Isostatic constraints on the assembly, stabilization, and preservation of cratonic lithosphere

Abstract

Cratonic uplift or subsidence, as recorded geologically by erosion or deposition, indicates the net change in lithospheric buoyancy. Density change is primarily controlled by temperature change, such that the geologic record reveals the net lithospheric heating or cooling. Our modeling shows that thermal stabilization of lithosphere can occur in <100 m.y., largely determined by the time for crustal radioactivity to warm the entire lithosphere. Longer-term thermal evolution (350–400 m.y.) is controlled by the feedback between isostatic and thermal responses. Crustal heat production is the dominant factor for evolution of crustal geo therms, whereas the detailed initial thermal structure is less important. The fact that many cratons are eroded to only modest depths indicates that they have a history of warming slightly since their formation, implying a rather cool 700–1000 °C for initial lithospheric mantle. This is consistent with the growth of cratonic lithosphere by progressive thickening during continental assembly and/or subduction stacking of oceanic lithosphere. Relatively warm early lithosphere due to elevated crustal heat production or higher initial lithospheric mantle temperatures may have had a lower preservation potential due to preferential deformation and destruction. Study of the Proterozoic orogen of the southwestern United States indicates that the distinct thermal and isostatic records of adjacent crustal blocks are compatible with observed differences in crustal heat production, but also with similar initial lithospheric mantle temperatures.