Plate Motion History Research Articles

Mantle circulation models with variational data assimilation: inferring past mantle flow and structure from plate motion histories and seismic tomography

SUMMARY Mantle convection models require an initial condition some time in the past. Because this initial condition is unknown for Earth, we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent Mesozoic and Cenozoic geological history of our planet. Here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation. We derive the generalized inverse of mantle convection and explore the initial condition problem in high-resolution, 3-D spherical mantle circulation models for a time period of 100 Myr, roughly comparable to half a mantle overturn. We present a synthetic modelling experiment to demonstrate that mid-Cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling, assuming present-day mantle structure is well-known, even if an initial first guess assumption about the mid-Cretaceous mantle involved only a simple 1-D radial temperature profile. We also demonstrate that convecting present-day mantle structure back in time by reversing the time-stepping of the energy equation is insufficient to model the mantle structure of the past. The difficulty arises, because such backward convection calculations ignore thermal diffusion effects, and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time. Inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle.

Mantle circulation models with variational data assimilation: inferring past mantle flow and structure from plate motion histories and seismic tomography

SUMMARY Mantle convection models require an initial condition some time in the past. Because this initial condition is unknown for Earth, we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent Mesozoic and Cenozoic geological history of our planet. Here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation. We derive the generalized inverse of mantle convection and explore the initial condition problem in high-resolution, 3-D spherical mantle circulation models for a time period of 100 Myr, roughly comparable to half a mantle overturn. We present a synthetic modelling experiment to demonstrate that mid-Cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling, assuming present-day mantle structure is well-known, even if an initial first guess assumption about the mid-Cretaceous mantle involved only a simple 1-D radial temperature profile. We also demonstrate that convecting present-day mantle structure back in time by reversing the time-stepping of the energy equation is insufficient to model the mantle structure of the past. The difficulty arises, because such backward convection calculations ignore thermal diffusion effects, and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time. Inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle.

Mantle-circulation models with sequential data assimilation: inferring present-day mantle structure from plate-motion histories.

Data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow. Such an approach is essential in mantle circulation models, where we seek to constrain an unknown initial condition some time in the past, and thus cannot hope to use first-principles convection calculations to infer the flow history of the mantle. One of the most important observables for mantle-flow history comes from models of Mesozoic and Cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle-buoyancy forces due to subducted oceanic slabs. Here we present five mantle circulation models with an assimilated plate-motion history spanning the past 120 Myr, a time period for which reliable plate-motion reconstructions are available. All models agree well with upper- and mid-mantle heterogeneity imaged by seismic tomography. A simple standard model of whole-mantle convection, including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation, reveals downwellings related to Farallon and Tethys subduction. Adding 35% bottom heating from the core has the predictable effect of producing prominent high-temperature anomalies and a strong thermal boundary layer at the base of the mantle. Significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep, factor 100, viscosity rise from the upper to the lower mantle results in substantial transition-zone heterogeneity, enhanced by the effects of trench migration implicit in the assimilated plate-motion history. An expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km, which is influenced by Jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the Cretaceous. This result implies that sequential assimilation of past plate-motion models is ineffective in studying the temporal evolution of core-mantle-boundary heterogeneity, and that a method for extrapolating present-day information backwards in time is required. For short time periods (of the order of perhaps a few tens of Myr) such a method exists in the form of crude 'backward' convection calculations. For longer time periods (of the order of a mantle overturn), a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model.

Tomographic images of a mantle circulation model

Sampling convection in the Earth's mantle by seismic tomography is difficult as evidenced by the uneven distribution of seismic stations and events with some regions being well imaged compared to others. Here we quantitatively explore tomographic filtering on Earth structure by tracing ISC P‐body‐wave data through a computer simulation of mantle circulation which accounts for internal heating of the mantle by radioactive decay, heatflux from the core, a depthwise increase in viscosity, and plate motion history of the past 120 million years. The travel time residuals are inverted by solving jointly for structure and hypocentral parameters with explicit damping and smoothing. We recover the Farallon and Tethys slabs as well as some low velocity anomalies associated with hot upwelling flow suggesting that tomographic filtering is probably minor in areas of high ray density.

