SUMMARY Plate motions relative to the hotspots over the past 4 to 7 Myr are investigated with a goal of determining the shortest time interval over which reliable volcanic propagation rates and segment trends can be estimated. The rate and trend uncertainties are objectively determined from the dispersion of volcano age and of volcano location and are used to test the mutual consistency of the trends and rates. Ten hotspot data sets are constructed from overlapping time intervals with various durations and starting times. Our preferred hotspot data set, HS3, consists of two volcanic propagation rates and eleven segment trends from four plates. It averages plate motion over the past ≈5.8 Myr, which is almost twice the length of time (3.2 Myr) over which the NUVEL-1A global set of relative plate angular velocities is estimated. HS3-NUVEL1A, our preferred set of angular velocities of 15 plates relative to the hotspots, was constructed from the HS3 data set while constraining the relative plate angular velocities to consistency with NUVEL-1A. No hotspots are in significant relative motion, but the 95 per cent confidence limit on motion is typically ±20 to ±40 km Myr −1 and ranges up to ±145 km Myr −1 . The uncertainties of the new angular velocities of plates relative to the hotspots are smaller than those of previously published HS2-NUVEL1 (Gripp & Gordon 1990), while being averaged over a shorter and much more uniform time interval. Nine of the fourteen HS2-NUVEL1 angular velocities lie outside the 95 per cent confidence region of the corresponding HS3NUVEL1A angular velocity, while all fourteen of the HS3-NUVEL1A angular velocities lie inside the 95 per cent confidence region of the corresponding HS2-NUVEL1 angular velocity. The HS2-NUVEL1 Pacific Plate angular velocity lies inside the 95 per cent confidence region of the HS3-NUVEL1A Pacific Plate angular velocity, but the 0 to 3 Ma Pacific Plate angular velocity of Wessel & Kroenke (1997) lies far outside the confidence region. We show that the change in trend of the Hawaiian hotspot over the past 2 to 3 Myr has no counterpart on other chains and therefore provides no basis for inferring a change in Pacific Plate motion relative to global hotspots. The current angular velocity of the Pacific Plate can be shown to differ from the average over the past 47 Myr in rate but not in orientation, with the current rotation being about 50 per cent faster (1.06 ± 0.10 deg Myr −1 ) than the average (0.70 deg Myr −1 ) since the 47-Myr-old bend in the Hawaiian‐Emperor chain.

/pdf/young-tracks-of-hotspots-and-current-plate-velocities-4qm8vsz8fm.pdf

Young tracks of hotspots and current plate velocities

[1] Three approaches are used to reduce the error in the satellite-derived marine gravity anomalies. First, we have retracked the raw waveforms from the ERS-1 and Geosat/GM missions resulting in improvements in range precision of 40% and 27%, respectively. Second, we have used the recently published EGM2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean. Third, we have used a biharmonic spline interpolation method to construct residual vertical deflection grids. Comparisons between shipboard gravity and the global gravity grid show errors ranging from 2.0 mGal in the Gulf of Mexico to 4.0 mGal in areas with rugged seafloor topography. The largest errors of up to 20 mGal occur on the crests of narrow large seamounts. The global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate. For rates less than about 60 mm/a the typical ridge segment is 50–80 km long while it increases dramatically at higher rates (100–1000 km). This transition spreading rate of 60 mm/a also marks the transition from axial valley to axial high. We speculate that a single mechanism controls both transitions; candidates include both lithospheric and asthenospheric processes.

/pdf/global-marine-gravity-from-retracked-geosat-and-ers-1-cc0qiwepyy.pdf

Global marine gravity from retracked Geosat and ERS‐1 altimetry: Ridge segmentation versus spreading rate

The North China Craton (NCC) is a classical example of ancient destroyed cratons. Since the initiation of the North China Craton Destruction Project by the National Natural Science Foundation of China, numerous studies have been conducted on the timing, scale, and mechanism of this destruction through combined interdisciplinary research. Available data suggest that the destruction occurred mainly in the eastern NCC, whereas the western NCC was only locally modified. The sedimentation, magmatic activities and structural deformation after cratonization at ~1.8 Ga indicate that the NCC destruction took place in the Mesozoic with a peak age of ca 125 Ma. A global comparison suggests that most cratons on Earth are not destroyed, although they have commonly experienced lithospheric thinning; destruction is likely to occur only when the craton has been disturbed by oceanic subduction. The destruction of the NCC was coincident with globally active plate tectonics and high mantle temperatures during the Cretaceous. The subducted Pacific slab destabilized mantle convection beneath the eastern NCC, which resulted in cratonic destruction in the eastern NCC. Delamination and/or thermal-mechanical-chemical erosion resulted from the destabilization of mantle convection.

