Access point frame of reference (2023)

Access points or no access points?

Where do hotspots come from? How deep is your source? Do they offer a fixed frame of reference? The hot spot tracks were used to calculate the motion of the plate relative to the mantle. For this, it is essential to know if the hotspots are i) fixed in relation to the mantle, ii) fixed to each other and iii) at what depth they originate. However, hotspots are often used uncritically and with little regard for their true nature. Volcanic footprints on Earth's surface may be the result of clouds within the plate (z.B.Hawaii), subduction plate retrogradation, reverse arc extension migration, longitudinal crack propagation (z.B.East Africa) or the spread of a transformation disturbance with a transtensive component (Chagos Ridge? see alsodecanbook page). All these volcanic footprints can have different depths than their mantle sources and must be differentiated (Figure 1). Plate boundaries, by definition, move relative to each other and relative to the underlying mantle. Therefore, a hotspot located on a plate boundary cannot be used for a fixed hotspot reference frame.

Figure 1. The main volcanic chains on the Earth's surface can have different origins and depths. The red arrows indicate the direction of volcanic migration over time. Filled triangles represent recent volcanic products. Volcanic trails originating from summits can be wet patches (Bonatti himself, 1990) and arise from a fluid-rich asthenosphere. Hot spots located at plate boundaries are not fixed by definition, as both crests and trenches move relative to each other and relative to the mantle. Pacific hotspots are located within the plate, regardless of the depth of their source, and are virtually the only ones that can be considered reliable for a hotspot benchmark.

Deep springs or shallow rises? Solid or not solid? A growing body of evidence suggests that hotspots are mostly flat features (good luck, 1900;Smith e Lewis, 1999;anderson, 2000;dirtier, 2002;Fouger and ai., 2005). For example, Atlantic hot spots can be interpreted as wet spots (Figure 2) rather than hot lines, as suggested bygood luck(nineteen ninety). A richer asthenosphere source of fluids that lower the melting point may be responsible for the overproduction of magma. Propagating cracks (hot lines, etc.) are shallow features that are not fixed relative to the deeper mantle. The only hotspots relevant to a fixed hotspot frame of reference are those inside the plates. For example,Norton(2000) grouped the hotspots into three main families, each with very little internal relative movement (the Pacific, Indo-Atlantic, and Icelandic (single hotspot) families). It concluded that a global hotspot framework is insufficient as Pacific hotspots move relative to Indo-Atlantic and Icelandic hotspots. As the Indo-Atlantic and Icelandic hotspots are on ridges, they do not meet the strength requirement.Norton(2000), the Pacific hotspots were relatively well fixed to each other over the last 80 million. As a result, detecting volcanic footprints that can be used for the fixed hotspot frame of reference leaves a very limited number of hotspots, with only those in the Pacific meeting the requirements. Decoupling in the asthenosphere The origin of Pacific magmatism within the plate is also unclear, and the depth of the source and the mechanism of its melting are still debated (Fouger and ai.2005). Since the Pacific is the fastest moving plate, shear heating along the basal divide has been proposed as a possible mechanism to generate localized hotspot bands (Figure 3). Areas of higher than normal viscosity in the asthenospheric divide should generate more shear heating. Figure 2 Hypothetical reconstruction of migratory volcanic mountain ranges in the South Atlantic. An abnormally water-rich asthenospheric mantle, or wet line, oblique to the absolute movement of the African plate and migration of the Mid-Atlantic Ridge (MAR) could produce a volcanic trail that narrows to the southwest. Similar mirror-shaped orbits (NW trend, SE spread) may form in the South American plate. This model could explain why volcanic tracks of progressive age tend to transform faults.

Figure 3 The trajectory of the Hawaiian volcano indicates a decoupling between the magma source and the lithosphere moving relative to WNW. If the source is below the asthenosphere (for example, in the subsanshenosphere, option 1), the trail records all shear between the lithosphere and the mantle. In the case of an asthenospheric source for the Hawaiian hotspot (option 2), the volcanic trace does not capture all the shear between the lithosphere and the sub-sustainen mantle, as part of it acts below the source (deep missing shear). Furthermore, greater decoupling implies greater shear heating, which may explain the diffuse and punctual intraplate magmatism of the Pacific (afterDoglioni and outros, 2005). [See toohawaiiYjacket temperaturePages.]