Time Scales and Heterogeneous Structure in Geodynamic Earth Models

Computer models of mantle convection constrained by the history of Cenozoic and Mesozoic plate motions explain some deep-mantle structural heterogeneity imaged by seismic tomography, especially those related to subduction. They also reveal a 150-million-year time scale for generating thermal heterogeneity in the mantle, comparable to the record of plate motion reconstructions, so that the problem of unknown initial conditions can be overcome. The pattern of lowermost mantle structure at the core-mantle boundary is controlled by subduction history, although seismic tomography reveals intense large-scale hot (low-velocity) upwelling features not explicitly predicted by the models.

The dynamics of Cenozoic and Mesozoic plate motions

Our understanding of the dynamics of plate motions is based almost entirely upon modeling of present‐day plate motions. A fuller understanding, however, can be derived from consideration of the history of plate motions. Here we investigate the kinematics of the last 120 Myr of plate motions and the dynamics of Cenozoic motions, paying special attention to changes in the character of plate motions and plate‐driving forces. We analyze the partitioning of the observed surface velocity field into toroidal (transform/spin) and poloidal (spreading/subduction) motions. The present‐day field is not equipartitioned in poloidal and toroidal components; toroidal motions account for only one third of the total. The toroidal/poloidal ratio has changed substantially in the last 120 Myr with poloidal motion decreasing significantly after 43 Ma while toroidal motion remains essentially constant; this result is not explained by changes in plate geometry alone. We develop a self‐consistent model of plate motions by (1) constructing a straightforward model of mantle density heterogeneity based largely upon subduction history and then (2) calculating the induced plate motions for each stage of the Cenozoic. The “slab” heterogeneity model compares rather well with seismic heterogeneity models, especially away from the thermochemical boundary layers near the surface and core‐mantle boundary. The slab model predicts the observed geoid extremely well, although comparison between predicted and observed dynamic topography is ambiguous. The midmantle heterogeneities that explain much of the observed seismic heterogeneity and geoid are derived largely from late Mesozoic and early Cenozoic subduction, when subduction rates were much higher than they are at present. The plate motion model itself successfully predicts Cenozoic plate motions (global correlations of 0.7–0.9) for mantle viscosity structures that are consistent with a variety of geophysical studies. We conclude that the main plate‐driving forces come from subducted slabs (>90%), with forces due to lithospheric effects (e.g., oceanic plate thickening) providing a very minor component (<10%). For whole mantle convection, most of the slab buoyancy forces are derived from lower mantle slabs. Unfortunately, we cannot reproduce the toroidal/poloidal partitioning ratios observed for the Cenozoic, nor do our models explain apparently sudden plate motion changes that define stage boundaries. The most conspicuous failure is our inability to reproduce the westward jerk of the Pacific plate at 43 Ma implied by the great bend in the Hawaiian‐Emperor seamount chain. Our model permits an interesting test of the hypothesis that the collision of India with Asia may have caused the Hawaiian‐Emperor bend. However, we find that this collision has no effect on the motion of the Pacific plate, implying that important plate boundary effects are missing in our models. Future progress in understanding global plate motions requires (1) more complete plate reconstruction information, including, especially, uncertainty estimates for past plate boundaries, (2) better treatment of plate boundary fault mechanics in plate motion models, (3) application of numerical convection models, constrained by global plate motion histories, to replace ad hoc mantle heterogeneity models, (4) better calibration of these heterogeneity models with seismic heterogeneity constraints, and (5) more comprehensive comparison of global plate/mantle dynamics models with geologic data, especially indicators of intraplate stress and strain, and constraints on dynamic topography derived from the stratigraphic record of sea level change.

Paleomagnetic evidence for motion of the Hawaiian hotspot during formation of the Emperor seamounts

The bend in the Hawaiian-Emperor chain is the best example of a change in plate motion recorded in a fixed-hotspot frame of reference. Alternatively, the bend might record primarily differences in motion of the Hawaiian hotspot relative to the Pacific lithosphere. New paleomagnetic data from the Emperor chain support the latter view. Althouth the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain, the best available paleomagnetic data suggest Pacific hotspots may have moved at rates comparable to those of lithospheric plates (> 30 mm yr −1) in late Cretaceous to early Tertiary times (81-43 Ma). If correct, this requires a major change in how we view mantle dynamics and the history of plate motions. In the early to mid-Cretaceous (128-95 Ma), hotspots in the Atlantic moved at similar rates. These episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion, such as the past 5 m.y.

