Stage Pole Research Articles

Kinematics of Deformable Blocks: Application to the Opening of the Tyrrhenian Basin and the Formation of the Apennine Chain

We describe the opening of back-arc basins and the associated formation of accretionary wedges through the application of techniques of deformable plate kinematics. These methods have proven to be suitable to describe complex tectonic processes, such as those that are observed along the Africa–Europe collision belt. In the central Mediterranean area, these processes result from the passive subduction of the lithosphere belonging to the Alpine Tethys and Ionian Ocean. In particular, we focus on the opening of the Tyrrhenian basin and the contemporary formation of the Apennine chain. We divide the area of the Apennine Chain and the Tyrrhenian basin into deformable polygons that are identified on the basis of sets of extensional structures that are coherent with unique Euler pole grids. The boundaries between these polygons coincide with large tectonic lineaments that characterize the Tyrrhenian–Apennine area. The tectonic style along these structures reflects the variability of relative velocity vectors between two adjacent blocks. The deformation of tectonic elements is accomplished, allowing different rotation velocities of lines that compose these blocks about the same stable stage poles. The angular velocities of extension are determined on the basis of the stratigraphic records of syn-rift sequences, while the rotation angles are obtained by crustal balancing.

Quantitative mRNA Imaging with Dual Channel qFIT Probes to Monitor Distribution and Degree of Hybridization.

Fluorogenic oligonucleotide probes facilitate the detection and localization of RNA targets within cells. However, quantitative measurements of mRNA abundance are difficult when fluorescence signaling is based on intensity changes because a high concentration of unbound probes cannot be distinguished from a low concentration of target-bound probes. Here, we introduce qFIT (quantitative forced intercalation) probes that allow the detection both of probe-target complexes and of unbound probes on separate, independent channels. A surrogate nucleobase based on thiazole orange (TO) probes the hybridization status. The second channel involves a nonresponsive near-IR dye, which serves as a reporter of concentration. We show that the undesirable perturbation of the hybridization reporter TO is avoided when the near-IR dye Cy7 is connected by means of short triazole linkages in an ≥18 nucleotides distance. We used the qFIT probes to localize and quantify oskar mRNA in fixed egg chambers of wild-type and mutant Drosophila melanogaster by wash-free fluorescence in situ hybridization. The measurements revealed a relative 400-fold enrichment of oskar within a 3000 μm3 large volume at the posterior pole of stage 8-9 oocytes, which peaked at a remarkably high 1.8 μM local concentration inside 0.075 μm3 volume units. We discuss detection limits and show that the number of oskar mRNA molecules per oocyte is independent of the oocyte size, which suggests that the final levels are attained already during the onset of oskar localization at stage 8.

Spreading behaviour of the Pacific-Farallon ridge system since 83 Ma

We present improved rotations, complete with uncertainties, for the Pacific-Farallon Ridge (PFR) between geomagnetic chrons 34y (83 Ma) and 10y (28.28 Ma). Despite substantial shortening since ∼55 Ma, this ridge system and its remnants (e.g. the East Pacific Rise) have produced as much as 45per cent of all oceanic lithosphere created since the Late Cretaceous, but reconstructions face the twin challenges of extensive subduction of Farallon crust—which precludes reconstruction by fitting conjugate magnetic anomaly and fracture zone (FZ) traces—and asymmetric spreading behaviour for at least the past 51 Myr. We have calculated best-fit ‘half’-angle stage rotations between nine geomagnetic chron boundaries (34y, 33y, 29o, 24.3o, 20o, 18.2o, 17.1y, 13y and 10y) using combined anomaly and FZ data from both the northern and southern Pacific Plate. For rotations younger than chron 24.3o, estimates for spreading asymmetry, derived using anomaly picks from yet-to-be subducted Farallon/Nazca crust in the south Pacific, allow full stage rotations to be calculated. Between 50 and 83 Ma, where no direct constraints on spreading asymmetry are possible, a ‘best-fit’ full stage rotation was calculated based on the net Nazca:Pacific spreading asymmetry (Pacific spreading fraction f PAC =0.44) over the past 50 Myr, with conservative lower and upper bounds, based on variability in the degree of spreading asymmetry over periods of<15 Myr, assuming f PAC s of 0.5 and 0.36, respectively. Synthetic flowlines generated from our new stage rotation produce a better match to Pacific FZ trends than previously published rotations. With the exception of the chron 18o–20o rotation, the six stage poles for rotations between chrons 33y and 13y (74–33 Ma) all cluster tightly at 60–75°E, 60–68°N, consistent with the relatively constant trend of the major Pacific FZs. This stability spans at least one episode of Farallon Plate fragmentation caused by subduction of PFR segments beneath the Americas, at 55–48 Ma, which appears to have greatly accelerated divergence on the surviving ridge without significantly affecting the location of the instantaneous rotation pole. Together with quasi-periodic 15–20 Myr variations in the degree of spreading asymmetry that also appear to correlate with changes in spreading rate, this indicates that forces other than slab pull may be a factor in determining Pacific-Farallon Plate motions.

