Sites were assigned a quality ranking based on the drilled depth and the quality of the data. Wells with depths deeper than 25 meters were examined. The deeper wells receive the higher quality rankings and on a regional basis only wells deeper than 100 m were used. The general ranking criteria are described by Blackwell et al. (1991). Wells with “geothermal” scale gradients/heat flow or those drilled within a known geothermal area are given a separate ranking. Plotted heat flow values by symbol on the map are for regional wells. A cut off heat flow of 120 mWm-2 was used to differentiate “geothermal” versus “regional” sites.
All of the heat flow values considered regional from the different data sets and countries were merged into a single point file and contoured using a minimum curvature algorithm with a tension factor of 0.5, averaging all points within a 0.02° spacing of each other, and a grid interval of 5’ (0.08333°) of latitude/longitude. Points were added to constrain the contours to follow structural/province trends in areas of sparse data coverage. The gridding was done using Golden Software program Surfer8.
Heat flow contour levels are increased from the 1992 GSA map of 10 mWm-2 to 5 mWm-2. By increasing number of heat flow points, the knowledge on heat flow contouring has improved with them. This increased detail amplifies the information for a region. By using colors and contours that blend from one interval to the next, the map is designed to show the heat flow values for specific areas where detailed information is known, i.e., the Basin and Range, but still allow for the greater generality in places with little to no specific data, i.e., Kentucky. The overall heat flow contouring represents the regional heat flow levels of North America. Although, where the geology, tectonics, and data represent more specific area detail, we have contoured it accordingly.
The Pacific and Atlantic Oceans were contoured based on the oceanic crustal age, rather than the individual heat flow data. Although there are many more ocean data points, the amount of scatter in ocean heat flow and large areas lacking data still makes it difficult to contour according to actual data points for these two oceans.
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