Seafloor Spreading Rates Research Articles

The Cretaceous opening of the South Atlantic Ocean

The separation of South America from Africa during the Cretaceous is poorly understood due to the long period of stable polarity of the geomagnetic field, the Cretaceous Normal Superchron (CNS, lasted between ∼121 and 83.6 Myr ago). We present a new identification of magnetic anomalies located within the southern South Atlantic magnetic quiet zones that have arisen due to past variations in the strength of the dipolar geomagnetic field. Using these anomalies, together with fracture zone locations, we calculate the first set of magnetic anomalies-based finite rotation parameters for South America and Africa during that period. The kinematic solutions are generally consistent with fracture zone traces and magnetic anomalies outside the area used to construct them. The rotations indicate that seafloor spreading rates increased steadily throughout most of the Cretaceous and decreased sharply at around 80 Myr ago. A change in plate motion took place in the middle of the superchron, roughly 100 Myr ago, around the time of the final breakup (i.e., separation of continental–oceanic boundary in the Equatorial Atlantic). Prominent misfit between the calculated synthetic flowlines (older than Anomaly Q1) and the fracture zones straddling the African Plate in the central South Atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion (<100 km) within South America, west of the Rio Grande fracture zone. This process has lasted until ∼92 Myr ago after which both Africa and South America (south of the equator) behaved rigidly. The clearing of the continental–oceanic boundaries within the Equatorial Atlantic Gateway was probably completed by ∼95 Myr ago. The clearing was followed by a progressive widening and deepening of the passageway, leading to the emergence of north–south flow of intermediate and deep-water which might have triggered the global cooling of bottom water and the end for the Cretaceous greenhouse period.

Cyclostratigraphy and orbital tuning of the terrestrial upper Santonian–Lower Danian in Songliao Basin, northeastern China

The Songke-1 north (SK-1n) borehole recovered a continuous, 1541.66 m Late Santonian–Early Danian terrestrial succession in Songliao Basin (SB), northeastern China. It provides a unique record for improving our understanding of continental paleoclimate and ecological system in Cretaceous greenhouse world. Here we use thorium (Th) logging data as a paleoenvironmental and paleoclimatic proxy to conduct a detailed cyclostratigraphic study on the SK-1n core. Power spectra, evolutionary fast Fourier transformation and wavelet analysis all reveal significant decameter- to meter-scale sedimentary cycles in the Nenjiang (K2n), Sifangtai (K2s) and Mingshui (K2m) formations. The ratios of cycle wavelengths in these stratigraphic units are ∼20:5:2:1, and are interpreted as Milankovitch cycles of 405 kyr and 100 kyr eccentricity, 38.4 kyr obliquity and 20 kyr precession cycles, respectively. An astronomical time scale (ATS) is established by tuning filtered 405 kyr eccentricity cycles to a target curve of the astronomical solution La2010d based on the magnetostratigraphic time framework of the SK-1n borehole. This ATS provides precise numerical ages for stratigraphic boundaries, biozones, geological and geophysical events, and serves as a basis for correlation of strata and events between marine and terrestrial systems. The Cretaceous/Paleogene (K/Pg), Campanian/Maastrichtian, Santonian/Campanian boundaries are estimated at core depths of 318 m, 752.8 m and 1751.1 m, respectively. A ∼3.8 myr-long hiatus between the Nenjiang (K2n) and Sifangtai (K2s) formations occurs from 76.1 to 79.9 million years ago. The ages and durations of magnetochrons C33r to C30n are precisely estimated and provide new constraints on the Late Cretaceous Geomagnetic Polarity Time Scale (GPTS) and South Atlantic sea-floor spreading rates.

New insights into geochemical behaviour in ancient marine carbonates (Upper Jurassic Ammonitico Rosso): Novel proxies for interpreting sea‐level dynamics and palaeoceanography

AbstractElemental concentrations in Phanerozoic sea water are known to fluctuate both in time and space. With regard to carbonates precipitated from marine fluids, elemental concentrations in the carbonate crystal lattice are affected by a complex array of equilibrium and non‐equilibrium as well as post‐depositional alteration processes. To assess the potential of carbonate elemental chemostratigraphy, seven Upper Jurassic sections were investigated along a proximal to distal transect across the south‐east Iberian palaeomargin. The aim was to explore stratigraphic and spatial variations in calcium, strontium, magnesium, iron and manganese elemental abundances. The epicontinental geochemical record is influenced by the combination of continental runoff and a significant diagenetic overprint. In contrast, the epioceanic geochemical record agrees with reconstructed open marine sea water values, reflecting a moderate degree of syn‐depositional to early marine pore water diagenesis. Establishing a fair degree of preservation of matrix micrite, a thorough statistical approach was applied and elemental associations tested for their environmental significance. Principal component and hierarchical cluster analyses revealed a persistent relation between carbonate magnesium, iron and strontium abundances. Processes related to early diagenetic nodulation in Ammonitico Rosso facies most probably account for the incorporation of these elements in the calcium carbonate lattice. The clear decoupling of carbonate manganese abundance with respect to the remaining elements is documented and related to high sea floor spreading rates and hydrothermal activity during the Late Jurassic. The investigation of potential time‐fluctuation of geochemical patterns was approached through variogram computation. The observed temporal behaviour is most likely to be forced by relative sea‐level dynamics, reflecting Late Jurassic palaeoceanographic conditions and potential planetary interactions. The data obtained in this study highlight the utility of elemental data from carbonate matrix micrites as geochemical proxies for studying the influence of remote trigger factors.

Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering.

A shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered--contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500-500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time.

Variations in axial magma lens properties along the East Pacific Rise (9°30′N–10°00′N) from swath 3‐D seismic imaging and 1‐D waveform inversion

AbstractWe use three‐dimensional multistreamer seismic reflection data to investigate variations in axial magma lens (AML) physical properties along the East Pacific Rise between 9°30′N and 10°00′N. Using partial‐offset stacks of P‐ and S‐converted waves reflecting off the top of the AML, we image four 2–4 km long melt‐rich sections spaced 5–10 km from each other. One‐dimensional waveform inversion indicates that the AML in a melt‐rich section is best modeled with a low Vp (2.95–3.23 km/s) and Vs (0.3–1.5 km/s), indicating &gt;70% melt fraction. In contrast, the AML in a melt‐poor section requires higher Vp (4.52–4.82 km/s) and Vs (2.0–3.0 km/s), which indicates &lt;40% melt fraction. The thicknesses of the AML are constrained to be 8–32 m and 8–120 m at the melt‐rich and ‐poor sites, respectively. Based on the AML melt‐mush segmentation imaged in the area around the 2005–2006 eruption, we infer that the main source of this eruption was a 5 km long section of the AML between 9°48′N and 51′N. The eruption drained most of the melt in this section of the AML, leaving behind a large fraction of connected crystals. We estimate that during the 2005–2006 eruption, a total magma volume of 9–83 × 106 m3 was extracted from the AML, with a maximum of 71 × 106 m3 left unerupted in the crust as dikes. From this, we conclude that an eruption of similar dimensions to the 2005–2006, one would be needed with a frequency of years to decades in order to sustain the long‐term average seafloor spreading rate at this location.

The solid Earth's influence on sea level

Because it lies at the intersection of Earth’s solid, liquid, and gaseous components, sea level links the dynamics of the fluid part of the planet with those of the solid part of the planet. Here, I review the past quarter century of sea-level research and show that the solid components of Earth exert a controlling influence on the amplitudes and patterns of sea-level change across time scales ranging from years to billions of years. On the shortest time scales (10 0 –10 2 yr), elastic deformation causes the ground surface to uplift instantaneously near deglaciating areas while the sea surface depresses due to diminished gravitational attraction. This produces spatial variations in rates of relative sea-level change (measured relative to the ground surface), with amplitudes of several millimeters per year. These sea-level “fingerprints” are characteristic of (and may help identify) the deglaciation source, and they can have significant societal importance because they will control rates of coastal inundation in the coming century. On time scales of 10 3 –10 5 yr, the solid Earth’s time-dependent viscous response to deglaciation also produces spatially varying patterns of relative sea-level change, with centimeters-per-year amplitude, that depend on the time-history of deglaciation. These variations, on average, cause net seafloor subsidence and therefore global sea-level drop. On time scales of 10 6 –10 8 yr, convection of Earth’s mantle also supports long-wavelength topographic relief that changes as continents migrate and mantle flow patterns evolve. This changing “dynamic topography” causes meters per millions of years of relative sea-level change, even along seemingly “stable” continental margins, which affects all stratigraphic records of Phanerozoic sea level. Nevertheless, several such records indicate sea-level drop of ∼230 m since a mid-Cretaceous highstand, when continental transgressions were occurring worldwide. This global drop results from several factors that combine to expand the “container” volume of the ocean basins. Most importantly, ridge volume decrease since the mid-Cretaceous, caused by an ∼50% slowdown in seafloor spreading rate documented by tectonic reconstructions, explains ∼250 m of sea-level fall. These tectonic changes have been accompanied by a decline in the volume of volcanic edifices on Pacific seafloor, continental convergence above the former Tethys Ocean, and the onset of glaciation, which dropped sea level by ∼40, ∼20, and ∼60 m, respectively. These drops were approximately offset by an increase in the volume of Atlantic sediments and net seafloor uplift by dynamic topography, which each elevated sea level by ∼60 m. Across supercontinental cycles, expected variations in ridge volume, dynamic topography, and continental compression together roughly explain observed sea-level variations throughout Pangean assembly and dispersal. On the longest time scales (10 9 yr), sea level may change as ocean water is exchanged with reservoirs stored by hydrous minerals within the mantle interior. Mantle cooling during the past few billion years may have accelerated drainage down subduction zones and decreased degassing at mid-ocean ridges, causing enough sea-level drop to impact the Phanerozoic sea-level budget. For all time scales, future advances in the study of sea-level change will result from improved observations of lateral variations in sea-level change, and a better understanding of the solid Earth deformations that cause them.

