A sheeted dike complex (SDC) is a unique geological formation consisting entirely of parallel, near-vertical igneous intrusions, known as dikes. This structure is composed of one dike directly against another, sometimes reaching hundreds of meters in thickness. The SDC is a fundamental layer within the oceanic crust, representing the ancient plumbing system that carries molten rock upward from deep below the seafloor.
Geological Setting of Formation
The creation of a sheeted dike complex (SDC) is confined to a specific tectonic environment: the Mid-Ocean Ridge (MOR) or spreading center. This divergent plate boundary involves two tectonic plates pulling away from each other. The continuous separation of the plates creates the necessary space and tensional stress for the structure to form.
This extensional setting is characterized by extremely high heat flow and the upwelling of hot mantle material. The rate at which the plates spread apart influences the final structure, with faster-spreading ridges developing the most well-defined complexes. The process requires a dynamic balance between the speed of plate separation and the rate of molten rock supply.
Magma Source and Initial Intrusion
The source of the rock material is a persistent, lens-shaped magma chamber located beneath the ridge axis. This chamber acts as a reservoir, holding the basaltic melt that has risen from the mantle. The dikes themselves are fractures that have been filled and solidified by this melt.
The formation begins when pressure within the magma chamber exceeds the strength of the overlying oceanic crust, causing a rupture. This high pressure forces the molten rock vertically into the crust, a process called dike injection. The melt preferentially moves upward along the path of least resistance, utilizing the extensional fractures created by the separating plates. This action creates the initial, singular dike, which is a tabular intrusion of igneous rock.
This upward movement of magma forms an underground plumbing system connecting the deep reservoir to the seafloor. This system feeds the volcanic eruptions that create pillow lavas on the ocean floor surface. As the molten rock cools quickly within the narrow fracture walls, it solidifies into fine-grained basalt.
Sequential Generation of the Dike Complex
The defining characteristic of the sheeted dike complex is its continuous, self-perpetuating growth, driven by plate divergence. As the tectonic plates move apart, solidified dikes are carried away from the central ridge axis. This movement creates a central void, which is the weakest structural point in the crust.
New molten rock preferentially injects into this central fracture, effectively splitting the older dike material symmetrically. This continuous process means every new dike intrusion splits a previous one, pushing the older halves away from the center. This constant cycle of fracturing, magma injection, and solidification ensures the entire volume is composed only of dikes.
The result is a structure where a new dike is always intruded into the center, pushing previously formed dikes laterally outward. This continuous splitting ensures that no intervening “country rock” or original oceanic crust remains between the dikes. Each individual dike observed is a half-dike, representing one-half of a pair split by a later intrusion, creating the characteristic parallel, layered structure.
Structure and Identification
The final sheeted dike complex is a distinct, layered structure within the oceanic crust, typically ranging from 1 to 2 kilometers in thickness. Geologists identify this complex by examining ophiolites, which are fragments of ancient oceanic crust uplifted onto continental landmasses. The SDC occupies a specific position in the idealized ophiolite sequence.
It is situated beneath the extrusive layer of pillow lavas and above the massive, coarse-grained gabbro layer, which represents the crystallized magma chamber. The dikes are characterized by a near-vertical orientation and a basaltic to diabasic composition. A key field feature for identification is the presence of one-sided “chilled margins,” confirming the sequential intrusion process away from the spreading center.