/pdf/timing-scale-and-mechanism-of-the-destruction-of-the-north-24ei0cb53t.pdf

Timing, scale and mechanism of the destruction of the North China Craton

An in-depth Sr-Nd-Pb-He-Os isotope and trace element study of the EMII-defining Samoan hot spot lavas leads to a new working hypothesis for the origin of this high 87Sr/86Sr mantle end-member. Systematics of the Samoan fingerprint include (1) increasing 206Pb/204Pb with time - from 18.6 at the older, western volcanoes to 19.4 at the present-day hot spot center, Vailulu'u Seamount, (2) en-echelon arrays in 206Pb/204Pb – 208Pb/204Pb space which correspond to the two topographic lineaments of the 375 km long volcanic chain – this is much like the Kea and Loa Trends in Hawai'i, (3) the highest 87Sr/86Sr (0.7089) of all oceanic basalts, (4) an asymptotic decrease in 3He/4He from 24 RA [Farley et al., 1992] to the MORB value of 8 RA with increasing 87Sr/86Sr, and (5) mixing among four components which are best described as the “enriched mantle”, the depleted FOZO mantle, the (even more depleted) MORB Mantle, and a mild HIMU (high 238U/204Pb) mantle component. A theoretical, “pure” EMII lava composition has been calculated and indicates an extremely smooth trace element pattern of this end-member mantle reservoir. The standard recycling model (of ocean crust/sediment) fails as an explanation for producing Samoan EM2, due to these smooth spidergrams for EM2 lavas, low 187Os/188Os ratios and high 3He/4He (>8 RA). Instead, the origin of EM2 has been modeled with the ancient formation of metasomatised oceanic lithosphere, followed by storage in the deep mantle and return to the surface in the Samoan plume.

http://www.matthewgjackson.com/uploads/3/6/6/5/3665856/2_workman_samoa_gcubed_2003gc000623.pdf

Recycled metasomatized lithosphere as the origin of the Enriched Mantle II (EM2) end‐member: Evidence from the Samoan Volcanic Chain

Fundamentals of Rock Mechanics

The large-scale geometry and age progression of many hotspot island chains, such as the Hawaiian–Emperor chain, are well explained by the steady movement of tectonic plates over stationary hotspots But on a smaller scale, hotspot tracks are composed of discrete volcanic islands whose spacing correlates with lithospheric thickness1 Moreover, the volcanic shields themselves are often not positioned along single lines, but in more complicated patterns, such as the dual line known as the Kea and Loa trends of the Hawaiian islands2, 3 Here we make use of the hypothesis that island spacing is controlled by lithospheric flexure1 to develop a simple nonlinear model coupling magma flow, which feeds volcanic growth, to the flexure caused by volcanic loads on the underlying plate For a steady source of melt underneath a moving lithospheric plate, magma is found to reach the surface and build a chain of separate volcanic edifices with realistic spacing If a volcano is introduced away from the axis of the chain, as might occur following a change in the direction of plate motion, the model perpetuates the asymmetry for long distances and times, thereby producing an alternating series of edifices similar to that observed in the Kea and Loa trends of the Hawaiian island chain

Discrete alternating hotspot islands formed by interaction of magma transport and lithospheric flexure

