Non-Volcanic Passive Margins
Encyclopedia
Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins
; the other end member being volcanic passive margin
s (VPM). Transitional crust welds continental crust
to oceanic crust
along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rift
s to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.
NVPM are the result of rifting when a continent breaks up to form an ocean, producing transitional crust without volcanism
. Extension causes a number of events to occur. First is lithospheric thinning, which allows asthenospherc upwelling; heating further erodes the lithosphere
, furthering the thinning process. The extensional forces also cause listric faults and continentward dipping reflectors that help identify NVPM and distinguish them from VPM, characterized by seaward-dipping seismic reflectors. The main difference between NVPM and VPM is that in the latter case, the mantle is hot enough to melt and produce voluminous basalts, whereas in the former case the mantle doesn't melt and there is little or no volcanism. Instead, extension simply pulls the crust away, exposing or "unroofing" the mantle, exposing serpentinized peridotite
. The mantle doesn't melt because it is cold or upwells slowly, so there are no igneous rocks like there are in VPM. The basalts and granites are replaced with serpentinized peridotite, accompanied by unique serpentothemal and hydrothermal activity. Increasing density
of the lithosphere as it cools and sediment accumulation causes subsidence.
NVPM also display distinct p-wave
velocity structures that differentiate them from VPM. Typical NVPM exhibit a high velocity, high gradient lower crust (6.4-7.7 km/s) overlain by a thin, low velocity (4–5 km/s) upper crustal layer. The high velocity shallow layer is usually interpreted as the serpentinzed peridotite associated with NVPM. In some cases, an extremely thick igneous underplating of a VPM will display similar P-wave
velocity (7.2-7.8 km/s, but with a lower gradient). For this reason, velocity structure alone cannot be used to determine the nature of a margin.
y feature associated with any continent-ocean transition
, including NVPM, is the free-air edge effect anomaly, which consists of a gravity high and a gravity low associated with the contrast between the thick continental and thin oceanic crust. There are also subsurface variations in density that cause significant variations across the continent-ocean transition. The crust, as well as the entire lithosphere
, is thinned due to mechanical extension. The Moho
marks a large density contrast between crust
and mantle
, typically at least 0.35 g/cm3. The highest amplitudes of the gravity anomaly
occur seaward of the continent-ocean transition. High-density upper mantle material is elevated relative to the more landward crustal root. The oceanic crust density is then further enhanced with gabbros and basalts and additionally contributes to the regional gravity trend.
Where the thickness of the crust and lithosphere varies, equilibrium must be reached. Isostatic
compensation and gravity anomalies result from balance between mass excess of the extra mantle beneath the thinned lithosphere and the overlying low-density crust. Positive gravity anomalies result from the relatively low flexural strength of the lithosphere during the beginning of rifting. As the passive margin matures, the crust and uppermost mantle become colder and stronger, so that the compensating deflection in the base of the lithosphere is broader than the actual rift. Higher flexural strength results in a broadening of the gravity anomaly with time.
During the Late Jurassic-Early Cretaceous, tectonic extensional forces created a shallow angle east-dipping detachment fault
. This fault cut from what is now the Flemish Cap margin in Nova Scotia
, eastern Canada to the Galicia margin, which is located west of the Iberian Peninsula
. This fault penetrated the upper portion of the continental crust and merged into the transition between brittle upper and plastic lower crust. In time, displacement along this detachment fault decreased to zero at a point under the Galicia margin. East of this detachment fault, the structure of the Galicia NVPM is entirely pure shear resulting in rotated fault blocks, normal faults, and continent-ward dipping seismic reflectors. Simple shear is only evident in the western edge of the Galicia margin and the upper crust of the Flemish Cap margin where the crust is brittle. Below this brittle crust, the ductile crust follows McKenzie’s pure shear model. Mantle material composed of peridotites is serpentinized by circulating seawater after it rises close enough to the upper crust due to its low density and isostatic forces. After sufficient thinning of the lithosphere, this serpentinized material is emplaced at the continent-ocean transition. This is why the transitional crust of NVPM are made of serpentinized peridotite instead of magmatic structures seen in VPM. Since the emplacement of the peridotite, oceanic crust has been forming at the Mid-Atlantic Ridge
and driving the two NVPM apart. The simple shear detachment became a deactivated detachment fault once this rifting process began the formation of new oceanic crust. This process explains the structures seen at the Galicia margin today.