Hot tracks

Continental plates don't leave tracks—or do they? As hot material wells up from deep in the Earth's mantle at sites called hotspots, seamounts are produced. Chains of seamounts, like those running northwestward from the Hawaiian islands, reveal the direction and rate of tectonic plate movement— as long as their age can be determined.Paul Wessel and Loren Kronke of the University of Hawaii recently described a simple and labor‐saving technique for locating extinct hotspots and tracing the history of plate motions without having to know how old the seamounts are. By using the geometric relationship linking hotspots to the seamounts that they produce, the researchers traced the motion of tectonic plates without identifying the age of seamounts. Using this method,

Conference on the magnetization of the oceanic crust steers future research

Because marine magnetic anomalies arise from the combination of seafloor spreading and geomagnetic polarity reversals, they delineate a history of global plate motions and geomagnetic field behavior. Thirty years ago, interpretation of sea surface magnetometer profiles led to the plate tectonics revolution. Recent developments in high resolution magnetic studies are similarly changing our view of the structure and evolution of oceanic crust and beginning to answer basic questions concerning geomagnetic field behavior.In response to these developments, the Conference on the Magnetization of Oceanic Crust was held September 21–24,1996, on Orcas Island in Washington State. Forty‐seven scientists representing 20 institutions in seven countries attended the conference, which was funded by the National Science Foundation, the Ridge Interdisciplinary Global Experiment (RIDGE), and the United States Science Advisory Committee (USSAC).

Hot-spotting in the Pacific

Combining the geometry of plate tectonics, and the computational advantage of digital sea-floor topography, provides a new method for locating plumes rising from the Earth's deep mantle and for tracing the history of plate motions.

Antarctic Tectonics: Constraints From an ERS-1 Satellite Marine Gravity Field

A high-resolution gravity field of poorly charted and ice-covered ocean near West Antarctica, from the Ross Sea east to the Weddell Sea, has been derived with the use of satellite altimetry, including ERS-1 geodetic phase, wave-form data. This gravity field reveals regional tectonic fabric, such as gravity lineations, which are the expression of fracture zones left by early (65 to 83 million years ago) Pacific-Antarctic sea-floor spreading that separated the Campbell Plateau and New Zealand continent from West Antarctica. These lineations constrain plate motion history and confirm the hypothesis that Antarctica behaved as two distinct plates, separated from each other by an extensional Bellingshausen plate boundary active in the Amundsen Sea before about 61 million years ago.

Cordillera formation, mantle dynamics, and the Wilson cycle

Research Article| June 01, 1996 Cordillera formation, mantle dynamics, and the Wilson cycle R. M. Russo; R. M. Russo 1Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, D.C. 20015 Search for other works by this author on: GSW Google Scholar P. G. Silver P. G. Silver 1Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, D.C. 20015 Search for other works by this author on: GSW Google Scholar Geology (1996) 24 (6): 511–514. https://doi.org/10.1130/0091-7613(1996)024<0511:CFMDAT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. M. Russo, P. G. Silver; Cordillera formation, mantle dynamics, and the Wilson cycle. Geology 1996;; 24 (6): 511–514. doi: https://doi.org/10.1130/0091-7613(1996)024<0511:CFMDAT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We propose that the Andes Cordillera formed by a “collision” between the trenchward-moving South American plate and the Nazca slab and subslab mantle. Resistance to trenchward motion, in the form of horizontal compressive normal stress, deforms South America's leading edge. The advancing continent also perturbs the subslab mantle flow field as mantle is displaced. This flow field, detected via shear-wave splitting, includes trench-parallel flow away from a central stagnation point and corner flow around the northern and southern ends of the continent. The corresponding spatial variation in applied stress accounts for the large-scale characteristics of Andean deformation: buckling at the stagnation point (maximum normal stress) forming the Central Andes and Bolivian orocline, and corner flow associated with the eastward motion of the Caribbean and Scotia plates. The high stresses required to form and maintain the Andes imply that South America's trenchward motion is primarily driven by deep mantle flow coupled to its base; no other driving force of sufficient magnitude is available. Similarities between North and South American plate motion history and large-scale Cordillera structure indicate that the western North American Laramide Rockies formed in a similar manner and that the North American plate is also driven westward by deep mantle flow. We infer that Atlantic spreading itself is similarly driven. By contrast, slab rollback in a closing ocean basin opens marginal basins at convergent margins, as in the western Pacific. We suggest that there is a causal link between the opening and closing of ocean basins and the formation of cordilleras and marginal basins. Cordillera formation occurs at the leading continental margin during the spreading phase of the Wilson cycle, whereas marginal basins form during the closing phase. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geophysics of the Pitman Fracture Zone and Pacific-Antarctic Plate Motions During the Cenozoic

