (Minier, 1987). Even a rapid sedimentation rate

57

for a prolonged period of time disturbs the BHT based temperature gradient calculations only 214% (Rio Grande rift, Reiter et al., 1986). Most of the heat-flow data presented in this study are considered shallow data and as such are more likely to be influenced by near surface heat-flow pe~urbations than are deeper data. Deeper wells for temperature measurement are not available in the study area. Perhaps the strongest perturbation of heat flow is that which is introduced by groundwater movement (e.g., Donaldson, 1962; Mansure and Reiter, 1979; Majorowicz and Jessop, 1981). It has been suggested that, due primarily to generally decreasing permeability with depth (Magara, 1980), deeper heat-flow data are less likely to be influenced by groundwater movement than are shallow data (Reiter et al., 1979; Reiter and Mansure, 1983). Although shallow data sets generally show greater variance than deep data sets, a number of shallow data may provide a mean value of heat flow which has regional significance and is in reasonable agreement with deep data (Reiter and Mansure, 1983; Chapman et al., 1984). Because useful information may be obtained from temperature data over short depth intervals from drillholes as shallow as 50 m (see Blackwell et al., 1982; Bode11 and Chapman, 19821, all available data have been included in this study. The increased uncertainty associated with shallow data is noted. The transport of heat by groundwater movement has been discussed by Stallman (19631, Philip and De Vries (1957) and others. Bredehoeft and Papadopulos (1965) have presented a method for estimating the rate of steady, vertical groundwater movement from temperature data. This method has been used to estimate vertical groundwater discharge in the vadose and saturated zones (Sorey, 1971; Boyle and Saleem, 1979; Sammis et al, 1982). Mansure and Reiter (1979) estimate vertical specific discharge by examining plots of temperature gradient versus temperature. The method does not, however, incorporate vertical variations of thermal conductivity. Reiter et al. (1989) present a method to estimate vertical specific discharge across the zone of temperature measurement which incorporates

vertical variations in thermal conductivity. The method uses plots of conducted heat flow versus temperature. Examples from their study demonstrate that estimates of vertical groundwater movement obtained from this method may be quite different from estimates where thermal conductivity is assumed to be constant (Reiter et al., 1989). They also show that by using the estimated specific discharge to correct heat-flow data, reasonable estimates of regional or geologically representative heat flow may be obtained. The groundwater corrections to heat-flow data in Table 1 have been calculated using the method described by Reiter et al. (1989). Heat-flow data, observations and interpretations Heat-flow data in west-central New Mexico, on the southern boundary of the Colorado Plateau, exhibit a complex heat-flow pattern that does not appear to have a straight-forward correlation to recent volcanism (Table 1, Fig. 3). Sites with relatively high heat flow (> 90 mW/m’) are often located near sites with significantly lower heat flow (< 70 mW/m2). Although high heatflow sites are located near centers of recent volcanism along the portion of the Jemez lineament within the study area, high heat-flow values also occur in the northern part of the study area away from recent volcanic activity (Fig. 3). The mean of heat-flow values within the Jemez zone does not appear to be significantly different than the mean of heat-flow values outside the Jemez zone (as determined by the “t” test; 94.7 & 52.4 k 10.9 mW/m2, mean heat flow t_ standard deviation + standard error, n = 23 and 87.8 f 36.1 + 5.8 mW/m2, IZ= 39, respectively). Low to intermediate heat-flow values (< 70 mW/m2) are also located within the volcanically active Jemez zone (Figs. 1 and 3). From the data, the heat-flow pattern in the study area may be described as local areas of elevated heat flow superimposed on a regionally intermediate background heat flow. The intermediate background heat flow observed in the study area has implications with respect to crustal and lithospheric thinning models. If a regional thinning event is occurring under the study area, then initiation of the