Magnetic stripes of a transitional continental rift in Afar

Magnetic stripes parallel to mid-ocean ridges are one of the most signifi cant consequences of seafl oor spreading, and have played an essential role in the establishment of the plate tectonics theory and the determination of seafl oor spreading rates. Similar magnetic anomaly patterns have not been well documented subaerially in continental rifts transitioning into seafl oor spreading centers. Here, using high-resolution magnetic data that were collected across the Tendaho Graben in the Afar Depression, Ethiopia, we document one of the fi rst examples of subaerial magnetic lineations similar in pattern and amplitude to those that characterize seafl oor spreading centers. The ~50-km-wide graben is the southernmost structural and geomorphological expression of the on-land continuation of the Red Sea propagator, which is taken to represent the Arabian-Nubian plate boundary within Afar. The graben is bounded by northwest-trending border faults, with the footwalls dominated by ca. 1.7 Ma basalts and the downthrown blocks constituting progressively younger basalts toward the center of the graben, reaching ca. 35 ka. The Tendaho magnetic fi eld is characterized by an ~10-km-wide linear negative magnetic anomaly that corresponds to a normal-polarity zone that is fl anked by two parallel, ~20-km-wide linear positive magnetic anomalies of reversed polarity. This work shows that magnetic stripes can be developed in transitional continental rifts before the development of oceanic spreading centers. The common assumption that magnetic stripes can be used to date the onset of seafl oor spreading may need to be re-evaluated in light of the evidence provided here.

The eastern Jan Mayen microcontinent volcanic margin

The Jan Mayen microcontinent (JMMC) in the NE Atlantic was created through two Cenozoic rift episodes. Originally part of East Greenland, the JMMC rifted from NW Europe during the Early Eocene under extensive magmatism. The eastern margin is conjugate to the Møre-Faeroes volcanic margin. The western JMMC margin underwent prolonged extension before it finally separated from East Greenland during the Late Oligocene. Here we present the modelling by forward/inverse ray tracing of two wide-angle seismic profiles acquired using Ocean Bottom Seismometers, across the northern and the southern JMMC. Early Eocene breakup magmatism at the eastern JMMC produced an igneous thickness of 7-9 km in the north, and 12-14 km in the south. While the continent is clear in the north, the southern JMMC appears to be affected by later Icelandic magmatism. Reduced seismic velocity and increased crustal thickness are compatible with continental crust adjacent to the volcanic margin in the south, but the continental presence towards the Iceland shelf is less clear. Our magnetic track off the southern JMMC gives seafloor spreading rates comparable to that of the conjugate Møre Margin. Transition to ultraslow seafloor spreading occurs at ~43 Ma, indicating onset of major deformation of the JMMC. Calculating the igneous thickness-mean V P relationship at the eastern volcanic margin gives the typical positive correlation seen elsewhere on the NE Atlantic margins. The results indicate temperature driven breakup magmatism under passive mantle upwelling, with a maximum mantle temperature anomaly of ~50 °C in the north and 90-150 °C in the south.

A Wide Angle Ocean Bottom Seismometer Experiment Across Liyue Bank, the Southern Margin of the South China Sea

