Plate Motion

If the true plate motions (for the part of plates exposed on the Earth's surface) are on average gravitationally horizontal (with respect to the geoid), then on average the motion must also be horizontal with respect to the reference ellipsoid (which is defined to align with the geoid on average).

From: Treatise on Geophysics , 2007

Plate boundaries and driving mechanisms

Graeme Eagles , in Regional Geology and Tectonics (Second Edition), 2020

Summary

Plate motion is maintained due to the detailed balance of (1) buoyancy forces generated by their thickness variations, by the distribution and size of their subducted parts and by upwelling in the mantle beneath them, with (2) resistance at their various interfaces with other plates and the underlying mantle. The pattern of plate motion may dictate or be dictated by the pattern of mantle convection. The torque balance maintains, and is maintained by, steady plate motion and mantle convection over long periods. Relative to their shared margins, movements between pairs of plates may be divergent, forming continental rifts and mid-ocean ridges, convergent, giving rise to subduction and collision zones, or be parallel, causing the action of transform faults. The detailed structure and development of all these features depend on a wide variety of specific parameters that vary strongly according to the speed of relative motion, crustal and lithospheric rheology, preexisting structure, and composition, and the composition and structure of the shallow convecting mantle.

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Crustal and Lithosphere Dynamics

P. Wessel , R.D. Müller , in Treatise on Geophysics (Second Edition), 2015

6.02.4.6 APM Models, Paleomagnetics, and TPW

APM models have traditionally been determined for a single plate (e.g., Pacific) or a set of plates connected by ridges (e.g., Africa and neighbor plates in the Indo-Atlantic). It is therefore of interest to propagate these predictions into other oceans using the plate circuits mentioned earlier so that APM models may be compared. As mentioned earlier, Molnar and Stock (1987) first showed that the hot spots in the Pacific appear to have moved relative to the set of hot spots used to determine Indo-Atlantic APM; however, Andrews et al. (2006) found that this relative motion is only significant prior to 68   Ma. There are two important issues that make these comparisons challenging. First, a key problem area in designing global plate circuits is the connection via West Antarctica. Because the boundary is located in the Ross Sea, the motion between East Antarctica and West Antarctica remains difficult to determine with precision. Seafloor spreading in the Adare Trough was first identified by Cande et al. (2000), but data remain sparse and relatively small changes in the RPM estimates propagate to give relative large changes elsewhere. In particular, continental deformation within the Transantarctic Mountains is difficult to reconstruct accurately, contributing to the total uncertainty. Second, relatively few hot spot chains have adequate (or any at all) sampling for paleomagnetic analysis, making the models incomplete.

A benchmark test for all global APM modeling has been how well an Africa-based APM, after projection via the plate circuit, is able to reconstruct the trend of the Hawaiian–Emperor hot spot trail in the Pacific. Early efforts (Cande et al., 1995; Raymond et al., 2000) showed significant mismatch for reconstructions older than ~   40   Ma and were unable to account for the geometry of the HEB. One approach to address this failure was to consider the effect of moving hot spots (Steinberger, 2000). The latitudinal component of such motions can be constrained by paleolatitudes, at least in the case of the Hawaiian plume (Tarduno et al., 2003). It is expected that recently published data from the Louisville chain (Koppers et al., 2012) will be used to constrain its hot spot motion as well. Steinberger et al. (2004) found that by using a moving hot spot APM model for Africa, the predictions in the Pacific depended on which plate circuit was used for the reconstruction. Using the East–West Antarctica circuit employed by Cande et al. (1995) again failed to match the HEB geometry, but if a new circuit that connected Australia to the Pacific via Lord Howe Rise was used (thus bypassing West Antarctica entirely), the fit improved somewhat. This improvement led Doubrovine et al. (2012) to extend the approach of O'Neill et al. (2005) and determine a global APM that satisfied the geometry, age progression, and paleolatitude of five hot spot chains (Hawaii and Louisville on the Pacific Plate, Réunion and Tristan da Cunha on the African Plate, and the New England chain on the North American Plate). Their model championed the Lord Howe Rise circuit and represents a truly global moving hot spot APM model, despite some shortcomings in fitting the five individual hot spot trails. The Lord Howe Rise plate circuit is based on the assumption that there was no subduction east of the Lord Howe Rise from about 100 to 50   Ma. However, recent work (Matthews et al., 2012) shows there is petrologic evidence for subduction in this region after 90   Ma, and there is clear seismological evidence of subducted slab material in the lower mantle underneath the Lord Howe Rise region that requires active subduction between the Pacific and the Lord Howe Rise after ~   90–85   Ma. This argues for using the East–West Antarctica circuit despite its shortcomings.