Changes in plate boundary kinematics: Punctuated or smoothly varying — Evidence from the mid-Cenozoic transition from lithospheric extension to shortening in New Zealand

Abstract The marine magnetic anomaly record and plate kinematics derived from that data provide evidence of numerous cases of significant changes in plate motions and plate interactions. However, in most cases the temporal resolution provided by the marine record does not discriminate between punctuated (abrupt) or smooth variations in plate motions. During a 10 million year period (~ 30 Ma to 20 Ma), the Euler poles (stage poles) describing Australia–Pacific relative plate motion migrated more than 20° in latitude. The New Zealand segment of the plate boundary is close to the location of the Euler poles and their migration making the tectonics of its plate boundary structures particularly sensitive to the history of plate kinematics. Based on the rapid southward migration of the Euler (stage) poles from north of New Zealand to south of the landmass, there is an expected rapid, migrating transition from an extensional (stage pole north of the site) to transpressional (stage pole south of the site) plate boundary system. The geologic signature of this rapid change in plate boundary kinematics is preserved in the stratigraphic record of a suite of basin deposits that span the latitudinal sweep of the migrating Euler poles. Analyses of these deposits indicate that the timing of the transition from extensional to transpressional tectonics shows a continuous and systematic southward sweep, indicating that the changes in Australia–Pacific plate motions during the 30 Ma–20 Ma interval were smoothly continuous and not punctuated.

A new model for the Paleogene motion of Greenland relative to North America: Plate reconstructions of the Davis Strait and Nares Strait regions between Canada and Greenland

A simplified plate kinematic model for the Paleogene motion of Greenland relative to North America has been developed to provide a new framework for modeling the oceanic spreading system in Baffin Bay and the intraplate tectonic development of the Davis Strait and Nares Strait regions of the Arctic. A single Euler rotation pole was calculated for the C13N to C24N Eocene motion of the Greenland Plate relative to North America using spreading centers and fracture zones interpreted from satellite derived gravity data in Baffin Bay combined with fracture zones in Labrador Sea from published sources. A single stage pole is proposed for the C25N to C27N portion of the Paleocene and a short‐lived stage pole was found necessary to accommodate the C24N to C25N interval. This kinematic model has been used to reinterpret published shipborne magnetic profiles in central Baffin Bay to reveal a Paleocene spreading center and limits of both Eocene and Paleocene oceanic crust. Aeromagnetic data over northeastern Baffin Bay have been used to identify a new fracture zone in northern Baffin Bay. Plate reconstructions are presented incorporating constraints on plate boundaries from onshore and offshore geological and geophysical mapping. Within the Davis Strait, Paleocene oceanic crust was emplaced in an elongated rift that was subsequently inverted by approximately 300 km of Eocene transpression along the Ungava Fault Zone. In the Nares Strait Region, a “microplate” scenario is presented to explain the simultaneous formation of the Lancaster Sound Rift Basin and complex deformation within the Eurekan Orogenic Belt.