AbstractThe P wave velocity model of wide angle seismic profile‐OBS973‐2 that extends in NW‐SE direction for 369 km long from the northeastern Liyue Bank to the central sub‐basin is established by the trial‐and‐error 2D ray‐tracing method. This profile model is compared with that in the northern continental margin for the discussion of the conjugate relationship between the northern margin and the southern margin of the South China Sea (SCS). The velocity model consists of six layers, i.e. three sediments layers, upper crust, lower crust and upper mantle. The velocities of the three sediments layers are 1.8~2.0 km/s, 2.0~2.7 km/s and 3.5~4.0 km/s, respectively. The sediment layers are much thinner in general or even absent in some parts. There are some small volcanoes on the top of crust in Liyue Bank and P wave velocities increase downward from 5.5~6.4 km/s in the upper crust (9~10 km thick) to 6.6~7.1 km/s in the lower crust (11 km thick). In the transition zone and sea basin P wave velocities increase downward from 5.9~6.1 km/s in the upper curst (4~5 km thick) to 6.6~6.9 km/s in the lower crust (2~4 km thick). The crust in the transition zone and sea basin shows some tectonic effects of stretching and thinning. The velocity structure of the model indicates that its continental crust is a standard one but its oceanic upper crust is of relatively higher velocity than standard one. The seaward rising of the buried depth of Moho is 23 km in Liyue Bank and 8~12 km in the sea basin. The velocity beneath Moho is 8.0 km/s in the sea basin obviously smaller than 8.2 km/s in Liyue Bank.The comparison of profile OBS973‐2 with a previous profile OBS2006‐1 in the northern margin shows the much more similarity between them. This result and detailed discussion suggest a possibility of conjugate relationship of Liyue Bank with Zhongsha Massif, separated by the central seamount chain of the SCS. Furthermore, the full rates of seafloor spreading of the northwestern sub‐basin and the southwestern sub‐basin are calculated.

From pull-apart basins to ultraslow spreading: Results from the western Barents Sea Margin

This paper describes results from a geophysical study in the area between the ultraslow Knipovich Ridge and Bear Island, western Barents Sea. The objective was to map the crustal structure along a profile crossing a pull-apart rifted continental margin and oceanic crust generated by ultraslow spreading. The results are based on modeling of wide-angle seismic and gravity data, together with interpretation of multichannel reflection data. Our results show a two layered oceanic crust in the western part of the profile. The thickness of the oceanic crust is variable in the western 130 km, ranging from 3.5 to 5.5 km. East of km 130 the crustal thickness is relatively constant, with values close to the global average for oceanic crust. The oceanic crust is buried by a thick package of Cenozoic sedimentary rocks. The continent–ocean transition (COT) is placed in the interval 207–255 km, between unequivocal oceanic crust and the foot of the westernmost fault in the Hornsund Fault Zone. It is not possible to conclude whether this interval is oceanic crust or thinned and intruded continental crust, but we favor the latter interpretation, at least for the eastern part of the COT. Stretched continental crust is observed between Hornsund Fault Zone and the Knølegga Fault. Here the sedimentary rocks have high velocities and are interpreted to be mainly of Mesozoic and Late Paleozoic age. In this interval Moho depths increase abruptly from 15 km in the west to 27 km in the east. Crystalline basement velocities are observed close to the seafloor east of the Knølegga Fault. We suggest that continental breakup north of Greenland–Senja Fracture Zone occurred around 33 Ma, after a period of pull-apart tectonics. The spreading rate of the earliest seafloor spreading may have been higher than the present day spreading, creating thicker oceanic crust close to the COT.

Evaluation of the astronomical time scale for the Paleocene and earliest Eocene

The astronomical-tuned time scale is rapidly extended into the Paleogene but, due to the existence of an Eocene gap, different tuning options had to be presented for the Paleocene. These options differ both in number and tuning of ~ 405-kyr eccentricity related cycles and are only partially consistent with recalculated 40Ar/ 39Ar constraints for the Cretaceous/Paleogene (K/Pg) and Paleocene/Eocene (P/E) boundaries. In this paper, we evaluate the cyclostratigraphic interpretation of records from ODP Leg 198 and 208 sites, and the Zumaia section to solve the problem of the different tuning options. We found that the interval between the K/Pg boundary and the early Late Paleocene biotic event (ELPE) comprises 17 instead of 16 × ~ 405-kyr eccentricity related cycles as previously proposed, while the entire Paleocene contains 25 × ~ 405-kyr cycles. Starting from 40Ar/ 39Ar age constraints for the K/Pg boundary, a new tuning to 405-kyr eccentricity is presented for the Paleocene and earliest Eocene, which results in ages of ~ 66.0 and ~ 56.0 Ma for the K/Pg and P/E boundaries, respectively. This tuning introduces considerable differences in age for a number of nannofossil events at ODP Sites 1209 and 1262 in the interval between 61 and 63 Ma, but eliminates large and abrupt changes in the seafloor spreading rate. The tuning seems further consistent with recalculated 40Ar/ 39Ar ages for ash layer −17 of early Eocene age. However, despite this apparent consistency with existing radio-isotopic constraints, an alternative 405-kyr younger or, less likely, older tuning cannot be excluded at this stage.