The availability of paleolatitudes from the Louisville chain means two chains on the same plate now have considerable latitudinal constraints for their oldest sections. Combined with the time series of inter-hot spot distances (Wessel and Kroenke, 2009), there are now several types of data constraints available for APM models. Models of hot spot motion must broadly satisfy these independent datasets. In particular, the inter-hot spot distances can be computed given dated rock samples from several hot spot chains; such data are much more plentiful than the limited paleolatitude dataset that requires expensive oceanic drilling expedition that hopefully sample enough volcanic flow units to average out the paleosecular variation. Being independent datasets, additions of new chronology and paleolatitude measurements will be exceptionally valuable for future APM studies.

As more paleomagnetic data suggest changes in the latitudes of hot spots through time, approaches such as that of Doubrovine et al. (2012) will likely become refined over time. However, many challenges remain. For reconstructions that go back before 70   Ma, severe numerical artifacts tend to develop in mantle backward advection calculations (e.g., Conrad and Gurnis, 2003). Doubrovine et al. (2012) thus made the choice to limit the backward advection for their plumes to 70   Ma; for older times (i.e., up to 130   Ma), a constant flow field was assumed defined by the density structure reconstructed at 70   Ma. This necessary simplification creates additional uncertainty for the modeled hot spot motions. Further work will clearly be needed to refine global plate circuits and their effects on global APM models, as well as improvements to the APM models themselves.

TPW is defined as the wholesale rotation of the Earth relative to its spin axis. TPW is believed to occur in response to changing mass distributions about the Earth's spin axis (e.g., Tsai and Stevenson, 2007) and has been called upon to explain tectonic and paleomagnetic puzzles in the Earth's past (e.g., Li et al., 2004). The possibility of TWP complicates the determination of global APM. For instance, in fitting their global APM model, Doubrovine et al. (2012) found evidence for significant amounts of TPW. In particular, they identified two nearly equal and antipodal rotations of the Earth relative to its spin axis for the 90–60 and 60–40   Ma intervals. Other workers have identified episodes of TPW around 100–110   Ma and several instances in the more distant past (e.g., Besse and Courtillot, 2002; Greff-Lefftz and Besse, 2012; Steinberger and Torsvik, 2008 ). Paleolatitude measurements from oceanic hot spot chains may contain components of TPW; hence, a key challenge of future work is to resolve the contributions from plume motions, TWP, and plate motions.

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Basin Structure, Tectonics, and Stratigraphy

A.M. Dayal , in Shale Gas, 2017

4.2 Plate Motion

Plate motion is an important phenomenon of our Earth system. The various geological and geophysical activities on land and ocean are the result of plate motion. Plate boundaries can be divided as convergent, divergent, or transform. They are related with compression, extension, and strike-slip faults. Compression of plates is responsible for subduction activity. Mountain building is the result of subduction of one plate under another plate. There is a collision of continental plates and also continental plate to oceanic plate. In the case of compression of plates, there is crustal shortening, and the thickness of the crust increases. In the case of divergent activity of plates, new ocean floors are created and also related with large-scale volcanic activity and formation of new oceans. Convergence of oceanic plates with continental plates also results in mountain building activity. Motion of these plates is also responsible for major seismic activity as it activates many big fault systems. In fact, the active seismic zones at present are related with either tectonic activity at continental plates or the motion of a plate under continental or oceanic plates. In the case of divergent plate motion, new, smaller plates are moving apart. In the case of continental rifts, there is crustal thinning and faulting as the crust undergoes extension. Divergent movement of oceanic plates is responsible for the oceanic crustal thinning and formation of mid-ocean ridge basalt. The movement of plates is responsible for the formation of new continents and oceans in geological history.