Passive margin
A passive margin is the transition between oceanic and continental crust which is not an active plate margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic...
; the other end member being volcanic passive margin
Volcanic passive margin
Volcanic passive margins and non-volcanic passive margins are the two forms of transitional crust that lie beneath passive continental margins that occur on Earth as the result of the formation of ocean basins via continental rifting. Initiation of igneous processes associated with volcanic...
s (VPM). Transitional crust welds 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...
to oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent 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....
s to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.
Typical characteristics
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....
. Extension causes a number of events to occur. First is lithospheric thinning, which allows asthenospherc upwelling; heating further erodes the 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 :...
, furthering the thinning process. The extensional forces also cause listric faults and continentward dipping reflectors that help identify NVPM and distinguish them from VPM, characterized by seaward-dipping seismic reflectors. The main difference between NVPM and VPM is that in the latter case, the mantle is hot enough to melt and produce voluminous basalts, whereas in the former case the mantle doesn't melt and there is little or no volcanism. Instead, extension simply pulls the crust away, exposing or "unroofing" the mantle, exposing serpentinized peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
. The mantle doesn't melt because it is cold or upwells slowly, so there are no igneous rocks like there are in VPM. The basalts and granites are replaced with serpentinized peridotite, accompanied by unique serpentothemal and hydrothermal activity. Increasing density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the lithosphere as it cools and sediment accumulation causes subsidence.
Seismic characteristics
Seismic reflection lines across passive margins show many structural features common to both VPM and NVPM, such as faulting and crustal thinning, with the primary contra-indicator for volcanism being the presence of continent-ward dipping reflectors.NVPM also display distinct p-wave
P-wave
P-waves are a type of elastic wave, also called seismic waves, that can travel through gases , solids and liquids, including the Earth. P-waves are produced by earthquakes and recorded by seismographs...
velocity structures that differentiate them from VPM. Typical NVPM exhibit a high velocity, high gradient lower crust (6.4-7.7 km/s) overlain by a thin, low velocity (4–5 km/s) upper crustal layer. The high velocity shallow layer is usually interpreted as the serpentinzed peridotite associated with NVPM. In some cases, an extremely thick igneous underplating of a VPM will display similar P-wave
P-wave
P-waves are a type of elastic wave, also called seismic waves, that can travel through gases , solids and liquids, including the Earth. P-waves are produced by earthquakes and recorded by seismographs...
velocity (7.2-7.8 km/s, but with a lower gradient). For this reason, velocity structure alone cannot be used to determine the nature of a margin.
Gravity properties
Gravity data provides information about the subsurface density distribution. The most important gravitGravit
Gravitis a free and open source gravity simulator distributed under the GNU General Public License. The program is available for all major operating systems, including Linux and other Unix-like systems, Microsoft Windows and Mac OS X....
y feature associated with any continent-ocean transition
Continent-ocean boundary
The Continent-ocean boundary or continent-ocean transition is the boundary between continental crust and oceanic crust on a passive margin. The identification of continent-ocean boundaries is important in the definition of plate boundaries at the time of break-up when trying to reconstruct the...
, including NVPM, is the free-air edge effect anomaly, which consists of a gravity high and a gravity low associated with the contrast between the thick continental and thin oceanic crust. There are also subsurface variations in density that cause significant variations across the continent-ocean transition. The crust, as well as the entire 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 :...
, is thinned due to mechanical extension. The Moho
Mohorovičić discontinuity
The Mohorovičić discontinuity , usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle...
marks a large density contrast between crust
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...
and 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....
, typically at least 0.35 g/cm3. The highest amplitudes of the gravity anomaly
Gravity anomaly
A gravity anomaly is the difference between the observed acceleration of Earth's gravity and a value predicted from a model.-Geodesy and geophysics:...
occur seaward of the continent-ocean transition. High-density upper mantle material is elevated relative to the more landward crustal root. The oceanic crust density is then further enhanced with gabbros and basalts and additionally contributes to the regional gravity trend.
Where the thickness of the crust and lithosphere varies, equilibrium must be reached. Isostatic
Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
compensation and gravity anomalies result from balance between mass excess of the extra mantle beneath the thinned lithosphere and the overlying low-density crust. Positive gravity anomalies result from the relatively low flexural strength of the lithosphere during the beginning of rifting. As the passive margin matures, the crust and uppermost mantle become colder and stronger, so that the compensating deflection in the base of the lithosphere is broader than the actual rift. Higher flexural strength results in a broadening of the gravity anomaly with time.