In this report we examine how the waxing and waning of an ice sheet during a glacial cycle affects the state of stress in the Earth, and how those changes in stress influence the stability of faults. We focus on the stresses at repository depth in Forsmark and Oskarshamn, and on the stability field at seismogenic depth at the proposed repository sites and at the Paervie endglacial fault in northern Sweden. This study is a modelling study, where we use 3-dimensional ice and earth models to calculate the glacial isostatic adjustment (GIA), i.e. the response of the Earth to an ice load, examining both displacements and stresses. We use a flat-earth finite element approach, based on Wu with some modifications. The result presented here is a continuation of previous studies in 2 dimensions and complement those studies in assessing how the 3-dimensionality of the problem affects the conclusions. We use the Fennoscandian ice model of Naeslund, which is a dynamic ice sheet model based on climate reconstructions with constraints from geological observations. The ice model spans the entire Weichselian glaciation but we only use the last 68 kyr, which includes the 2 major periods of ice cover as depicted in this ice sheet reconstruction. For the GIA calculation we use a number of different earth models, both with flat horizontal layers and with various 3D structures of lithosphere thickness. We don't include lateral variations in the viscosity of the mantle. Comparing the current day rebound velocities predicted by our models with GPS observations from the BIFROST project, we note that in general, we can obtain a reasonable fit to the observations with our models, and that the results are rather sensitive to the assumed viscosity of the mantle. We find that the differences between data and model results, for all earth models, have common features which we interpret as due to the ice model. These observations are in agreement with numerous other GIA studies. Our flat layered models tend to fit the data better than the few models with laterally varying lithosphere thickness, where especially the horizontal velocities vary significantly between models and between the models and the data. The regional patterns of stress distribution and stress directions are remarkably similar for all earth models, while the magnitude of the induced stresses vary significantly between models, mainly due to variations in the stiffness of the uppermost layer. The temporal stress evolution at 500 m depth in Forsmark and Oskarshamn is determined by the ice sheet evolution whereas the magnitude of the induced stresses depend on the earth model. For models with realistic stiffness distributions, the induced horizontal stresses both in Forsmark and in Oskarshamn are similar to the magnitude of the vertical stress of the ice load. Stress histories for the Paervie fault, which is located close to the western edge of the ice sheet, show that although the Paervie fault is the largest known endglacial fault, the induced stress magnitudes are not very high, which is due to the relatively modest thickness of the ice sheet here all through the glacial history. In the fault stability analysis we use mainly two synthetic background stress fields, one reverse and one strike-slip. In agreement with previous studies we find that the background stress field is important for the resulting stability field. We show that in a reverse state of stress at 9.5 km depth, with a glacially induced pore pressure head of 50% of the local ice weight, both Forsmark and Oskarshamn would experience fault instability at the end of glaciation. In a strike-slip stress state, the stability field is more sensitive to variations in the direction of the background field, but for our reference field both Forsmark and Oskarshamn show mostly stable conditions. Stability analysis at the Paervie fault shows that in a strike-slip background field the Paervie fault would be stable all through the glaciation while in a reverse faulting background stress field our models show unstable conditions at the end of the glaciation, in general agreement with the observations. The assumed background stress field, with the direction of maximum horizontal stress in the direction of local plate motion, predicts a fault orientation in general agreement with the overall strike of the Paervie fault. Our simulations of fault stability show a very strong dependence of fault stability on the glacially induced excess pore pressure. Increasing the pressure head to 90% of the local ice weight will cause wide-spread instability during ice covered conditions in a strike-slip background field, while in a reverse field instability is promoted earlier in the glacial cycle. Our approach to estimating the induced pore pressure in this study has been one of very simple static conditions and high permeability, implying an immediate propagation of pressures at the base of the ice sheet to the studied depth.

Stress evolution and fault stability during the Weichselian glacial cycle

https://people.earth.yale.edu/sites/default/files/files/Bercovici/64nonhotspot_EPSL00.pdf

Non-hotspot formation of volcanic chains: control of tectonic and flexural stresses on magma transport

Magmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scaledependent and should be approximated by a probability distribution.

/pdf/weibull-distributed-dyke-thickness-reflects-probabilistic-18oqe1gmf3.pdf

Christoph F. Hieronymus

Papers

Discrete alternating hotspot islands formed by interaction of magma transport and lithospheric flexure

Stress evolution and fault stability during the Weichselian glacial cycle

Non-hotspot formation of volcanic chains: control of tectonic and flexural stresses on magma transport

Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength

A dynamical model for generating sharp seismic velocity contrasts underneath continents : Application to the Sorgenfrei-Tornquist Zone