Marshall Reiter

Terrestrial Heat Flow along the Rio Grande Rift, New Mexico and Southern Colorado

From heat-flow data obtained in New Mexico and southern Colorado, one finds: (1) a major geothermal anomaly with heat-flow values greater than 2.5 HFU (heat-flow unit, microcal/(sq cm) sec) coincident with the western part of the Rio Grande rift; (2) a complex heat-flow pattern in the eastern Colorado Plateau with values of 1.5 HFU and less, apparently associated with major structural basins, and values of 2.0 HFU and greater, apparently associated with some intrusions and perhaps major uplifts; and (3) a regional increase in heat-flow values from 1.5 to 2.0 HFU to values greater than 2.5 HFU in southwestern New Mexico, which may be coincident with the north-trending geothermal transition zone between the Colorado Plateau and the Basin and Range provinces.

Using precision temperature logs to estimate horizontal and vertical groundwater flow components

A new method of estimating the horizontal and vertical specific discharge components of groundwater flow from precision subsurface temperature measurements is presented. Plotting the vertical temperature gradient as a function of both Z and T and then fitting a plane to the data can provide coefficients from which the vertical and horizontal groundwater specific discharge components may be estimated. Alternatively, by considering the vertical component of groundwater to be zero, it is shown that quadric or cubic fits to temperature data provide coefficients from which horizontal groundwater flow may be estimated. These expressions along with expressions previously derived to estimate only vertical, only horizontal, or both vertical and horizontal flow components are fitted to observational data from four sites and five flow zones. It is learned for these examples (using an available fitting program) that although many of the expressions fit the observational data very well statistically, those expressions which incorporate both the vertical and horizontal flow components of specific discharge must generally be constrained so that resulting parameters will provide flow estimates consistent in direction and heat transfer with available piezometer data and/or the fundamental curvature of the temperature log. Expressions incorporating only νX or only νZ can sometimes yield estimates quite different from the estimates obtained from the expressions incorporating both νX and νZ. It is concluded that an estimate of the horizontal and vertical flow components requires both consideration of the flow calculations along with available piezometric data and the fundamental curvature of the temperature log.

Terrestrial heat flow and crustal radioactivity in northeastern New Mexico and southeastern Colorado

New heat-flow data obtained in northeastern New Mexico and southeastern Colorado show three regional trends: (1) A broad heat-flow anomaly associated with the southern Rocky Mountains contrasts with a narrow heat-flow anomaly between lat 35.5/sup 0/ and 34/sup 0/N, apparently associated only with the Rio Grande rift. (2) The high heat-flow anomaly apparently associated with the southern Rocky Mountains extends 200 to 300 km onto the Great Plains of northeastern New Mexico and southeastern Colorado. (3) Areas of extensive volcanic activity do not necessarily have high heat flow. In addition, measurements of crustal radioactivity in the vicinity of the Rio Grande rift suggest that the radioactive heat generation contributes uniformly to the surface heat flow. This implies that the heat-flow anomaly observed along the Rio Grande rift is caused by tectonic and volcanic sources and not by anomalously high crustal radioactivity.

Geothermal anomalies along the Rio Grande rift in New Mexico

Heat-flow data suggest that there are regions along the Rio Grande rift where crustal temperatures are above those in neighboring areas. Magma bodies at 15 to 30 km, as predicted by several investigators, seem to provide reasonable sources of heat that could increase heat-flow values from 1.8 to about 2.5 HFU and somewhat higher. However, heat-flow values of 6.0 to 16.0 HFU occur at four locations along the Rio Grande rift; these values occur within geologic environments such as recent volcanic centers and the intersections of cauldron boundaries with large normal faults where upward heat transport by magmatic and/or ground-water movement is plausible. Test drillings of several kilometres are necessary to confirm the continuity with depth of these very high geothermal gradients and to relate them to their possible sources in the upper crust.

Geothermal characteristics of the colorado plateau

New heat-flow measurements in the Colorado Plateau varying in depth from 400 to 1900 m suggest that the heat flux throughout the region is 1.5 HFU and greater (1 HFU = 41.8 mW/m2). Along the eastern and southern boundaries of the Plateau, near the San Juan volcanic field and the Mogollon Slope respectively, high heat flows (

Geothermal studies in the San Juan Basin and the Four Corners area of the Colorado Plateau I. Terrestrial heat-flow measurements

2.2 HFU) are observed to intrude 50–100 km into the Plateau. It is believed that the high heat flows are associated with the sources of the volcanics in those areas. In the interior areas of the Plateau, away from the major volcanic phenomena along its boundary (e.g., the Black Mesa—Kaiparowits synclinorium, the Four Corners area, and the Piceance and Uinta basins) heat flows are generally between 1.5 and 1.8 HFU, and appear to be rather uniform over large areas. This uniform heat-flow characteristic over large areas of the interior Plateau suggests the lack of large-scale, widespread, crustal thermal sources as in the Basin and Range or along the Rio Grande rift. It is possible that lithospheric temperatures within the Colorado Plateau were once similar to lithospheric temperatures within the Stable Interior. Present heat-flow differences between the two provinces (~0.4 HFU) may define the temperature change occurring in the lithosphere of the Colorado Plateau over the past 200 m.y. This temperature change may have contributed significantly to the uplifting of the Colorado Plateau by the process of thermal expansion.