Kennedy al.(2002) showed how mantle xenoliths record shear possibly located at the lithosphere-asthenosphere interface. This supports the notion of upper mantle flow and decoupling at the base of the lithosphere, which is also supported by seismic anisotropy (russian and silver, 1996;Doglioni et al., 1999;Bokelmann e Silber, 2000). The fastest board in the world in the hotspot framework(dh, Pacific) is also affected by the more widespread intraplate magmatism. It should be noted that the fast moving Pacific plate overlies the asthenosphere with the lowest average viscosity (5 x 10¹⁷No;Chick al., 1998) and possibly the least depleted mantle and therefore most subject to melting. Due to the melting properties of low-carbon + hydrogen peridotite (the Lherzolite–(C+H+O) system), the asthenosphere has already partially melted (B. Schubert et ai., 2001) and at a temperature of about 1430°C (B. Verde e Falloon, 1998;Green and others., 2001;See toojacket temperaturebook page). A temperature increase of a few tens of degrees increases the degree of melting, and this melt migrates to the surface in a deformable material. We postulate that locally the viscosity of the asthenosphere may also increase (z.B., to 10¹⁹Pa s) due to compositional anisotropy. Shear stress can be unevenly distributed in such a heterogeneous material, and hence higher shear heating (To look, 1973) can develop locally and produce punctual magmatism (Figure 4).

Figure 4. When the viscosity of the asthenosphere is locally higher than normal, the shear stress and shear heating are also higher, resulting in an increase in the temperature of the asthenosphere. Fluctuations in lithospheric velocity (100 or 200 mm per year^-1) and local increases in viscosity (4 x 10¹⁹o 10²⁰Pa·s) can determine different temperature excesses between 12°K and 120°K. Higher overtemperatures can lead to additional melting and possibly increased magmatism within the plate (postDoglioni and outros, 2005).

The frame of reference without net rotation

Das Modell NUVEL 1 (DeMets et al., 1990) and the Space Geodesy ITRF2000 and NASA databases (Heflin and ai., 2005) provide information about plate movements. They are based on the artificially imposed assumption of a reference frame without net rotation (NNR),dh, The plates move relative to a fixed center of the Earth, and the lithosphere does not move relative to the underlying mantle, or the sum of their movements is zero. However, we know that the lithosphere moves relative to the subsatenospheric mantle, and this is not only suggested by Hawaiian and similar volcanic trails, but also kinematically required by the relative migration of plate margins. The ITRF2000 reference structure (B. Heflin et ai., 2005) is excellent for describing the relative motions of plates and is also considered an absolute reference system (relative to the GPS satellite constellation and the Earth's center of mass). However, when magmatic sources are included in the kinematic analysis, the motion of the lithosphere relative to the mantle must be considered in an "absolute" plate motion analysis and the NNR must be abandoned.

drift west of the lithosphere

When plate movements are measured under the hotspot, the lithosphere shows a net "westward" drift (live flow, 1971;O'Connell et al., 1991;Ricardo e cols., 1991). This "western" drift also persists when plate movements are calculated relative to Antarctica (the pigeon, 1968;Knopoff e Leeds, 1972), which lies on a plate with almost no subduction and is therefore often considered stationary or slow moving relative to the underlying mantle. However, most of the hotspots used are not fixed nor do they represent a fixed reference system, since they are located at the edges of the plates, e.g.z.B.,Galapagos, Osterinsel,island, and Ascension), transformation errors (z.B., Réunion) or continental getaways (z.B., Afar), all features that move relative to each other and relative to the cloak. Although,griff y gordon(2002) calculated a net rotation of the lithosphere of 49 mm per year^-1(0.44 ± 0.11 graus Myr^-1around a pole of 56°S, 70°E). The WNW movement of the Pacific Plate relative to the underlying mantle is inferred from the Hawaiian footprints and other important points within the plate (z.B., Marquise, Society, Pitcairn,samoanoand MacDonald), which averages about 103-118 mm per year^-1. They also move along the same trend (290°-300°, WNW) and are therefore the only critical points that appear to be coherently fixed relative to each other, providing an apparently reliable frame of reference for the absolute movements of the boards.

