Global distribution of geothermal gradients in sedimentary basins

Abstract

The heat budget of sedimentary basins is determined by heat transfer across the lithosphere-asthenosphere boundary and lithospheric heat sources, such that the tectonic evolution of their host continental and oceanic lithosphere ultimately impact the present-day heat flow and average geothermal gradients. Recent increase in availability of publicly-accessible geothermal gradient measurements across the world provides an opportunity to globally assess the dominant controls on the thermal regime of sedimentary basins. Thus, we compile a global dataset of ∼ 152,000 unique data points with constrained bottomhole temperature measurements and assess the relationships between geothermal gradient and selected independent tectonic variables, including crustal and lithospheric thicknesses, crustal age, sediment thickness, and basin type. The results show that in both oceanic and continental settings, geothermal gradients exhibit a non-linear, systematic variation with the tectonic variables. We find that oceanic geothermal gradients decrease with increasing crustal age and lithospheric thickness. Geothermal gradients in the continents show no clear relationships with thermotectonic crustal age, but decrease with increasing crustal and lithospheric thicknesses. Gradients drop significantly at 1.5 km sediment-cover thickness, likely reflecting the effect of high sedimentation rates, but show a striking rise at > 12 km thicknesses, potentially influenced by thinned lithosphere and thermal blanketing effects. The commonly-assumed ‘normal’ gradient of 25 °C/km for continents is only valid for a narrow range (1.5–12.5 km) of sedimentary cover-thickness, and oceanic ‘normal’ gradient may be as high as 50–75 °C/km for > 20 Ma crustal age and > 50 km-thick lithosphere. We show that, conditionally, crustal age may best predict average geothermal gradients in oceanic settings, and lithosphere thickness in continents. Further, we observe that tectonic basin types exhibit distinct ranges of gradients that reflect their prevalent tectonic and geodynamic origins. Despite the complexities of determining shallow-crustal thermal conductivities, the results provide insights that fingerprint distinct tectonic settings based on the broad distribution of their geothermal gradients.