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Mantle Dynamics

D. Bercovici , ... Y. Ricard , in Treatise on Geophysics (Second Edition), 2015

7.07.5.3 Changes in Plate Motion

Plate motions evolve with various timescales. Some are clearly related to mantle convection, such as those associated with the Wilson cycle ( Wilson, 1966), that is, the periodic formation and breakup of Pangaea, approximately every 500   My. Various hypotheses have been proposed to explain the dispersal of supercontinents. The conventional view is that supercontinents provide an insulating blanket, and thus, with radiogenic heating, they warm the underlying mantle, eventually inducing a hot upwelling that weakens and breaks up the overlying lithosphere (Coltice et al., 2007, 2009; Gurnis, 1988; Lowman and Jarvis, 1996). However, this mechanism requires more radiogenic heating than may actually exist (Korenaga, 2008). In the same vein, the present-day continents do not suggest that they stand above hotter than normal mantle; indeed, their basal heat flux appears very low (e.g., Guillou et al., 1994). However, this recent observation might not be generally relevant; in particular, Rolf et al. (2012) showed that while small subcontinental temperature anomalies occur as continents are drifting, they can become significantly larger when continents assemble into a supercontinent. Regardless, whether supercontinent insulation induces sufficient heating to induce dispersal remains a matter of debate (see Bercovici and Long, 2014; Heron and Lowman, 2011; Lenardic et al., 2005, 2011).

Plate motion changes that have occurred on shorter timescales are even more difficult to understand. The plate-tectonic history recorded in paleomagnetic data and hot spot tracks consists of long stages of quasisteady motions separated by abrupt reorganizations.

Stages of quasisteady motion are reasonably well explained in terms of plate forces (slab pull, ridge push, mantle drag, etc; Forsyth and Uyeda, 1975) or equivalently convective buoyancy from large-scale heterogeneities (e.g., Lithgow-Bertelloni and Richards, 1995; Ricard et al., 1989). Gradual reversals can also be explained by convective motion; for example, the notion of thermal blanketing can be extended to allow for plate reversals in that hot subcontinental mantle anomalies can effectively annihilate slabs and thus cause radical changes in plate motion (King et al., 2002; Lowman et al., 2003) (see Figure 18 ).

Figure 18. The convection model of King et al. (2002) shows that reversals in plate motion can occur when converging flow over a cold downwelling (a) draws in hot subcontinental mantle (b) that annihilates the downwelling, subsequently changing the polarity of the convergent margin to a divergent one (c).

 Adapted from King SD, Lowman JP, and Gable CW (2002) Episodic tectonic plate reorganizations driven by mantle convection. Earth and Planetary Science Letters 203(1): 83–91.

Abrupt changes in plate motion, however, are not easily related to convective processes. The most dramatic plate motion change is recorded in the Hawaiian–Emperor bend, dated at 47   Ma (Sharp and Clague, 2006; Wessel and Kroenke, 2008); this bend suggests a velocity change of a major plate of ~   45° during a period no longer than 5   My, as inferred from the sharpness of the bend. Convective plate driving forces, such as due to sinking slabs, cannot change directions much faster than the time to lose or erase a thermal anomaly, which is of order several tens of millions of years, based on descent at a typical convective velocity (see Section 7.07.4.1.1 ). Therefore, changing convective motions in less than ~   5   My is physically implausible. Abrupt changes may be due to nonconvective sources such as rapid rheological response and fast adjustments in plate boundary geometries, such as due to fracture and rift propagation (e.g., Hey and Wilson, 1982; Hey et al., 1995). Plate reorganizations due to (a) the annihilation of a subduction boundary by rapid slab detachment and/or continental collision (e.g., Bercovici et al., 2015), (b) loss of a ridge and/or trench by subduction of a ridge (e.g., Thorkelson, 1996), or (c) the initiation of a new subduction zone (e.g., Hilde et al., 1977) possibly occur on relatively rapid timescales, although the timing of such mechanisms is not well constrained (Richards and Lithgow-Bertelloni, 1996). Offsets in plate age and thickness along already weak oceanic transform boundaries may provide excellent sites for the initiation of subduction (e.g., Hall et al., 2003; Stern, 2004; Stern and Bloomer, 1992; Toth and Gurnis, 1998) yielding a possible mechanism for plate motion changes; this, coupled with the notion of transform faults as long-lived weak 'motion guides' (see Section 7.07.6.1.2 ), emphasizes the importance of transform boundaries in the plate–mantle system. The possible mechanism for abrupt plate motion changes underscores the need to understand the interactions between the long timescale convective processes and the short timescale effects associated with the lithosphere's rheological response, for example, by fracture, fault sliding, and strain localization (see Section 7.07.6 ).