event occurred later than in the northwestern periphery of the plateau where high heat flows are consistent with seismic and gravity studies that suggest lithospheric thinning (Bode11 and Chapman, 1982; Eggleston and Reiter, 1984; Minier et al., 1988). Alternatively, if lithospheric thinning is occurring, it may be doing so at a slower rate, or greater depth, or further south, along the southern Colorado Plateau (Minier et al.. 1988). The distribution of heat-flow data in the study area suggests that the anomalous high values result from recent intrusions or groundwater movement at shallow depths. This heat-flow pattern would be consistent with the emplacement of relatively shallow (3-5 km), Quaternary ( < 1 Ma) intrusions of limited area1 extent (Minier et al., 1988). The presence of low to intermediate heatflow values, along with high heat-flow values, at sites near Pleistocene volcanic rocks may be consistent with the hypothesis that the Jemez lineament is acting as a zone of crustal weakness (Chapin et al., 1978; Baldridge, 1979; Aldrich and Laughlin, 1984). Periodic intrusion of magma at various locations could result in local areas of high heat flow with a lower background flux. Magma intrusion along the JL does not however account for heat-flow anomalies away from the lineament towards the interior of the CP, where high heat-flow values are observed near intermediate values, suggesting relatively shallow depths to the heat source. A model that may help to explain high heat-flow anomalies away from the JL is now discussed. The location of Oligocene dikes and diatremes in the Four Corners area have been included on the map of the inferred basement fracture system of the CP (Fig. 2). Volcanic centers are often located on, near, or at the intersection of inferred basement fault zones, many of which exhibit a northeast trend similar to that of the Jemez lineament (Figs. 1 and 2). This observation suggests that the basement fracture systems have acted as zones of weakness localizing magma emplacement or enhancing groundwater circulation. An intrusion, located east of Zuni, may have risen along one of the inferred basement fractures acting as a zone of weakness (J in Fig. 3; Anderson,

HEAT

FLOW

ON THE

SOUTHERN

COLORADO

PLATEAU

1986). Three mid-Tertiary dikes, located south of the Zuni-Bandera volcanic field, are approximately aligned with the inferred basement fractures associated with the Nutria and Gallestina monoclines (N and G, respectively, in Fig. 3; Laughlin et al., 1983). A thermal spring is located at the north end of the Atarque monocline at the intersection of two inferred basement fracture zones which may provide an avenue of locally increased permeability (A in Fig. 3 is the Atarque monocline; Levitte and Gambill, 1980). The high heat-flow values located just east and southeast of Zuni probably reflect the presence of recent, shallow intrusions or thermal waters (e.g., the high heat-flow values, 108 and 124 mW/m*; Fig. 3). Similar structural controls of magma emplacement and groundwater circulation in the Socorro area are described by Chapin et al. (1978). High heat-flow sites located in other parts of the study area where recent volcanic activity is not indicated, reflect the presence of additional or continued basement fracture zones. For example, consider the heat-flow sites located north of Springerville (sites with values of 86, 116 and 149 mW/m*, Fig. 3). These sites are located approximately along the extension of the southernmost

12 t

59

basement fracture zone inferred by Anderson (1986, 0 in Fig. 3). Farther north are two inferred basement fracture zones trending north to north-northeast (Davis, 1978; WD and T in Fig. 3). Sites with high heat flow are also located near or along the extension of these inferred fracture zones (sites with values of 94, 109 and 159 mW/m*). We also propose that high heat flows observed in the southern part of the study area reflect the presence of a basement fracture zone that trends northeast from the Springerville area (M in Fig. 3). The zone of high heat flow (8 of 12 sites > 90 mW/m*) is located in and approximately parallel to the JL. In addition, the zone of high heat flow is parallel to other inferred basement fracture zones to the north (i.e. 0 and J in Fig. 3). One should note that low to intermediate heat flows are also located on or near the inferred basement fracture zones. For example, intermediate heat flows are observed along the northeasttrending fracture zone inferred to exist near Springerville and Quemado, and along the northwest-trending fracture zone associated with the Nutria monocline (M and N, respectively; Fig. 3). With groundwater movement enhanced along

69

67

76

Fig. 4. Histogram of heat-flow values in the study area. Circled heat-flow values are located within 5 km of inferred basement fractures (see Fig. 3). Heat-flow values in squares are located within 5 km of an extension of a basement fracture. 17 of the 23 values 2 90 mW/m’ (74%) and only 10 of the 40 values < 90 mW/m’ (25%) are located within 5 km of an inferred basement fracture. Excluding data within 5 km of the southernmost inferred fracture zone, 10 of the 16 values 2 90 mW/m2 (63%) and 6 of the 36 values < 90 mW/m* (17%) are located within 5 km of an inferred basement fracture zone. Excluding the southernmost fracture zone but including all of the data in the study area (Fig. 31, 10 of 23 of the values 2 90 mW/m2 (43%) and 6 of the 40 values < 90 mW/m’ (15%) are located within 5 km of an inferred basement fracture.