True polar wander since 32 Ma B.P.: A paleomagnetic investigation of the skewness of magnetic anomaly 12r on the Pacific plate

We test the fixed hot spot and fixed spin axis hypotheses through a paleomagnetic investigation of the skewness of crossings of magnetic anomaly 12r (32 Ma B.P.) between the Galapagos and Clarion fracture zones on the Pacific plate. We focus on this region for three reasons. First, numerical experiments show that these crossings, of all those available from the Pacific plate, should contain the most information about the location of the 32 Ma B.P. paleomagnetic pole for the Pacific plate. Second, many of the available crossings are from vector aeromagnetic profiles, which have superior signal‐to‐noise ratios. Third, the rate of seafloor spreading recorded in these crossings exceeds the threshold (half rate of 50 mm a−1) above which anomalous skewness is negligible. The new pole (83.5°N, 44.6°E) has compact 95% confidence limits (ellipse with major semiaxis length of 3.1° toward 84° clockwise from north and minor semiaxis length of 1.2°) and is not subject to the biases inherent in other methods for estimating Pacific plate paleomagnetic poles. The pole differs significantly by ≈5° from the pole predicted if the Pacific hot spots have been fixed with respect to the spin axis, thus demonstrating, for the first time from paleomagnetic data, that Pacific hot spots have moved relative to the spin axis since the formation of the elbow in the Hawaiian‐Emperor chain. The pole is consistent, however, with previously published paleomagnetic poles in a reference frame fixed relative to Indo‐Atlantic hot spots. Thus, the new results require no motion between Pacific and Indo‐Atlantic hot spots since 32 Ma B.P. Instead, superimposed on whatever motion occurs between hot spots, as expected for true polar wander.

Geologically current plate motions

We describe best-fitting angular velocities and MORVEL, a new closure-enforced set of angular velocities for the geologically current motions of 25 tectonic plates that collectively occupy 97 per cent of Earth's surface. Seafloor spreading rates and fault azimuths are used to determine the motions of 19 plates bordered by mid-ocean ridges, including all the major plates. Six smaller plates with little or no connection to the mid-ocean ridges are linked to MORVEL with GPS station velocities and azimuthal data. By design, almost no kinematic information is exchanged between the geologically determined and geodetically constrained subsets of the global circuit—MORVEL thus averages motion over geological intervals for all the major plates. Plate geometry changes relative to NUVEL-1A include the incorporation of Nubia, Lwandle and Somalia plates for the former Africa plate, Capricorn, Australia and Macquarie plates for the former Australia plate, and Sur and South America plates for the former South America plate. MORVEL also includes Amur, Philippine Sea, Sundaland and Yangtze plates, making it more useful than NUVEL-1A for studies of deformation in Asia and the western Pacific. Seafloor spreading rates are estimated over the past 0.78 Myr for intermediate and fast spreading centres and since 3.16 Ma for slow and ultraslow spreading centres. Rates are adjusted downward by 0.6–2.6 mm yr−1 to compensate for the several kilometre width of magnetic reversal zones. Nearly all the NUVEL-1A angular velocities differ significantly from the MORVEL angular velocities. The many new data, revised plate geometries, and correction for outward displacement thus significantly modify our knowledge of geologically current plate motions. MORVEL indicates significantly slower 0.78-Myr-average motion across the Nazca–Antarctic and Nazca–Pacific boundaries than does NUVEL-1A, consistent with a progressive slowdown in the eastward component of Nazca plate motion since 3.16 Ma. It also indicates that motions across the Caribbean–North America and Caribbean–South America plate boundaries are twice as fast as given by NUVEL-1A. Summed, least-squares differences between angular velocities estimated from GPS and those for MORVEL, NUVEL-1 and NUVEL-1A are, respectively, 260 per cent larger for NUVEL-1 and 50 per cent larger for NUVEL-1A than for MORVEL, suggesting that MORVEL more accurately describes historically current plate motions. Significant differences between geological and GPS estimates of Nazca plate motion and Arabia–Eurasia and India–Eurasia motion are reduced but not eliminated when using MORVEL instead of NUVEL-1A, possibly indicating that changes have occurred in those plate motions since 3.16 Ma. The MORVEL and GPS estimates of Pacific–North America plate motion in western North America differ by only 2.6 ± 1.7 mm yr−1, ≈25 per cent smaller than for NUVEL-1A. The remaining difference for this plate pair, assuming there are no unrecognized systematic errors and no measurable change in Pacific–North America motion over the past 1–3 Myr, indicates deformation of one or more plates in the global circuit. Tests for closure of six three-plate circuits indicate that two, Pacific–Cocos–Nazca and Sur–Nubia–Antarctic, fail closure, with respective linear velocities of non-closure of 14 ± 5 and 3 ± 1 mm yr−1 (95 per cent confidence limits) at their triple junctions. We conclude that the rigid plate approximation continues to be tremendously useful, but—absent any unrecognized systematic errors—the plates deform measurably, possibly by thermal contraction and wide plate boundaries with deformation rates near or beneath the level of noise in plate kinematic data.