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Plate Tectonics☆

R.C. Searle , in Reference Module in Earth Systems and Environmental Sciences, 2015

Plates as Parts of the Mantle Convection Cycle

Plate motions are ultimately driven by the Earth's heat energy, and they are intimately related to the mantle convection that is driven by this heat. One view of plates is that they simply represent the surficial parts of mantle convection cells: as hot, ductile mantle rises to the surface, it cools and becomes brittle – a plate – and then moves as a rigid block over the surface before being subducted, gaining temperature and becoming ductile again. Recent results from seismic tomography suggest that, around the rim of the Pacific, sheets of cold material descend below subduction zones deep into the lower mantle, implying a strong coupling of mantle motion and subducted plates.

However, the coupling is not perfect. There are some parts of the mid-ocean ridge (divergent plate boundaries) where it seems that the deeper mantle (below the asthenosphere) may be descending rather than rising. One such place is the so-called Australo-Antarctic Discordance south of Australia. Moreover, some plates, such as Africa, are almost entirely surrounded by ridges and have very few subduction zones on their boundaries. In such cases, a rigid coupling of plates to convection cells would imply the unusual scenario of upwelling along an expanding ring, with a downwelling column inside it. In fact, one of the advantages of plate tectonics is that it allows partial decoupling of plate motions from deeper mantle flow via the ductile asthenosphere.

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Volcanic Reservoirs and Hydrocarbon Accumulations

Caineng Zou , in Unconventional Petroleum Geology, 2013

3 Tectonic Environment for the Formation of Volcanic Rocks

Plate motion is restricted by deep-seated processes. Volcanic eruption or intrusion is the reflection of upper mantle and deep crust convection on the ground surface or shallow crust. Therefore, it is important to consider the tectonic background when studying the distribution and features of volcanic rocks. In terms of plate tectonic theory, volcanism is generally developed in environments that are closely related to plate motion, such as basin margins and island arcs ( Figure 7-5).

FIGURE 7-5. Sketch map of the plate tectonic environment.

The rift zone, a canyon topography developed along parallel faults, is one of the major tectonic mobile belts on the ground, which is defined as large extensional tectonic units with deep affection and large extension (Ma, 1982). Because of upwelling hot mantle or magma, the lithosphere became thin during the extensional process. The arching process took place first, forming large dome structures or the trigeminal rifts and many fault-blocks, along with continental alkaline and weak-alkaline basalt eruptions in large areas. Magma activities in a subduction zone are mainly inside the range of magmatic arc, about 150–300 km away from the trench axis, and are distributed in an arc shape parallel to the trench. The major rock series include island-arc tholeiite series, calc-alkaline series, and island-arc alkaline series (or high-K shoshonite series). Mid-ocean ridge is characterized by the generation of tholeiite and lack of andesite, where the magma is generated in the relatively shallow depth along the mid-ocean with seismic activities, forming alkaline-poor tholeiite magma (Jokat et al., 1992). Igneous rocks of continental craton are related to certain intraplate extensional tectonic environment. Magma activities usually are related to hotspots or the plume of upwelling mantle in the area without obvious tectonic features. Magma eruptions inside the oceanic basins are mostly represented by volcanic islands and oceanic volcano, with two basic occurrences: (1) volcanic island chain, and (2) isolated volcanic island. Magma activities are poorly developed in passive continental margins. Except for the preserved igneous associations formed during the continental rifts in the early stage and intercontinental rift stage, there are a few magma activities that are related to mantle hotspots or mantle plume, forming some intraplate-like (continental plate and intraoceanic plate) igneous associations, which are dominated by intrusive activities of mafic, ultramafic, and vein rock activities, along with some centered or fissured volcanic eruptions.

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Volume 5

Vivien Gornitz , in Encyclopedia of Geology (Second Edition), 2021

Plate Tectonics

Tectonic plate motions affect paleoclimate change on timescales of 10–100  Ma. Geographic reconfiguration of continents and ocean basins produced marked shifts in North-South climate zonation, continental topography, and the opening or closing of ocean gateways that can alter ocean and atmospheric circulation patterns. Plate collisions raise mountains, while sea floor spreading changes ocean basin volume and sea level. Volcanism is closely tied to tectonism. It influences climate through atmospheric emissions of CO2, CH4, SO2, H2S, and other volatiles.