00

I MlNlbK

these fractures, heat flow may well be expected to increase or decrease relative to the regional heat flow (due to upward or downward groundwater flow transporting heat). A histogram of the heat-flow data in the study area is presented in Fig. 4. The most often occurring heat-flow values are between 60 and 80 mW/m* which we interpret as indicating a regional heat flow of about 70 mW/m2. The distribution of data is skewed toward higher heat-flow values. Many of the high heat-flow data are lo-

ANI)

M

l
I I II

cated within about 5 km of inferred basement fracture zones (74%, or 17 of 23, of the values > 90 mW/m”; Fig. 3). Heat-flow data with values less than 90 mW/m* indicate a decreased percentage of sites located within about 5 km of inferred basement fracture zones (25%, or 10 of 40, of the values less than 90 mW/m*). A similar conclusion is made if the data within 5 km of the southernmost inferred fracture zone are not included in the calculation (M, Fig. 3). This observation indicates that most of the high heat-flow

200+ .

w

’

‘* l

80-

l‘.’

7060-

-*

. .

.

I

*

l

.

.

8.

x

50-

.

. .

.

. .

. .

. .

.

.

.

40

I 10

0

I 20

I 30

1 50

40

I 60

I 70

/ 80

I 90

I 100

(km)

DISTANCE 200+ . 200-

(b)

1 go160170160-

.* .

”

150-

:

140-

L

1so-

s2

120- IlO-

.:

2

TOO-

.

%I

go-. -.

.

l

.

. .

08

80-.

.*

70-a.. ;;+.*;

40

.

%

. 0

I 10

) .

1’

I 20

.

l

8

.

I 30

. I 40

.

.

.

.

..

1 50

DISTANCE

I 60

I 70

I 90

I 90

I 100

(km)

Fig. 5. Plot of heat flow versus distance from proposed fractures in study area, south-central Colorado Plateau. See Fig. 3 for geographic locations. (a) Heat-flow values are preferred values in Table 1. x indicates two values plotted at the same point. (b) Heat-flow values not corrected for water flow, see Table 1.

HEAT

FLOW

(a)

ON THE

SOUTHERN

113”

426140

COLORADO

61

PLATEAU

112”

110”

Ill0

106’

107”

IOW

109”

42”

UTAH MIDDLE \I

EA:

40”

/--

BL 126.

I-

A

RA

PRO\

390

---

73 JUAN

‘.__&_.__-_+\; 109.*‘3sI WHITE

ARIZONA

IlAO

4” 1

“8. 1130

III0

“p

I ,”

VII

88..85 112”

t-

MOUNTAINS

110”

0

NEW

108”

109” 200

Q

I

.EX1$63d10

107”

km

Fig. 6a. Fracture and monocline system proposed by Davis (1978) superimposed on heat flow map of Colorado Plateau after Eggleston and Reiter (1984). Dashed lines (with some modification in southeast and west central area) indicates Colorado Plateau interior region.

data are associated with inferred basement fractures that appear to enhance magma cmplacement and/or groundwater circulation. The frequent proximity of high-heat flow data to inferred basement fractures is clearly illustrated by plotting heat flow versus distance from the nearest fracture (Figs. 3 and 5a). In Fig. 5a, data from this study and Minier et al. (1988) include a correction for groundwater transport. The remainder of the data in Fig. 5a are from other studies and do not include a groundwater correction (see references given in caption for Fig. 3). Figure 5b presents heat-flow data for the study area (i.e., the same sites as represented by Fig. 5a) neglecting groundwater corrections and using lower thermal conductivity estimates. Even without the groundwater correction all but one of the heat-flow data greater than 90 mW/m’ are located within 5-10 km of inferred basement fractures (compare Figs. 3, 5a, and 5b). This pattern differs markedly from the plot of heat flow versus distance from inferred basement fractures for the central Colorado Plateau (Fig. 6). As compared to the study area, heat flow in the central Colorado Plateau is relatively low and uniform (compare Figs. 3, 5b and 5c to Figs. 6a and 6b). This seems reasonable because the central region of the plateau is more removed from the extensive Cenozoic volcanism and tectonism around its periphery. The lack of an increase in heat flow towards inferred basement fracture

