Lineaments of Texas---Possible Surface Expressions of Deep-Seated Phenomena

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Woodruff, Jr., C. M.
Caran, S. Christopher

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Geologic structures are important controls on geothermal resources. Buried plutons, intrusions, or diapirs may provide either localized areas of high heat flow or zones of high thermal conductivity. Various structures such as faults, joints, folds, or buried massifs may affect underground fluid flow and thus may alter local thermal regimes. These subsurface hydrologic effects may either enhance or detract from geothermal potential because upwelling of deep-seated fluids increases local geothermal gradient, but recharge decreases geothermal gradient. Geologic structures provide direct controls on heat flow and indirect controls on geothermal gradients via hydrologic processes. Understanding geologic structures, especially buried structures, will therefore aid in assessing the geothermal potential of a given area.

Some lineaments are surface indicators of geologic structures. An analysis of lineaments and an awareness of general hydrologic regimes provide means for delimiting promising areas for geothermal development. Lineaments, however, are polygenetic; not all linear features are related to earth structures at depth. Some are expressions of surface processes alone. Others seem to be essentially random alignments of features such as drainage, topography, soils, or vegetation, and the cause of many such features is unknown or ambiguous. Because of the varying quality of information imparted by individual lineaments, a high "noise-to-signal" ratio exists in lineament data. The geologist's task, in collecting lineament data and in subsequently analyzing them, is to winnow noise from signal—that is, to eliminate patterns that impart no geologic information, and thereby to ascertain which features are significant with respect to local or regional geologic structures. This process entails correlation of lineaments with other mapped features. For geothermal assessment, special emphasis must be placed on recognition of buried structures or other irregularities that may control thermal conditions in subsurface fluids.

Using lineament analysis for ascertaining geothermal potential entails a consideration of the structural (subsurface) control on surface features—especially the recurring motifs of aligned features that constitute regional or statewide linear trends or "grain." In places, lineaments are clearly correlative with buried structures. But lineaments also occur in areas without known subsurface discontinuities, and lineaments also occur in many places that have no geothermal potential. Thus, lineament analysis as a prospecting tool must be used carefully. Some workers have tended to assume that the very presence of a lineament invariably implies geologic control. The data we obtained do not support such a view, but they do indicate that many lineaments are expressions of local structural conditions that may be otherwise hidden or subtle.


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