The onset of modern-style plate tectonics dates to between 3.2 and 2.5   Ga, as inferred from changes in igneous-metamorphic rock assemblages, deformationalstyles, and sulfur isotope anomalies in diamond inclusions (Cawood et al., 2018; Smit et al., 2019).

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South Atlantic Ocean

Webster Ueipass Mohriak , ... Andres C. Gordon , in Meso-Cenozoic Brazilian Offshore Magmatism, 2022

Potential field datasets and plate reconstructions

Absolute plate motions based on GPS data in past decades indicate an NW movement of the South American plate and an NE movement of the African plate. These directions are in agreement with traces of hot spots, particularly in Africa, where Walvis–Tristan da Cunha is characterized by an NE–SW direction, similar to the Cameroon lineament near the equatorial margin. According to Fairhead and Wilson (2005), the South Atlantic spreading ridge is moving north relative to the mantle, and flow lines associated with plate divergence trend approximately E–W, following the transform fault zones. The resultant vectors of the absolute plate motions point NW along the Brazilian margin and NE along the African margin (Fig. 1.1). These directions are characterized by major lineaments in the South Atlantic Ocean, such as the Walvis Ridge in the African plate and several NW lineaments in the South American plate.

The free-air anomaly map suggests two major NW–SE lineaments in the Brazilian margin: the Bahia seamounts in the northeastern margin, and the Cruzeiro do Sul lineament in the southeastern margin, extending from the RGR toward the Cabo Frio High at the boundary between the Campos and Santos basins (Fig. 1.1). The residual Bouguer gravity anomaly map of the eastern Brazilian margin (Fig. 1.2) shows that the Cruzeiro do Sul (Southern Cross) Lineament is marked by an NW-trending zone with postbreakup rift structures in the RGR and igneous intrusions from the oceanic crust toward the continental crust in the Cabo Frio High (Mohriak et al., 2008, 2010).

Several igneous plugs are located both onshore and offshore of the southeastern Brazilian margin, following the NW-trending magmatic lineament in the oceanic region. These volcanic edifices are associated with several large seamounts between the RGR and the Cabo Frio High. The Jean Charcot Seamounts (Fig. 1.2) correspond to the largest cluster of igneous structures adjacent to the distal limit in the Santos Basin. In the Cabo Frio Volcanic Province, between the Campos and Santos basins, several exploratory wells were drilled in volcanic buildups formed during the Late Cretaceous and Paleogene (Mohriak, 2003, 2020).

Some authors have interpreted that the NW-trending onshore lineament separated the Paraná and São Francisco basins, forming one of the largest intracontinental water divides in South America (Ribeiro et al., 2018). Other proposed an almost E–W magmatic lineament onshore, evidenced by several plugs forming a belt parallel to the coast, extending from Poços de Caldas in the west to Cabo Frio Island in the east (Sadwowski and Dias Neto, 1981). Because the radiometric age determinations for these plugs become younger eastward, some authors postulated a hot spot track for these alkaline intrusions (Thomaz Filho et al., 2000, 2008). Other authors have proposed changes in the hot spot track owing to variations in plate motion, correlating to igneous activity in the Santos Basin and to volcanic features in the Campos and Espírito Santos basins (Schattner and Michaelovitch de Mahiques, 2020). However, based on refined radiometric ages (Geraldes et al., 2013), some works have questioned these interpretations.