Thermo-mechanical model for incremental fracturing in cooling lava flows

Abstract Twenty-five new heat-flow measurements are presented for the San Juan Basin and the Four Corners area of the Colorado Plateau in the southwestern United States. Temperature gradients at most sites are calculated from temperature logs at depths between 1 km and 2 km. Resulting heat-flow values appear to be locally less variable than shallower data. These new data develop smooth, uniform trends consistent with the regional geology. As such, it is reasonable to suggest that these data may be somewhat less influenced by near surface perturbations to heat flow than are shallower data in the area; i.e., local hydrologie movement, weathered conductivity samples, topographic variations, and paleoclimate effects. Heat-flow increases going from the Four Corners area into the northern San Juan Basin; a trend consistent with other geophysical studies. Heat flow also increases as the San Juan Basin is traversed south to north, approaching the San Juan volcanic field. This observation suggests a unique thermal anomaly associated with the San Juan volcanics. Heat flow in areas of the San Juan Basin, quite away from extensive volcanics, is 70 mW/m 2 ; suggesting a small but statistically valid difference between regional heat flow in the non-volcanic regions of the San Juan Basin and in some of the other non-volcanic regions of the Colorado Plateau where the mean heat flow is 65 mW/m 2 . Again, this conclusion seems consistent with other geophysical data.

Terrestrial heat-flow estimates from petroleum bottom-hole temperature data in the Colorado Plateau and the eastern Basin and Range Province

Abstract This study presents an idealized quantitative model of stress development and incremental fracturing in cooling lavas. It appears that columnar jointing forms in lavas by a process of incremental fracturing driven by thermal stresses. The fractures progress inward from the cooling surfaces as time increases. The cooling of a horizontal layer of lava has been modeled analytically for three cooling regimes: conductive cooling with liquid magma present, conductive cooling after total solidification, and hydrothermal cooling. Horizontal thermal stresses develop and cause vertical fractures at the upper and lower surfaces of the lava. As the lava cools, layer after layer becomes elastic. Thermal stresses develop in these thin layers, between previously fractured lava (where, it is assumed, stresses have been released), and the hot interior lava where temperatures exceed the elastic temperature limit. Stresses are analyzed by treating each unfractured elastic layer as a slab, restrained in bending, unrestrained from contraction. The stress level at which incremental fracturing occurs, extending previous fractures, and the effect of stress and temperature conditions on fracture penetration, are discussed. Our analysis yields values for the length of incremental fracture as a function of time and material strength for both a thin lava flow cooling conductively (incremental fracture lengths increase with time), and for a thicker lava flow in which hydrothermal convection is important (incremental fracture lengths are constant with time). Our results are compared to observed striae widths measured from the base of basaltic lava columns (striae represent successive incremental fractures). The comparison shows agreement in magnitude and trend after some amount of cooling i.e., striae and incremental fracture lengths increase with time at an accelerating rate.

Estimates of terrestrial heat flow in northern Chihuahua, Mexico, based upon petroleum bottom-hole temperatures

Preliminary heat-flow estimates were calculated for 32 sites in the Colorado Plateau, the eastern Basin and Range Province, and the Middle Rocky Mountains. At 23 of these sites, the heat-flow estimates are believed to have merit. The heat-flow estimates often agree with nearby measurements and correlate reasonably well with interpretations from other geophysical techniques. The mean of the heat-flow estimates hi the northern Colorado Plateau interior (66 to 69 mWm −2 ) is within ∼5% or ∼18% of the mean of deep or shallow heat-flow measurements, respectively. Estimates in the peripheral areas of the plateau, in the San Juan Basin, agree with higher heat-flow measurements at sites approaching the San Juan volcanic field. Estimates along the western periphery of the Colorado Plateau are higher than in the interior and therefore suggest an anomalously shallow mantle (asthenosphere), extending ∼50 km under the plateau; this is consistent with previous studies. Above-average Basin and Range heat flows are estimated in the eastern Basin and Range Province (99–102 mWm −2 ), and this concurs with a high energy-flux zone proposed for the area. The transition from high to low heat-flow estimates between the eastern Basin and Range Province and the Middle Rocky Mountains seems to occur over a small distance (∼36 km). The narrow heat-flow transition implies shallow thermal sources in proximity to the Basin and Range–Middle Rocky Mountains transition; this may correspond to a shallow crustal low-velocity zone previously interpreted along the transition.

Geothermal studies in the San Juan Basin and the Four Corners Area of the Colorado Plateau II. Steady-state models of the thermal source of the San Juan volcanic field

Bottom-hole temperature data from deep petroleum tests are used in conjunction with estimated and measured thermal conductivity values to estimate heat flows at 12 sites in northern Chihuahua. The errors and uncertainties associated with these estimates and measurements are discussed. From these heat-flow estimates, several geothermal regions are proposed which appear reasonable in terms of data statistics and the geologic environments. In northeastern Chihuahua, an area of considerable tectonic activity with no Cenozoic volcanic activity, the heat-flow mean is ∼73 mW/m 2 . Westward in north-central Chihuahua, the heat-flow values become ∼85 mW/m 2 . Farther westward, in northwestern Chihuahua, heat flows >∼105 mW/m 2 are estimated; however, there is only a modest probability that mean heat-flow values for northwestern and north-central Chihuahua differ. Very high heat flows (>∼250 mW/m 2 ) and low heat flows ( 2 ) are not observed in north-central or in northwestern Chihuahua. The deep-temperature data may be removed from the effects of hydrothermal activity in sediments and along faults, which can cause very high or low values. It may also be possible that potential magma sources in the crust of the study area are not as young and hot as the sources associated with the volcanics along the Rio Grande rift in New Mexico, although heat flows of >∼105 mW/m 2 do suggest crustal magmatic sources. Additional geophysical data will be needed to consider these possibilities.