Astrochronology of the Mediterranean Langhian between 15.29 and 14.17 Ma

An integrated high-resolution magnetobiocyclostratigraphy including radioisotopic dating and astronomical tuning is presented for the interval between 15.29 and 14.17 Ma in the marine La Vedova section in northern Italy. The natural remanent magnetization is carried by the iron sulphide greigite and the resultant magnetostratigraphy can be correlated straightforwardly to the interval ranging from C5Bn.2n to C5ADn in the Astronomically Tuned Neogene Time Scale (ATNTS2004). Spectral analysis on high-resolution magnetic susceptibility and geochemical proxy records in the depth domain and, using our magnetobiostratigraphic age model, in the time domain demonstrate that the various scales of cyclicity in the section are related to astronomical climate forcing. Starting from our initial age model, larger-scale cycles were first tuned to eccentricity. This first-order tuning was followed by tuning the basic cycle to precession and boreal summer insolation using inferred phase relations between maxima in Ca/Al, redox-sensitive elements and Ba, and minima in magnetic susceptibility, and maxima in precession and minima in obliquity and boreal summer insolation. Our astronomical ages for reversal boundaries are supported by analysis of sea floor spreading rates and should replace the existing ages in the ATNTS2004 lacking direct astronomical control. Two major steps in the geochemical proxy records, astronomically dated at 15.074 and 14.489 Ma, coincide with abrupt changes in sedimentation rate, and are the result of the combined effect of the ∼ 400-kyr eccentricity cycle superimposed upon a longer-term climatic or tectonic induced trend.

Mid-Cretaceous seafloor spreading pulse: Fact or fiction?

Research Article| August 01, 2009 Mid-Cretaceous seafloor spreading pulse: Fact or fiction? M. Seton; M. Seton * 1EarthByte Group, School of Geosciences, Madsen Building F09, University of Sydney, Sydney, NSW 2006, Australia *E-mail: maria.seton@usyd.edu.au. Search for other works by this author on: GSW Google Scholar C. Gaina; C. Gaina 2Center for Geodynamics, Geological Survey of Norway, Leiv Eirikssons vei 39, N-7491 Trondheim, Norway Search for other works by this author on: GSW Google Scholar R.D. Müller; R.D. Müller 1EarthByte Group, School of Geosciences, Madsen Building F09, University of Sydney, Sydney, NSW 2006, Australia Search for other works by this author on: GSW Google Scholar C. Heine C. Heine 1EarthByte Group, School of Geosciences, Madsen Building F09, University of Sydney, Sydney, NSW 2006, Australia †Current address: StatoilHydro Global Exploration Technology, Drammensveien 264, N-0246 Oslo, Norway Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (8): 687–690. https://doi.org/10.1130/G25624A.1 Article history received: 23 Oct 2008 rev-recd: 18 Mar 2009 accepted: 22 Mar 2009 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation M. Seton, C. Gaina, R.D. Müller, C. Heine; Mid-Cretaceous seafloor spreading pulse: Fact or fiction?. Geology 2009;; 37 (8): 687–690. doi: https://doi.org/10.1130/G25624A.1 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 Two main hypotheses compete to explain the mid-Cretaceous global sea-level highstand: a massive pulse of oceanic crustal production that occurred during the Cretaceous Normal Superchron (CNS) and the “supercontinent breakup effect,” which resulted in the creation of the mid-Atlantic and Indian ocean ridges at the expense of subducting old ocean floor in the Tethys and the Pacific. We have used global oceanic paleo-age grids, including now subducted ocean floor and two alternative time scales, to test these hypotheses. Our models show that a high average seafloor spreading rate of 92 mm/a in the Early Cretaceous that decreased to 60 mm/a during the Tertiary, with peaks of 86 mm/a and 70 mm/a at 105 Ma and 75 Ma ago, respectively, correspond to the two observed sea-level highstands in the Cretaceous. Calculations using GTS2004 produce lower seafloor spreading rates during the same period and diminish the mid-Cretaceous spreading pulse. Global ridge lengths increased in the earliest Cretaceous but stayed relatively constant through time. However, we find that the average age of the ocean basins through time is only weakly dependent on the choice of time scale. The expansive mid- and Late Cretaceous epicontinental seas, coupled with warm climates and oxygen-poor water masses, were ultimately driven by the younger average age of the Cretaceous seafloor and faster seafloor spreading rather than a vast increase in mid-ocean ridge length due to the breakup of Pangea or solely by higher seafloor spreading rates, as suggested previously. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Horizontal thermal contraction of oceanic lithosphere: The ultimate limit to the rigid plate approximation