According to some researchers (Coutinho, 2008), the continental breakup in the South Atlantic margin is related to a triple junction system, as indicated by dyke swarms along the southeastern Brazilian margin. The south arm of the triple junction trends N–S and extends from Florianópolis Island toward the north, crossing the São Paulo and Rio de Janeiro states. Detailed geological maps of the onshore dykes have characterized a conspicuous NNE trend for the Early Cretaceous dykes along the southeastern margin, particularly in the Florianópolis dyke swarm The northern arm corresponds to the coastal dyke swarms that trend NNE in the eastern part of Rio de Janeiro state (Giro et al., 2021). The NW–SE arm corresponds to the Ponta Grossa Arch, which is clearly expressed on magnetic maps (Almeida et al., 2013). The E–W arm of the triple junction corresponds to the Poços de Caldas–Cabo Frio tectono-magmatic lineament (Sadwowski and Dias Neto, 1981), which runs north of the coastline between São Paulo and Rio de Janeiro. Near the São Sebastião Island, the coastline deflects to an E–W direction toward the Cabo Frio region (Fig. 1.3). According to Coutinho (2008), the N–S branch continued south toward the Etendeka volcanics in Namibia, where the dykes align with the N–S arm where the plates are restored to their predrift location. The Merluza Graben, which also trends N–S, is located at the offshore region of the southwestern Santos Basin, and its trend approximately corresponds to the northern continuation of this triple junction, resulting in a failed arm.

The residual Bouguer gravity anomaly (Fig. 1.2) shows the main elements of the southeastern margin. The map indicates that the pre-Aptian hinge line, corresponding to the proximal limit of the rift basins, is associated with a positive gravity anomaly. This gravity anomaly, which some authors attribute to necking of the continental crust (Meisling et al., 2001), is located offshore the Santos and Campos basins, but deflects toward the onshore region near the Abrolhos Volcanic Complex. The inflection of the Cretaceous hinge line indicates that the Early Cretaceous lacustrine syn-rift sediments are restricted to the continental platform of the Santos and Campos basins, whereas the Espírito Santo Basin accumulated a narrow trough of syn-rift sediments in the current onshore region. Landward of the Cretaceous hinge line, only postsalt Cenozoic sediments are observed covering prerift basalts and Precambrian basement rocks (Fig. 1.2).

Some authors interpreted the anomalous NS-trending feature at the southern Santos Basin, marked by conspicuous gravity and magnetic anomalies, to correspond to an aborted oceanic spreading center, known as the AR (Fig. 1.2) oceanic propagator (Mohriak, 2001; Mohriak et al., 2008; Dehler et al., 2016). Other authors interpreted the anomaly to correspond to continental crust based on potential field datasets (Karner, 2000), or to exhumed mantle based on analogies with the Iberian margin (Zalán et al., 2011). The mantle exhumation model assumes that the sedimentary basins in SE Brazil are associated with a magma-poor margin, similar to the Iberian margin, where peridotite ridges mark the transition from continental to oceanic crust (Boillot et al., 1980). However, the seismic interpretation of the southern Santos Basin suggests the presence of seaward-dipping reflectors at the transition to the oceanic crust, indicating a magmatic origin for these features (Gladczenko et al., 1997; Mohriak et al., 2008, 2010; Gordon and Mohriak, 2015; McDermott et al., 2018).

Based on analogies with the distal margin of the Red Sea, Mohriak (2014, 2018) suggested that the oceanic propagator penetrated the salt sequences in the southern Santos Basin but was aborted by Late Aptian/Early Albian, owing to shifting of the spreading center to the east. Plate reconstructions for the Late Aptian salt basin indicate a V-shaped structure in the southern Santos Basin (Müller et al., 2016, 2019). This feature has been interpreted as equivalent to the Gulf of Aden oceanic propagator, which is currently advancing toward the continental crust in the Afar region (Mohriak and Leroy, 2013). In the central Red Sea, embryonic spreading centers are interpreted at the axial trough, where the protuberant ridge was formed in the past 2 million years (Ligi et al., 2012). The seismic interpretation of the Red Sea axial trough suggests that salt masses are flowing toward an abyss that is floored by proto-oceanic volcanic basement (Mitchell et al., 2010, 2017; Mohriak, 2014, 2018; Feldens and Mitchell, 2015). Contrasting with the Red Sea, the oceanic propagator in the southern Santos Basin is characterized by a protuberant igneous feature that is locally overlain by thousands of meters of Cretaceous to Cenozoic sediments.

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Plate Tectonics

William R. Dickinson , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II Quantitative Measures of Plate Motion

Directions of plate motion can be determined from the orientations of transform faults, which lie parallel to the relative movement of adjacent plates, and from the "first" (initial) motions of seismic waves generated during earthquakes caused by sudden jerky slip of rock masses in contact along plate boundaries. Most earthquakes do occur along plate boundaries, which are delineated as faithfully by maps of global seismicity as if the plate margins had been traced out by some cosmic stylus. First motions of earthquakes along divergent plate boundaries reflect extensional deformation of plate edges, those from the subduction zones of convergent plate boundaries reflect contractional deformation, and those generated along transform faults indicate the sense of transform slip, whether dextral (right-lateral) or sinistral (left-lateral) with respect to the fault trend.