Multibeam bathymetry and magnetometer data from the Pitman fracture zone (FZ) permit construction of a plate motion history for the South Pacific over the past 65 million years. Reconstructions show that motion between the Antarctic and Bellingshausen plates was smaller than previously hypothesized and ended earlier, at chron C27 (61 million years ago). The fixed hot-spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early Tertiary, either between East Antarctica and West Antarctica or between the North and South Pacific. A plate reorganization at chron C27 initiated the Pitman FZ and may have been responsible for the other right-stepping fracture zones along the ridge. An abrupt (8°) clockwise rotation in the abyssal hill fabric along the Pitman flowline near the young end of chron C3a (5.9 million years ago) dates the major change in Pacific-Antarctic relative motion in the late Neogene.

Constraints on Arabian Plate motion and extensional history of the Red Sea : Hempton, M.R., 1987. Tectonics, 6(6):687–705

Constraints on Arabian Plate motion and extensional history of the Red Sea : Hempton, M.R., 1987. Tectonics, 6(6):687–705

Constraints on Arabian Plate motion and extensional history of the Red Sea

Analysis of the geologic and geophysical data from the western and northern margin of the Arabian plate combined with data from the Red Sea/Gulf of Aden leads to an interpretation of Arabian plate motion that supports an episodic extensional history in the Red Sea and Gulf of Aden. Phase 1 extension began in medial to late Eocene and was caused by the NW propagation of the reoriented Central Indian Ridge spreading center. It stretched and thinned continental crust along numerous wrench and normal faults that reactivated preexisting structures. First‐phase extension stopped when the Arabian plate terminally sutured to Eurasia in the medial Miocene and halted northward movement of Arabia relative to Africa. The combined African/Arabian plates moved north more slowly, constricting and thickening continental crust in the Bitlis/Zagros sutures while the Red Sea remained technically quiescent and accumulated salt. By the early Pliocene, continued northward movement was accommodated by lateral extrusion of large continental wedges north of the Bitlis/Zagros sutures. This allowed Arabia to move northward faster than Africa and reopen the Red Sea where phase 2 extension was expressed by seafloor spreading. Extension within the Red Sea/Gulf of Aden rift system has been controlled by the interactions and effects of two factors: (1) convergent processes between the north margin of the Arabian plate and the Eurasian plate to the north and (2) northwest propagation of extension associated with the medial Eocene reorganization of the Central Indian spreading center.

Introduction to the Special Issue on Correlations Between Plate Motions and Cordilleran Tectonics

This special issue contains papers that grew out of the American Geophysical Union Symposium “Models for motion of North America and plates of the Pacific basin, and the tectonic history of the western Cordillera—a search for correlations.” This symposium was held in San Francisco on December 13 and 14, 1982. Forty‐two papers were presented on such diverse topics as plate motion and microplate accretion histories, paleomagnetism of Cordilleran terranes and their displacement histories, the volcanic and plutonic history of the western Cordillera, and the timing of important geological episodes of regional extent (onset of extensional tectonics, onset and termination of episodes of strike slip faulting, etc.). Mechanisms of deformation within the Cordilleran continental margin were also discussed. The reader is referred to Eos Trans. AGU (vol. 63, no. 45, 1982) for the full set of abstracts.