There is a contradiction between two widely accepted pillars of global tectonics, (1) the central plate tectonic assumption of plate rigidity and (2) the explanation of the relief of the seafloor as being due to lithospheric subsidence from thermal contraction. Here we quantify the rate of predictable horizontal thermal contraction of the lithosphere using depth averages of widely accepted thermal models. Depth‐averaged cooling rate, and thus depth‐averaged contraction rate, is proportional to t−1, where t is the age of the lithosphere. Depth‐averaged thermal contraction rate of old (i.e., 167 Ma old) lithosphere is 10−5 Ma−1 (3 × 10−19 s−1), which is an order of magnitude greater than the average strain rate inferred from seismic moment release. Newly created (i.e., 0.1 Ma old) lithosphere is displaced by thermal contraction toward lithosphere in old ocean basins at a rate that is 1.35% of the half rate of seafloor spreading, giving displacement rates of 0.1 to 1.1 km Ma−1 (or, equivalently, 0.1 to 1.1 mm a−1). Predicted displacement rates parallel to midocean ridges depend strongly on lithospheric age and are proportional to the distance along which contractional strain rates are integrated. Displacement rates can be significant (≥1 km Ma−1) for young lithosphere. In particular the displacement of oceanic lithosphere adjacent to southern Baja California relative to lithosphere near the Pacific‐Antarctic Rise has an indicated north‐south displacement rate between ≈3 and ≈10 km Ma−1. This suggests that global plate motion circuits based on the assumption of plate rigidity may be biased by a substantial velocity.

How supercontinents and superoceans affect seafloor roughness

Seafloor roughness varies considerably across the world's ocean basins and is fundamental to controlling the circulation and mixing of heat in the ocean and dissipating eddy kinetic energy. Models derived from analyses of active mid-ocean ridges suggest that ocean floor roughness depends on seafloor spreading rates, with rougher basement forming below a half-spreading rate threshold of 30-35 mm yr(-1) (refs 4, 5), as well as on the local interaction of mid-ocean ridges with mantle plumes or cold-spots. Here we present a global analysis of marine gravity-derived roughness, sediment thickness, seafloor isochrons and palaeo-spreading rates of Cretaceous to Cenozoic ridge flanks. Our analysis reveals that, after eliminating effects related to spreading rate and sediment thickness, residual roughness anomalies of 5-20 mGal remain over large swaths of ocean floor. We found that the roughness as a function of palaeo-spreading directions and isochron orientations indicates that most of the observed excess roughness is not related to spreading obliquity, as this effect is restricted to relatively rare occurrences of very high obliquity angles (>45 degrees ). Cretaceous Atlantic ocean floor, formed over mantle previously overlain by the Pangaea supercontinent, displays anomalously low roughness away from mantle plumes and is independent of spreading rates. We attribute this observation to a sub-Pangaean supercontinental mantle temperature anomaly leading to slightly thicker than normal Late Jurassic and Cretaceous Atlantic crust, reduced brittle fracturing and smoother basement relief. In contrast, ocean crust formed above Pacific superswells, probably reflecting metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, is not associated with basement roughness anomalies. These results highlight a fundamental difference in the nature of large-scale mantle upwellings below supercontinents and superoceans, and their impact on oceanic crustal accretion.

Stresses that drive the plates from below: Definitions, computational path, model optimization, and error analysis

We divide the torques on each surface plate into three parts: lithostatic pressure, side strength, and basal strength. We compute each part for each of 52 plates using a thin shell finite element model of the lithosphere with topography, variable heat flow, variable crust and lithosphere thicknesses from seismic data, transient geotherms, nonlinear rheology, and weak faults. We present an iterative method of adjusting boundary conditions that results in correct plate velocities without the need for models of deep mantle flow. Uncertainty remains because side strength torques, and therefore inferred basal strength torques, depend on the effective friction of faults. Therefore, we compute a two‐parameter suite of models with differing trench resistance and differing fault friction, and we evaluate their misfits relative to seafloor spreading rates, geodetic velocities, intraplate stress directions, and azimuths of seismic anisotropy. The minimum misfit occurs at effective fault friction of 0.1 and trench resistance of 2 × 1012 N m−1. In this preferred model, computed values of mean basal strength traction systematically increase for smaller plates. We analyze error sources and find that the largest source is unmodeled variation in effective friction of plate boundary faults. Discounting highly uncertain results, we find mean basal shear tractions of no more than 1 MPa for the six largest slabless plates: Africa 0.2 MPa; Antarctica 0.1 MPa; North America 0.6 MPa; Eurasia 1.0 MPa; South America 1.0 MPa; Somalia 0.9 MPa. The directions of basal shear traction on these plates are generally forward, meaning subparallel to absolute velocity. Basal strength torques on plates with subducting slabs represent the sum of net slab pull and distributed basal shear traction; if these torques are attributed to net slab pull alone, net slab pull is generally toward the trench and of order 5 × 1012 N m−1. Thus, present plate motions on Earth appear to be driven primarily by deep mantle convection rather than by topography and associated lithostatic pressures.

