Moho (Mohorovicic Discontinuity)

Key Takeaways

• The nature of the Moho as a physical boundary is more complex than a simple sharp discontinuity: in some geological settings (particularly beneath active magmatic arcs and recently thickened orogenic crust), the Moho is a transitional zone several kilometers thick where mafic lower crustal rocks grade downward into peridotite mantle through a sequence of intermediate compositions including garnet pyroxenite and eclogite; in other settings (beneath cratonic continental lithosphere and some ocean basins), the Moho is a sharp, first-order velocity discontinuity that generates a strong seismic reflection and is interpretable as a compositionally abrupt boundary; the physical mechanism maintaining the Moho — whether it is primarily a compositional boundary (igneous/metamorphic mineralogy changes) or a phase boundary (the same composition but at different metamorphic facies) — has been debated for decades, and the answer varies by tectonic setting; in stable cratonic lithosphere, the Moho is thought to be a primary compositional boundary established during Archean crustal formation, while in recently tectonized crust, the Moho may be actively maintained by partial melting and melt extraction processes at the crust-mantle boundary.
• Basin subsidence modeling, which underpins the petroleum system timing calculations in sedimentary basin analysis, is fundamentally tied to the depth and thermal properties of the Moho because lithospheric stretching during rifting (the most common mechanism for creating sedimentary basins) thins the crust by stretching and reducing the depth to the Moho beneath the basin center; the McKenzie stretching model (1978) and its descendants describe how lithospheric extension creates a basin by thinning the crust (which reduces crustal thickness and creates accommodation space for sediment) and heating the lithosphere (which brings hot mantle closer to the surface and elevates heat flow through the basin); the predicted thermal subsidence that follows the initial mechanical subsidence in a rift basin is controlled by the depth to Moho and the temperature difference between the stretched thin lithosphere (where the mantle is closer to the surface) and the surrounding unstretched lithosphere (where the mantle is deeper); errors in the Moho depth estimate beneath a basin propagate directly into errors in the predicted subsidence history and thermal maturity of the source rocks, affecting the accuracy of hydrocarbon generation and migration timing models.
• The Moho Project (formally known as Project Mohole, 1958-1966) was the first scientific attempt to drill through the oceanic crust to retrieve samples of the uppermost mantle from below the Moho, driven by the recognition that the Moho is the most fundamental compositional boundary in the planet but has never been sampled by direct drilling in an undisturbed state; the project's technical achievement of drilling in deep ocean water (testing in 3,500 meters of water depth off the coast of Mexico in 1961, long before deep-ocean drilling was considered practical) demonstrated the feasibility of dynamic positioning of a drill ship and established many of the deep-ocean drilling techniques that the subsequent Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) relied on; the project was cancelled in 1966 due to cost overruns and political controversy, and the goal of sampling Moho rocks by drill bit has not yet been achieved, though the International Ocean Discovery Program (IODP) has periodically revisited the science case for a SloMo (Slow-spreading Moho) drilling project targeting the thin crust of the Pacific Basin where the Moho is shallowest.
• The Pn seismic wave that propagates along the Moho surface (as a head wave that travels at mantle velocity along the top of the mantle and returns to the surface at the critical angle) provides a direct measurement of mantle P-wave velocity beneath any region where refraction seismic surveys have been conducted; Pn velocity variations across a region (which typically range from 7.6-7.8 km/s in cold, stable cratonic mantle to 7.8-8.2 km/s or higher in cool depleted lithosphere) reflect lateral variations in the composition and temperature of the uppermost mantle; slower-than-normal Pn velocities in a region (7.2-7.6 km/s) can indicate partial melting in the uppermost mantle, elevated heat flow, or the presence of hydrated or metasomatized mantle rocks; seismic tomography of the mantle using teleseismic P-wave delay times provides a three-dimensional image of mantle velocity structure that, combined with Moho depth from reflection and refraction surveys, reveals the thickness and compositional variability of the lithospheric mantle beneath oil-producing sedimentary basins and constrains the thermal models that govern hydrocarbon generation timing.
• Isostatic compensation across the Moho is the physical mechanism by which the earth's surface topography is supported at depth: the buoyancy of lower-density crustal rocks floating in the denser mantle creates the lithostatic pressure equilibrium (isostasy) that relates surface elevation to Moho depth through the Airy isostasy model (thicker crust beneath mountains) or the Pratt isostasy model (less dense crust beneath elevated terrain); the Airy isostasy model predicts that the Moho is shallowest beneath ocean basins (where the thin dense oceanic crust requires little crustal root) and deepest beneath high mountain ranges (where the light granitic crustal root extends deep into the mantle to support the elevated topography); the isostatic rebound that occurs when a crustal load is removed — by erosion of a mountain range, melting of an ice sheet, or depletion of a reservoir — is a Moho-controlled phenomenon that can affect the stress state of the overlying crust and influence fault reactivation and induced seismicity risk in depleted petroleum basins.