Rates of relative plate motion are recorded by arrays of linear geomagnetic anomalies, where earth's magnetic field is greater or less than expected, which lie parallel to loci of seafloor spreading within ocean basins. From independent studies of the magnetization retained by lavas exposed by erosion of volcanic fields on land, it is known that the earth's geomagnetic field has reversed repeatedly through geologic time, to impart alternately normal and reversed magnetization to lavas erupted at different times. The lavas preserve a faithful record of the alternating geomagnetic field because they cooled through the temperature at which the imprint of an ambient geomagnetic field is frozen into solid rock, as remanent (permanent) magnetization, during successive intervals ("chrons") of normal and reversed geomagnetic polarity. Because the timing of geomagnetic reversals is irregular (episodic), rather than regular (periodic), the pattern of normal and reversed polarity chrons in geologic time defines a unique pattern. If normal chrons are denoted by black stripes, and reversed chrons by white or blank stripes, the reversal pattern is geometrically similar to the bar codes used for labeling many commercial products (Fig. 4).

FIGURE 4. Diagram illustrating the correlation of geomagnetic polarity chrons spaced in geologic time with seafloor geomagnetic anomalies spaced geographically as magnetic "stripes" detected by magnetometers sailed or flown over the ocean floor. The correlation allows the rate of formation of new seafloor by plate divergence at a midocean spreading ridge to be determined without ambiguity.

Analysis of geomagnetic anomalies at sea reveals that the geographic spacing of linear anomalies, positive and negative, mimics the spacing in time of past polarity chrons, normal and reversed (Fig. 4). As successive increments of new oceanic lithosphere form by seafloor spreading, the lavas of the seafloor are magnetized with normal or reversed polarity, depending on the nature of the chron during which each segment of new seafloor was created. Normally magnetized seafloor reinforces the strength of the current geomagnetic field, to produce positive magnetic anomalies, whereas reversely magnetized seafloor counteracts the strength of the current geomagnetic field, to produce negative magnetic anomalies.

Once each specific geomagnetic anomaly is identified as the record of a particular polarity chron, the geographic spacing of the parallel magnetic anomalies can be used as a magnetic tape recorder documenting the rate of seafloor spreading induced by plate divergence. Arrays of parallel geomagnetic anomalies are displayed as mirror images on opposing flanks of each midocean ridge marking a divergent plate boundary (Fig. 4). Each increment of seafloor is split down its middle, where it is hottest and weakest along the plate boundary, by continued plate divergence. Because of that characteristic geodynamic behavior, each midocean ridge generates two identical bar codes, one displayed on each flank, with each recording half the full spreading rate of plate motion. Correlations of geomagnetic anomalies through the various modern ocean basins allow the relative motions of multiple plates to be established with confidence. The geometric similarity of the "bar codes" delineated in space by geomagnetic anomalies at sea and the "bar codes" delineated in time by polarity chrons shows that seafloor spreading commonly proceeds at rates that are nearly uniform for millions of years (Fig. 4).

Linear axes of seafloor spreading are termed spreading "centers" from the central positions of the youngest geomagnetic anomalies in axisymmetric arrays of magnetic "stripes" (Fig. 4), and from the locations of those central anomalies along the crests of midocean ridges as viewed in transverse profile. The persistent linearity of the magnetic anomalies as they move away from spreading centers indicates that plates of oceanic lithosphere are indeed rigid, not deforming internally as they move laterally with respect to one another. Patterns of seafloor geomagnetic anomalies indicate, however, that the evolution of spreading centers may include discrete shifts in axes of seafloor spreading ("ridge jumps") into positions breaking older oceanic lithosphere, episodic abandonment or initiation of transform linkages between midocean ridge segments, longitudinal propagation and complementary termination of spatially overlapping axes of seafloor spreading, and development of subordinate local "microplates" bounded by subparallel, simultaneously active spreading systems.

