Slab gap hypothesis
Encyclopedia
In geology
, the slab gap hypothesis is one of the explanations put forward to explain several instances of crustal
extension that occur inland near former subduction zones.
Standard plate tectonic theory holds that once a trench
is closed by an overriding plate reaching a rift/spreading center
, the plate that has just been fully consumed continues to descend beneath the overriding plate for some time, transmitting compressive pressures to the overriding plate above as well as occasional volcanism
. Meanwhile, the descending plate leaves behind it a "window" of inactivity. In this view, there is no mantle
upwelling, so once the crustal rift
is overridden, the only residual effects are from the remnant descending plate slab. However, actual observations of the crust in western North America where the Farallon Plate
's trench and rift was snuffed out millions of years ago by the westward movement of the North American Plate
, and replaced by the San Andreas Fault
, show not compression inland, but extension. This is most evident in the Great Basin
where the continental crust
and lithosphere
as a whole is becoming increasingly thin. The crust here is also warming from below.
The slab gap hypothesis proposes that instead of a "window", the descending slab
leaves behind a "gap" through which the asthenospheric mantle
of the former spreading zone continues to act beneath the overriding plate. This hypothesis then assumes that a crustal spreading zone is also underpinned by a corresponding asthenospheric mantle spreading zone or upwelling of warmer material. The gap is created because instead of the old subducted plate continuing to sink, it quickly melts, allowing the asthenospheric upwelling zone to act directly on the underside of the overriding plate, heating it and causing it to spread apart. The fast melt is because the portion of the subducted plate nearest the spreading zone is thin and still warm from its recent creation.
The slab gap hypothesis goes on to state that the upwelling can form very deep cracks, which in turn lets very fluid basalt
lava
quickly spread over the land surface forming shield volcano
es and vast volcanic plains called "flood basalt
s". If, however, the extension is spread over a very large area then these flood basalt events may not occur.
This idea has been used to explain the extension and very large flood basalts that occurred in what is now southern Washington, Oregon
and northern California
about 17 million years ago (see Columbia River Plateau
). Slab gap has also been used to help explain the earlier creation of the Basin and Range Province.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, the slab gap hypothesis is one of the explanations put forward to explain several instances of crustal
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
extension that occur inland near former subduction zones.
Standard plate tectonic theory holds that once a trench
Oceanic trench
The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....
is closed by an overriding plate reaching a rift/spreading center
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
, the plate that has just been fully consumed continues to descend beneath the overriding plate for some time, transmitting compressive pressures to the overriding plate above as well as occasional volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
. Meanwhile, the descending plate leaves behind it a "window" of inactivity. In this view, there is no mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
upwelling, so once the crustal rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
is overridden, the only residual effects are from the remnant descending plate slab. However, actual observations of the crust in western North America where the Farallon Plate
Farallon Plate
The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period...
's trench and rift was snuffed out millions of years ago by the westward movement of the North American Plate
North American Plate
The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...
, and replaced by the San Andreas Fault
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...
, show not compression inland, but extension. This is most evident in the Great Basin
Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds in North America and is noted for its arid conditions and Basin and Range topography that varies from the North American low point at Badwater Basin to the highest point of the contiguous United States, less than away at the...
where the continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
and lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
as a whole is becoming increasingly thin. The crust here is also warming from below.
The slab gap hypothesis proposes that instead of a "window", the descending slab
Slab (geology)
In geology, a slab is the portion of a tectonic plate that is being subducted.Slabs constitute an important part of the global plate tectonic system. They drive plate tectonics both by pulling along the lithosphere to which they are attached in a processes known as slab pull and by inciting...
leaves behind a "gap" through which the asthenospheric mantle
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
of the former spreading zone continues to act beneath the overriding plate. This hypothesis then assumes that a crustal spreading zone is also underpinned by a corresponding asthenospheric mantle spreading zone or upwelling of warmer material. The gap is created because instead of the old subducted plate continuing to sink, it quickly melts, allowing the asthenospheric upwelling zone to act directly on the underside of the overriding plate, heating it and causing it to spread apart. The fast melt is because the portion of the subducted plate nearest the spreading zone is thin and still warm from its recent creation.
The slab gap hypothesis goes on to state that the upwelling can form very deep cracks, which in turn lets very fluid basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
quickly spread over the land surface forming shield volcano
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
es and vast volcanic plains called "flood basalt
Flood basalt
A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...
s". If, however, the extension is spread over a very large area then these flood basalt events may not occur.
This idea has been used to explain the extension and very large flood basalts that occurred in what is now southern Washington, Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
and northern California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
about 17 million years ago (see Columbia River Plateau
Columbia River Plateau
The Columbia Plateau is a geologic and geographic region that lies across parts of the U.S. states of Washington, Oregon, and Idaho. It is a wide flood basalt plateau between the Cascade Range and the Rocky Mountains, cut through by the Columbia River...
). Slab gap has also been used to help explain the earlier creation of the Basin and Range Province.