Magnetic properties
The magnetic signature of a passive continental margin is influenced by the volume of material with a high magnetic susceptibility and the depth of the material below the surface. Large amplitude magnetic anomalies are associated with high magnetic susceptibility (~0.06 emu) igneous rocks of VPM. In contrast, NVPM exhibit only small amplitude anomalies associated with the edge effect at the boundary between the exhumed mantle (~0.003 emu) at the transition zone, and the true oceanic crust basalt (~0.05 emu). This anomaly can be used to locate the boundary between transitional crust and oceanic crust. The absence of large amplitude anomalies is a very strong indication that a margin is non-volcanic.Passive rifting
Passive rifting, unlike active rifting, occurs principally by extensional tectonic forces as opposed to magmatic forces originating from convection cells or mantle plumes. Isostatic forces allow mantle material to rise under the thinning lithosphere. Subsidence and sedimentation occur during both the initial rifting stage and the post rifting stages. Only after initial rifting does any mantle melting occur. Continued extension of the lithosphere will eventually lead to decompression melting of the mantle and the formation of a mid-ocean ridge. This process results in the creation of an ocean basin, and possibly conjugate NVPM (Geoffroy 2005).Rifting models
There are several models for forming NVPM. Passive rifting can follow McKenzie’s pure shear model, Wernicke’s simple shear model, or a composite model combining features of both, as observed at the Galicia bank NVPM.McKenzie pure shear model
Pure shear describes “homogeneous flattening” of rocks without rotations, while maintaining a constant volume. If a cube undergoes pure shearing, the result will be a rectangular prism with sides parallel to those of the initial cube. McKenzie’s model predicts symmetric structures on either side of the rift zone composed of rotated fault blocks bounded by normal faults.Wernicke simple shear model
In contrast to pure shear, simple shear describes constant volume strain with rotations. If a cube undergoes simple shearing, the result will be a parallelogram with sides that increase in length and are no longer parallel to the sides of the original cube. The top and bottom of the cube will neither stretch nor shorten. In a simple shear model, a basin is stretched asymmetrically by a large scale detachment fault extending from the upper crust to the lower lithosphere and even asthenosphere.Composite model formation
Detachment fault
Detachment faulting is associated with large-scale extensional tectonics. Detachment faults often have very large displacements and juxtapose unmetamorphosed hanging walls against medium to high-grade metamorphic footwalls that are called metamorphic core complexes...
. This fault cut from what is now the Flemish Cap margin in Nova Scotia
Nova Scotia
Nova Scotia is one of Canada's three Maritime provinces and is the most populous province in Atlantic Canada. The name of the province is Latin for "New Scotland," but "Nova Scotia" is the recognized, English-language name of the province. The provincial capital is Halifax. Nova Scotia is the...
, eastern Canada to the Galicia margin, which is located west of the Iberian Peninsula
Iberian Peninsula
The Iberian Peninsula , sometimes called Iberia, is located in the extreme southwest of Europe and includes the modern-day sovereign states of Spain, Portugal and Andorra, as well as the British Overseas Territory of Gibraltar...
. This fault penetrated the upper portion of the continental crust and merged into the transition between brittle upper and plastic lower crust. In time, displacement along this detachment fault decreased to zero at a point under the Galicia margin. East of this detachment fault, the structure of the Galicia NVPM is entirely pure shear resulting in rotated fault blocks, normal faults, and continent-ward dipping seismic reflectors. Simple shear is only evident in the western edge of the Galicia margin and the upper crust of the Flemish Cap margin where the crust is brittle. Below this brittle crust, the ductile crust follows McKenzie’s pure shear model. Mantle material composed of peridotites is serpentinized by circulating seawater after it rises close enough to the upper crust due to its low density and isostatic forces. After sufficient thinning of the lithosphere, this serpentinized material is emplaced at the continent-ocean transition. This is why the transitional crust of NVPM are made of serpentinized peridotite instead of magmatic structures seen in VPM. Since the emplacement of the peridotite, oceanic crust has been forming at the Mid-Atlantic Ridge
Mid-Atlantic Ridge
The Mid-Atlantic Ridge is a mid-ocean ridge, a divergent tectonic plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South...
and driving the two NVPM apart. The simple shear detachment became a deactivated detachment fault once this rifting process began the formation of new oceanic crust. This process explains the structures seen at the Galicia margin today.