Transform plate boundaries in the oceans include the system of ridge-ridge transform faults that are an integral part of the mid-ocean ridge system. Magma upwells along the ridge segments, cools and crystallizes, becoming part of one of the diverging plates. The two plates then slide past each other along the transform fault between the two ridge segments, until the plate on one side of the transform meets the ridge on the other side of the transform. At this point, the transform fault is typically intruded by mid-ocean ridge magma, and the apparent extension of the transform, known as a fracture zone, juxtaposes two segments of the same plate that move together horizontally. Fracture zones are not extensions of the transform faults, and they are no longer considered plate boundaries. After the ridge/transform intersection is passed, the fracture zone juxtaposes two segments of the same plate. There is typically some vertical motion along this segment of the fracture zone, since the two segments of the plate have different ages, and subside at different rates.
The transform and ridge segments preserve an orthogonal relationship in almost all cases, because this geometry creates a least work configuration, creating the shortest length of ridge possible on the spherical Earth.
Transform faults generate very complex geological relationships. They juxtapose rocks from very different crustal and even mantle horizons, show complex structures, exhibit intense alteration by high-temperature metamorphism, and have numerous igneous intrusions. Rock types along oceanic transforms typically include suites of serpentinite, gabbro, pillow lavas, lherzolites, harzburgites, amphibolite-tectonites, and even mafic granulites.
Transform faults record a very complex history of motion between the two oceanic plates. The relative motion includes dip-slip (vertical) motions due to subsidence related to the cooling of the oceanic crust. A component of dip-slip motion occurs all along the transform, except at one critical point, known as the
^Transform^ Ridge Ridge
^Transform^ Ridge Ridge
Three-dimensional view of a transform fault in the ocean basin, apparently offsetting a segment of the mid-ocean ridge. The sense of motion on the transform is opposite the apparent offset. Note that the lateral motion between the two segments of the oceanic crust ceases once the opposite ridge segment is passed. At this point, magmas from the ridge intrude the transform, and the contact becomes an igneous contact.
crossover point, where the transform juxtaposes oceanic lithosphere of the same age formed at the two different ridge segments. This dip-slip motion occurs along with the dominant strike-slip motion, recording the sliding of one plate past the other.
Fracture zones are also called nontransform extension regions. The motion along the fracture zone is purely dip-slip, due to the different ages of the crust with different subsidence rates on either side of the fracture zone. The amount of differential subsidence decreases with increasing distance from the ridge, and the amount of dip-slip motion decreases to near zero after about 60 million years. Subsidence decreases according to the square root of age.
Transform faults in the ocean may juxtapose crust with vastly different ages, thickness, temperature, and elevation. These contrasts often lead to the development of a deep topographic hole on the ridge axis at the intersection of the ridge and transform. The cooling effects of the older plate against the ridge of the opposing plate influences the axial rift topography all along the whole ridge segment, with the highest topographic point on the ridge being halfway between two transform segments. Near transform zones, magma will not reach its level of hydrostatic equilibrium because of the cooling effects of the older cold plate adjacent to it. Therefore, the types and amounts of magma erupted along the ridge are influenced by the location of the transforms.
Transform faults are neither typically vertical planes nor are they always straight lines connecting two ridge segments. The fault planes typically curve toward the younger plate with depth, since they tend to seek the shortest distance through the lithosphere to the region of melt. This is a least energy configuration, and it is easier to slide a plate along a vertically short transform than along an unnecessarily thick fault. This vertical curvature of the fault causes a slight change in the position and orientation of the fault on the surface, causing it to bend toward each ridge segment. These relationships cause the depth of earthquakes to decrease away from the crossover point, due to the different depth of transform fault penetration. Motion on these curved faults also influences the shape and depth of the transform-ridge intersection, enhancing the topographic depression and in many causing the ridge to curve slightly into the direction of the transform. Faults and igneous dikes also curve away from the strike of the ridge, toward the direction of the transform in the intersection regions.
Many of the features of ridge-transform intersections are observable in some ophiolite complexes (on-land fragments of ancient oceanic lithosphere), including the Arakapas transform in Troodos ophio-lite in Cyprus and the Coastal complex in the Bay of Islands ophiolite in Newfoundland.
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