Assuming that the ESE velocity (110-120°) of the Pacific lithosphere is less than that of the underlying subasthenospheric mantle Vm (Vm > Vl), the relative motion corresponding to the WNW delay of the lithosphere is Vm - Vl = 103 mm yr^-1. However, if shear is distributed along the asthenosphere channel (Figure 3) and the Hawaiian melting point is within the asthenosphere (Figure 3, Option 2) and not in the lower mantle (Figure 3, Option 1), then the shear is only upward if the hotspot recorded on the hotspot trace, the total displacement between lithosphere and mesosphere is greater than suggested by the surface volcanic chain. Therefore, if the location of Hawaiian magmatism is in or on top of the asthenosphere, another deep shear component would have to be missing to increase the overall relative velocity. This higher velocity has two fundamental consequences: 1) it increases the global westward drift of the lithosphere and 2) it increases the shear heat released in the asthenosphere. When the source of the Pacific hotspots is in the middle of the asthenosphere, half of the relative motion between the lithosphere and the sub-asthenospheric mantle is not recorded, meaning that the total relative change would be about 200 mm per year.^-1. At this rate, the net "westerly" rotation of the lithosphere increases to about 9 cm/year. In this frame, no plate moves “eastwards” relative to the mantle (Figure 5, option 2).

Figure 5. Simplified kinematic relationship of the Pacific-Nazca-South American plates. Relative motion vectors (top) according to Heflin et al. (2005). Option 1 shows the "absolute" movements relative to the Hawaii hotspot, which moves about 103 mm per year.^-1(Gripp and Gordon, 2002). Option 2 (below) is the case where the source of the hotspot is in the asthenosphere and the relative movement between the Pacific plate and the sub-basthanosphere mantle is assumed to be ≥ 200 mm per year^-1(see Figure 1). In this last configuration, all three plates move "West" relative to the mantle.

crazy tracks

There is evidence that the rate of propagation of Pacific "hot spots" or seamount footprints varied over time, including with forward and backward bounces and oblique propagation relative to the "absolute" motion of the plates. , which casts doubt on the absolute movement of the calculated plates under the critical point. of reference and the nature of the magmatism itself (deep plume or more superficial uplifts produced by fissures or boudins of the lithosphere,winter and sand pit, 1987;Sandwell and others., 1995;lynching, 1999;Natland e Winterer,2003) that originated in a mantle with heterogeneous composition and no detectable thermal anomalies in hot-spot magmatism compared to normal mid-ocean ridges (See toojacket temperaturebook page).Janney and ai.(2000) described the speed ofbooksVolcanic mountain range (interpreted as a hot spot trace or leaking fracture zone) and located in the east-central Pacific, between 5 and 12 Ma of approximately 200-300 mm yr^-1. They also inferred a surface mantle source for the Pacific hotspots based on their geochemical characteristics.

These results are consistent with the anisotropy measured by shear wave division (russian and silver, 1994) and the low slope of the Andean Plate, which indicate a relative eastward flow of the mantle. A similar eastern mantle flow has been proposed for the North American Plate (prata e holt, 2002). The low slope of the Andean Plate has also been attributed to the age of the lithosphere under subduction. However, ocean age has been shown to be insufficient to explain the asymmetry between subduction zones that point "west" (steep and deep) and those that point "east" (low slope and shallow).Cruciani et al., 2005). In fact, geographic asymmetry persists even where the same lithosphere (oceanic or continental) undergoes subduction from both sides, as in the Mediterranean orogens (Doglioni et al., 1999).

Global tectonic asymmetry: Earth's rotation?