Contrasting modes of supercontinent formation and the conundrum of Pangea

Repeated cycles of supercontinent amalgamation and dispersal have occurred since the Late Archean and have had a profound influence on the evolution of the Earth's crust, atmosphere, hydrosphere, and life. When a supercontinent breaks up, two geodynamically distinct tracts of oceanic lithosphere exist: relatively young interior ocean floor that develops between the dispersing continents, and relatively old exterior ocean floor, which surrounded the supercontinent before breakup. The geologic and Sm/Nd isotopic record suggests that supercontinents may form by two end-member mechanisms: introversion (e.g. Pangea), in which interior ocean floor is preferentially subducted, and extroversion (e.g. Pannotia), in which exterior ocean floor is preferentially subducted. The mechanisms responsible remain elusive. Top–down geodynamic models predict that supercontinents form by extroversion, explaining the formation of Pannotia in the latest Neoproterozoic, but not the formation of Pangea. Preliminary analysis indicates that the onset of subduction in the interior (Rheic) ocean in the early Paleozoic, which culminated in the amalgamation of Pangea, was coeval with a major change in the tectonic regime in the exterior (paleo-Pacific) ocean, suggesting a geodynamic linkage between these events. Sea level fall from the Late Ordovician to the Carboniferous suggests that the average elevation of the oceanic crust decreased in this time interval, implying that the average age of the oceanic lithosphere increased as the Rheic Ocean was contracting, and that subduction of relatively new Rheic Ocean lithosphere was favoured over the subduction of relatively old, paleo-Pacific lithosphere. A coeval increase in the rate of sea floor spreading is suggested by the relatively low initial 87Sr/ 86Sr in late Paleozoic ocean waters. We speculate that superplumes, perhaps driven by slab avalanche events, can occasionally overwhelm top–down geodynamics, imposing a geoid high over a pre-existing geoid low and causing the dispersing continents to reverse their directions to produce an introverted supercontinent.

Magma productivity and early seafloor spreading rate correlation on the northern Vøring Margin, Norway — Constraints on mantle melting

Continental rifting at the Vøring Margin off mid-Norway was initiated during the earliest Eocene (~ 54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, a marine survey collecting ocean bottom seismometer, single-channel reflection, and magnetic data was conducted on the Norwegian Margin to constrain continental breakup and early seafloor spreading processes. The profile described here crosses the northern part of the Vøring Plateau, and the crustal velocity model was constructed through a combination of ray-tracing and forward gravity modeling, the latter corrected for the thermal effects remaining from the seafloor spreading. We found a maximum igneous crustal thickness of 18 km, decreasing to 6.5 km over the first ~ 6 M.y. after continental breakup. Both the volume and the duration of excess magmatism are about twice as large as that of the Møre Margin south of the East Jan Mayen Fracture Zone, which offsets the two margin segments by ~ 170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~ 150 km to the Lofoten Margin, but without a margin offset. Both the geochemical data and the mean P-wave velocity indicate that there is active mantle upwelling combined with a moderate temperature increase during the earliest mantle melting at the Vøring Margin. The mean P-wave velocity versus crustal thickness also indicates that there is a transition from convection dominated to temperature dominated magma production ~ 2 M.y. after breakup. The magnetic data were used to derive plate half-spreading rates for the Northern Vøring Margin, which are very similar to that obtained at the Møre Margin. There is a strong correlation between magma productivity and early plate spreading rate, suggesting a common cause. A model for the breakup-related magmatism should be able to explain this correlation, but also the magma production peak at breakup, the along-margin magmatic segmentation, and the active mantle upwelling. Proposed end-member hypotheses comprise elevated upper-mantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection fluxing mantle rocks through the melt zone. Edge-driven convection does not easily explain these observations, but a mantle plume model in which buoyant plume material flows laterally to pond in the rift-topography at the base of the lithosphere close to breakup time is promising: When the continents break apart, the hot and buoyant plume-material can flow up into the rift zone from surrounding areas as the rift transits to drift, and the excess temperature of this material will then cause excess magmatism which dies off as the rift-restricted material is spent. The buoyancy of the plume-material may in addition cause active upwelling which can increase the melting furthermore, and also increase the force on the plate boundaries to enhance plate spreading rate. This conceptual model explains how both excess magmatism and spreading rate will be reduced similarly with time as the plume material is consumed by plate spreading, and thus correlate.