Spreading ridges and hot spot swells are identified by their high surface heat flow, shallow seafloor, and high geopotential. To understand these and other thermotectonic features, the oceanic lithosphere is modeled as a thermomechanical boundary layer moving through a three-dimensional, time-independent heat source. The heat source mimics the heat advection associated with a spreading ridge or hot spot without introducing the nonlinearities of these flow processes. The Fourier transforms of three Green's functions (response functions), which relate the three observable fields to their common heat source, are determined analytically. Each of these response functions is highly anisotropic because the lithosphere is moving with respect to the source. However, the ratio of the gravity response function to the topography response function (i.e., gravity/topography transfer function) is nearly isotropic and has a maximum lying between the flexural wavelength and 2&pgr; times the thickness of the thermal boundary layer. The response functions are most useful for determining the surface heat flow, seafloor topography, and geopotential for complex lithospheric thermal structures. In practice, these three observables are calculated by multiplying the Fourier transform of the heat source by the appropriate response function and inverse transforming the products. Almost any time-independent thermotectonic feature can be modeled using this technique. Included in this report are examples of spreading ridges and thermal swells, although more complex geometries such as ridges offset by transform faults and RRR-type triple junctions can also be modeled. Because forward modeling is both linear and computationally simple, the inverse of this technique could be used to infer some basic characteristics of the heat source directly from the observed fields.