The geological and geophysical signatures of the subduction and rift zones show a global pattern, suggesting an "easterly" movement of the mantle relative to the lithosphere, independent of the hotspot frame of reference, to support an uneven origin. The plates move in a sinusoidal pattern (Figure 9), which is amply confirmed by plate kinematics derived from spatial geodesy (B. Heflin et al.,2005). Along the flowlines, westward-dipping subduction zones are steeper than those trending E or NE, and the associated orogens are characterized by lower structural and topographic uplift and back-arc basins, or by a higher structural and morphological elevation and without accentuated posterior. arch basins. any (Doglioni et al.,1999). The asymmetry is striking when comparing subduction zones in the western and eastern Pacific, and has generally been interpreted as being related to the age of the descending oceanic lithosphere.dh, older, colder, and denser in the west. However, these differences persist elsewhere, regardless of the age and composition of the descending lithosphere.z.B., in the Mediterranean Apennines and Carpathians of the Alps and Dinarides, or in the Banda and Sandwich Arcs, where even zero-age continental or oceanic lithosphere is oriented almost vertically along westward-facing subduction zones. The rift zones are also asymmetrical, with the eastern flanks about 100-300 m higher globally (Doglioni et al., 2003).

The westward drift of the lithosphere implies that the plates have a general sense of motion and are not moving randomly. If we accept this postulate, we conclude that the plates are moving “westwards” along the streamlines shown in Figure 9 at different speeds relative to the mantle. In this view, the plates are approximately separated from the mantle as a result of decoupling at their base. The degree of decoupling is mainly controlled by the thickness and viscosity of the asthenosphere. Lateral variations in the degree of decoupling can control the variable velocity of the overlying lithosphere (Figure 10). When a plate moves faster to the west compared to a neighboring plate to the east, the edge of the resulting plate can be stretched; When a plate moves faster to the west compared to an adjacent plate to the west, their common edge converges (Figure 10).

Figure 9. Combining the directions of absolute plate motions that we can infer from large-scale rift zones or convergent belts over the past 40 Ma, we observe a global coherent sinusoidal flow field along which plates appear to move at different rates. Relative speeds in geographic coordinate system (according toDoglioni, 1993).

Figure 10. Caricature illustrating that the plates (carts) move west along a common path (for example, the lines in Figure 9), but at different speeds, caused by the "westward" displacement of the lithosphere relative to the mantle. Differential velocities control the tectonic environment and result from different viscosities on the uncoupling surface, ie the asthenosphere. There is extension when the western plate moves more rapidly westward relative to the eastern plate, while convergence occurs when the eastern plate moves more rapidly westward relative to the western plate. When the car is "subducted" in the middle, the tectonic regime changes to extension because the car is moving faster westwards, for example.Doglioni, 1993).

The kinematic structure of the shallow Pacific hotspots (Figure 8) constrains plate movements because they are fully polarized westward relative to the deep mantle. This table provides a fundamental observation along the subduction zones that run E or NE. In fact, the plates of this framework tend to come out of the mantle, but are canceled out by the upper plates. This compromises plate traction as a mechanism to drive plate movements, since the plate does not sink into the mantle. From this point of view, the plates are rather passive elements (Figure 11).

The asymmetry of tectonic features on a global scale and the westward drift of the lithosphere support a rotational component to the origin of tectonic plates (Scopola and ai.2006). The westward drift of the lithosphere may be the result of three combined processes: (1) tidal twisting forces act on the lithosphere, creating a westward twist that slows the Earth's rotation; (2) the downward flow of denser material deep in the mantle and core slightly lowers the moment of inertia and accelerates Earth's rotation, only partially offsetting tidal drag; and (3) thin layers (3–30 km) of very low viscosity hydrate channels occur in the asthenosphere. It is believed that shear heating and mechanical fatigue self-perpetuate one or more of these channels, providing the necessary lithosphere decoupling zone (Scoppola and outros, 2006).

Figure 11. Drawing assuming a Pacific plate (A) moving at 16 cm/year. If plate movements are considered relative to the hotspot frame of reference, plates in E or NE subduction zones can move out of the mantle. This is clearly the case for Hellenic subduction and, in the shallow hot spot frame of reference, also for Andean subduction. This kinematic evidence of plates moving out of the mantle casts doubt on plate pull as the mechanism driving plate movement.