Geodynamic Peaks in Alpinotype Orogenies and Changes in Ocean-Floor Spreading During Late Jurassic-Late Tertiary Time

Modern data indicate that peaks of tectonic displacement alternate with relative tectonic quiescence and that dates of changes in plate motion coincide with times of orogeny. These relations seem to be substantiated by contemporaneity of major discontinuities in seafloor spreading and widespread angular unconformities related to mountain building. Examples are the spreading discontinuities 112, 77, 40, and 10 m.y. ago which are coeval with intense orogenic phases. They are turning points in geotectonic development of the earth--major orogenies occur contemporaneously with first-order readjustments in the plate-tectonic pattern and are reflected in sea-level changes. Thus a close kinematic relation of the processes is suggested. Evidently, orogenic paroxysms are geneticall associated with episodic reorganizations in direction and rate of plate motion. Alpinotype mountain building occurred along mobile continental margins of plates as soon as these were affected by intense compressive orogenic stress caused by a change to convergent interplate motions. Thus, phases of geodynamic activity become apparent in the history of lithospheric plate motions. The specific timing of these phases is an essential geologic parameter and base for further reproduction of plate-tectonic evolution in time and space.

Mesozoic magnetic lineations, the magnetic quiet zone, and sea floor spreading in the northwest Atlantic

The magnetic basement topography and associated magnetic anomaly lineation pattern in an area north of the New England seamounts lying between anomaly 31 and the continental margin were examined in light of the Mesozoic sea floor spreading history of the northwest Atlantic. Within the magnetic quiet zone the crust is typically oceanic as far landward as the slope anomaly and exhibits a normally polarized magnetism in the inner quiet zone. Magnetic lineations have been correlated with the Keathley sequence near Bermuda, and, based upon best estimates of the reversal time scale, cross-strike spreading rates are found to be about 1.7 cm yr−1 from −172 m.y. to −136 m.y. and between 0.9 and 1.0 cm yr−1 from −136 m.y. to −72 m.y. Landward of the Keathley sequence, magnetic anomalies decrease in amplitude, although the intensity of magnetization of the normally polarized crust exhibits average values. Magnetic observations within a large area of the quiet zone surveyed in detail can be fitted to the basement topography if a Jurassic pole position of normal polarity is assumed. However, three zones trending subparallel to the Keathley lineations require a weak reversely polarized magnetism. These low values of remanent magnetization are attributed to either viscous remanence or contamination of the original crust by widespread volcanic flows or intrusives during periods of predominantly normal magnetic polarity. The normal and reversed polarity sequence resulting from this study is correlated with the sequence obtained from paleomagnetic measurements on land. Trends of isochrons within this relatively limited oceanic area necessitate an alteration in the shape of the ridge axis. A difference in spreading rate and direction is obvious on either side of the New England seamount chain and is discussed with respect to the history of early plate motions in the northwest Atlantic.

Evidence for sea-floor spreading in the Cayman Trough

New seismic reflection and bathymetric data reveal that the rough topography in the central quarter of the Cayman Trough consists predominantly of north-south oriented ridges and valleys. Ridges with a relief of 200–2,000 m lie on either side of a north-south trending V-shaped valley which extends from the north to the south wall of the trough. We suggest that this valley and the flanking ridges represent an axis of sea-floor spreading for which we propose the name Mid-Cayman Rise. Although overall depths are greater for the Mid-Cayman Rise (rift valley, 5,500–6,000 m; flanking ridges, 3,500–4,000 m) than are typical of the mid-oceanic rise, striking similarities in morphology and sediments are noted. Both rises exhibit an axial rift valley, rough sediment-free basement near the axis, increasing depth and increasing burial by sediments with distance from the axial rift, and symmetry in basement morphology about the rift. Earthquake epicenters are clustered near the ends of the rift, where it intersects linear deeps along the north and south walls of the trough. The rift in the Cayman Trough is approximately equidistant from the two ends of the trough, suggesting that the Mid-Cayman Rise may have been spreading symmetrically for a long time, in response to relative motion between the Americas and Caribbean plates. Our interpretations impose important constraints on tectonic models for the Cenozoic history of plate motions in the Caribbean region.