zones in the central plateau may reflect the absence of post-Oligocene magmatic activity. From Fig. ha it may be noticed that in the eastern part of the CP high heat tlows occur, some rather close to proposed fracture zones. The hypothesis that high heat-flow data reflect magma emplacement and/or groundwater circulation along localized zones of weakness rather than a large-scale thermal event is consistent with coal maturation data. Coal maturation data in the study area indicate a relatively uniform, low level of thermal maturation equivalent to low rank bituminous (Minier and Reiter, 1989). The uniform distribution and relatively low level of thermal maturation suggest that any post-Cretaceous thermal events which may be associated with the southern Colorado Plateau boundary have not yet influenced shallow crustal temperatures or are occurring in the form of relatively small, widely spaced intrusions. Groundwater

considerations

Many of the heat-flow data in the study area have been measured at relatively shallow depths and therefore may be perturbed by groundwater circulation (Reiter and Tovar, 1982; Chapman et al., 1984). However, a few deep heat-flow estimates in the study area (Huckleberry Federal No. 1, and Spanel and Heinze SFP #l-9617, Table 1) which are less likely to be perturbed by groundwater circulation than shallow data, support the

. . .

.

.

.

.

. . . . 5 .

x

.

f 0

I 10

.

.

f

.

*

.

. t .

.

”

I 40

1 30

I 20

DISTANCE

Fig. 6b. Plot of heat flow versus distance

l

.

.

. 40

. .

.

from proposed fractures two values plotted

I

I 50

60

(km)

in the Colorado Plateau at the same point.

interior

region;

see Fig. 6a. X indicates

HEAT

FLOW

ON THE

SOUTHERN

COLORADO

63

PLATEAU

hypothesis that the background heat flow is intermediate (55-65 mW/m2; Table 1). Upward groundwater circulation may increase shallow heat flow by transporting warm water closer to the surface thereby raising near surface temperature gradients (Donaldson, 1962). Alternatively, downward groundwater movement may decrease shallow heat flow by moving cool water deeper thereby lowering near surface temperature gradients (Bredehoeft and Papadopulos, 1965). A correction has been applied to the heatflow data to account for heat transported by vertical groundwater movement within the zone of measurement (Mansure and Reiter, 1979; Reiter et al., 1989). The heat-flow correction for groundwater movement can be quite large (up to 100 mW/m2, Table 1; see Lachenbruch and Sass, 1977). The specific discharges calculated from the groundwater flow correction (up to about 1 m/yr) are not inconsistent with the limited amount of available hydrologic data (Summers, 1972; McGurk and Stone, 1986 in Stone and McGurk, 1987). The correction applied to data in Table 1 does not, however, consider heat transport beneath the zone of measurement. It is also possible that advection of heat occurs in the study area in the presence of magma intrusions and high-permeability zones. For example, thermal springs with water temperatures of 22 o C (versus mean annual air temperature of 12 o C; Levitte and Gambill, 1980) are present at the north end of the Atarque monocline (A in Fig. 3) where permeability is probably increased locally due to fracturing. Geochemical data obtained from thermal waters in the study area suggest that these waters have not circulated to great depths (i.e. probably not deeper than 400600 m; Levitte and Gambill, 1980). The lateral extent of heat-flow anomalies derived from hydrodynamic processes depends upon a number of factors such as permeability, duration of the hydrodynamic event, hydraulic gradient, depth of circulation, temperature gradient and other factors. Unfortunately, there is very little hydrologic information presently available for the study area. Until such data become available, it is not possible to separate hydrodynamic effects from magmatic effects. Furthermore