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Regional tectonics and basin formation: the role of potential field studies – an application to the Mesozoic West and Central African Rift System

James Derek Fairhead , in Regional Geology and Tectonics (Second Edition), 2020

Offshore plate tectonic links to the West and Central Africa Rift System

The relative plate motion for the Central Atlantic, as represented by the Kane fracture zone ( Fig. 20.3), is between the North America and Northern Africa plates. The change in curvature of all the fracture zones indicates subtle changes in relative plate motion between these plates.

The fracture zones close to the mid-ocean ridge axis are devoid of sediment and are major bathymetric troughs. Farther from the ridge, the fracture zones start to accumulate low-density sediment and their bathymetric depth increases as the mid-ocean ridge progressively cools and subsides away from its axis. These two factors result in sedimentation progressively masking the fracture zones towards the continental margin, making the gravity response of the fracture zones more difficult to identify. However, some of the fracture zones can be clearly tracked close to the continental margins even at the scale of the page image. The isochrones, shown in Fig. 20.5 as black lines with 10   Ma separation, have been derived from the geomagnetic reversal data (not shown here due to the sparseness of data and since geomagnetic reversals occur on average every 0.5 million year).

Figure 20.5. Free-air gravity map for the Central Atlantic Ocean. Superimposed on the map are the isochrons after Müller et al. (2008) at 10   Ma intervals with the 0   Ma isochron at the mid-ocean ridge. Below the map are two fracture zone profiles (Kane and Ascension) derived from the fracture zone azimuths and isochrones east of the mid-oceanic ridge. The dashed red lines are a subjective attempt to track the linear segments of the azimuth-age plots representing smooth plate opening about an individual or slowly moving Euler pole. The red dots are ages of the unconformities identified within the basins of the West and Central African Rift System.

Inspection of Fig. 20.3 shows the Equatorial fracture zones, that is the St. Paul's fracture zone and those to its immediate south, have a strong gravity response along their entire length suggesting they have not been passive, but have been active subsequent to their original development (Mascle et al., 1988). Farther south in the northern part of the South Atlantic, the Ascension fracture zone (Fig. 20.3) reflects relative motion between the South America–Southern Africa plates (defined in the lower profile shown in Fig. 20.5). Here, the fracture zones appear to be passive and the amplitude of the free-air anomaly decreases eastwards away from the ridge axis until it is cut by the SW–NE trending Cameroon volcanic seamount chain. To the east of the volcanic chain, the fracture zones are difficult to trace due to the Cameroon volcanic line (CVL) acting as a barrier, with sedimentation being mainly restricted to the east. Unlike the Kane fracture zone, the curvature of the Ascension fractures zone is considerably more subtle, indicating the South America–Southern Africa plate pair has not undergone large changes in relative plate motion during its development. This is clearly seen in the time-azimuth profile plot of Fig. 20.5. These findings will be shown in Evolution of the WCARS section to provide evidence of the close plate tectonic linkage with the tectonic evolution of the WCARS.

Physical crustal tectonic linkage between the oceanic and continental domains is essential if changes in direction of plate momentum are to be propagated into Africa. Here, gravity and magnetic data play a significant role in the Gulf of Guinea, at identifying the on- and offshore tectonic linkage beneath the Niger Delta. Here, the Chain and Charcot fracture zones of the Equatorial set of fracture zones, identified by the white arrows in Fig. 20.6, cut the continent–ocean boundary beneath the Niger Delta before becoming part of the Benue Trough rift structures. Since the Niger Delta is located close to and south of the magnetic equator (~10°N), the total magnetic intensity anomalies tend to follow the boundaries of the SW–NE trending fracture zones, rather than imaging the geomagnetic reversal pattern (Fig. 20.6B). The smooth spectral response of the magnetic data also indicates that deep oceanic crust existing beneath a major portion of the Niger Delta while farther onshore (NE from arrowhead), the magnetic field is responding to shorter-wavelength anomalies coming from shallow continental volcanic and basement structures.

Figure 20.6. (A) The free-air (offshore) and Bouguer (onshore) gravity field over the Niger Delta region of Nigeria. (B) The total magnetic intensity (TMI) field response over the Niger Delta. The white arrows identify the Chain and Charcot fracture zones and the dashed white line the approximate position of the continent–ocean boundary.

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https://www.sciencedirect.com/science/article/pii/B9780444641342000183