heat-flow anomalies may result as a consequence of combined magmatic and hydrodynamic effects, for example hydrothermal convection above magma bodies (Turcotte and Schubert, 1982). Whether due to groundwater convection, magma intrusion, or some combination of hydrodynamic and magmatic processes; high heat flows appear to be associated with the inferred basement fracture zones acting as preferential pathways for movement of magma and/or groundwater. Conclusions New heat-flow data on the southern periphery of the CP demonstrate considerable spatial variability not related to recent volcanism in a straightforward manner. Sites with relatively high heat flow (> 90 mW/m2) are located within the JL zone, where radiometric dates suggest volcanic activity during Miocene and Pleistocene times. High heat-flow values are also present outside the Jemez zone away from recent volcanism. Conversely, sites with low to intermediate heat flow (I 70 mW/m2) are located both in proximity to, and away from, areas of recent volcanic activity, Results of heat conduction analyses, which have been applied to the heat-flow data, indicate the possibility of young (< 1 Ma), shallow intrusions (Minier et al., 1988). The heat-flow data are, however, relatively shallow and thus may be perturbed by groundwater movement within and beneath the zone of heat-flow measurement. Corrections that account for heat transport by vertical water movement within the zone of heat-flow measurement indicate that the magnitude of groundwater advection of heat may be considerable. Two different processes can be proposed to describe the tectonic evolution of the southeastern boundary of the Colorado Plateau. A first hypothesis may suggest that the geophysical boundary of the plateau is migrating toward the plateau interior as a result of crustal/ lithospheric thinning, e.g., as seems to be the case along the western CP-Basin and Range Province boundary (Bode11 and Chapman, 1982; Eggleston and Reiter, 1984). A rather volumetrically extensive subsurface phenomenon would be envisioned as associated with this process. A second hypothesis

suggests that much of the volcanic activity along the southeastern plateau boundary occurs in more definitive and spatially restricted pre-existing zones of weakness in the lithosphere (i.e. the JL which has leaked magma to the surface; Chapin et al., 1978; Aldrich and Laughlin, 1984). Several observations derived from this study tend to favor the latter hypothesis. First, regional trends in heat flow are not observed in the study area (although deeper heat-flow data are surely needed). Rather than a gradual increase of heat flow from the plateau interior across the transition zone to the Rio Grande rift/Basin and Range Province, the data instead present a heatflow pattern consisting of local anomalies of relatively high heat flow superimposed on a regional intermediate heat-flow setting. The mean heat flow within the Jemez zone does not appear to be statistically different from the mean heat flow for the remainder of the study area. It is also possible that a more definitive transition exists in the southern part of the study area where few heattlow data are present. Deeper heat-flow data which are less likely to be perturbed by groundwater flow will probably be required to define any heat-flow transitions between the southern Colorado Plateau and the Basin and Range Province. Sites with relatively high heat flow are located towards the plateau interior and away from recent volcanic activity of the Jemez zone. Present in the interior of the plateau are monoclines from which a basement fracture pattern has been inferred (Davis, 1978). The high heat flows observed towards the the plateau interior may reflect magma intrusion and/or groundwater movement along crustal zones of weakness associated with Laramide deformation (monoclines). Sufficient data to differentiate magmatic from hydrodynamic effects near the inferred basement fractures are presently not available. Observations associated with the heat-flow data are in general consistent with conclusions based on coal maturation data. The lack of profound regional trends in coal maturation across the study area suggests that any post-Cretaceous thermal events which may be associated with the southern plateau boundary or Jemez lineament have been initiated relatively recently and/or are

occurring at relatively great depths; or the thcrma1 events are in the form of relatively small. widely spaced intrusions (Minier and Reiter, 1989). It is possible that a more uniform transition between the CP and BRP could occur in the southern part of the study area where few data exist. Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant No. EAR-8308367 and by the New Mexico Bureau of Mines and Mineral Resources. The following are thanked for permission to collect and present the data in this report: the Bureau of Land Management, the New Mexico Bureau of Mines and Mineral Resources, J.C. Brown, W. Green, D. Parker and H. Towner. Field and laboratory assistance was provided by M. Barroll, B. Broadwell, F. Campbell and S. Jarpe. We wish to thank O.J. Anderson, F. Campbell, C. Chapin, A. Gutjahr, F. Phillips, A. Sanford and W. Stone for valuable discussions. Support from Daniel B. Stephens and Associates, Inc. is gratefully acknowledged. Dr. A. Jessop and an anonymous reviewer are thanked for their comments. References Aldrich,

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