Reconstructing Ontong Java Nui: Implications for Pacific absolute plate motion, hotspot drift and true polar wander

Abstract The Taylor (2006) hypothesis suggesting a common origin for the Ontong Java, Manihiki, and Hikurangi large igneous provinces provides an opportunity for a quantitative reconstruction and reassessment of the Ontong Java–Louisville hotspot connection. Our plate tectonic reconstructions of the three plateaus into Ontong Java Nui, or greater Ontong Java, combined with models for Pacific absolute plate motion (APM), allow an analysis of this connection. A new survey of the central Ellice Basin confirms easterly fracture zones, northerly abyssal hill fabric, as well as an area of sigmoidally-southeast-trending fracture zones associated with a late-stage spreading reorientation. From the fracture zone trends we derive new rotation poles for a two-stage model of Ellice Basin opening between the Ontong Java and Manihiki Plateaus. We use these and a single stage pole for separation of the Manihiki and Hikurangi Plateaus, together with three different Pacific APMs, to reconstruct the Ontong Java Nui super plateau back to 123 Ma and compare its predicted location with paleolatitude data obtained from the Ontong Java and Manihiki plateaus. Discrepancies between our Ontong Java Nui reconstructions and Ontong Java and Manihiki paleolatitudes are largest for the fixed Pacific hotspot APM. Assuming a Louisville hotspot source for Ontong Java Nui, remaining disparity between Ontong Java Nui's paleo-location at 123 Ma and published paleomagnetic latitudes for Ontong Java plateau imply that 8°–19° of Louisville hotspot drift or true polar wander may have occurred since the formation of Ontong Java Nui. However, the older portions of the Pacific APMs could easily be biased by a similar amount, making a firm identification of the dominant source of misfit difficult. Prior studies required a combined 26° of hotspot drift, octupole bias effects, and true polar wander just to link the Ontong Java Plateau to Louisville. Consequently, we suggest the super plateau hypothesis and our new reconstructions have considerably strengthened the case for a Louisville plume origin for Ontong Java Nui.

Targeting and Anchoring Tudor in the Pole Plasm of the Drosophila Oocyte

BackgroundGermline formation is a highly regulated process in all organisms. In Drosophila embryos germ cells are specified by the pole plasm, a specialized cytoplasmic region containing polar granules. Components of these granules are also present in the perinuclear ring surrounding nurse cells, the nuage. Two such molecules are the Vasa and Tudor proteins. How Tudor localizes and is maintained in the pole plasm is, however, not known.Methodology/Principal FindingsHere, the process of Tudor localization in nuage and pole plasm was analyzed. The initial positioning of Tudor at the posterior pole of stage 9 oocytes was found to occur in the absence of a structurally detectable nuage. However, in mutants for genes encoding components of the nuage, including vasa, aubergine, maelstrom, and krimper, Tudor was detached from the posterior cortex in stage 10 oocytes, suggesting a prior passage in the nuage for its stability in the pole plasm. Further studies indicated that Valois, which was previously shown to bind in vitro to Tudor, mediates the localization of Tudor in the pole plasm by physically interacting with Oskar, the polar granule organizer. An association between Tudor and Vasa mediated by RNA was also detected in ovarian extracts.Conclusions/SignificanceThe present data challenge the view that the assembly of the polar granules occurs in a stepwise and hierarchical manner and, consequently, a revised model of polar granule assembly is proposed. In this model Oskar recruits two downstream components of the polar granules, Vasa and Tudor, independently from each other: Vasa directly interacts with Oskar while Valois mediates the recruitment of Tudor by interacting with Oskar and Tudor.

The HhH(2)/NDD Domain of the Drosophila Nod Chromokinesin-like Protein Is Required for Binding to Chromosomes in the Oocyte Nucleus

Nod is a chromokinesin-like protein that plays a critical role in segregating achiasmate chromosomes during female meiosis. The C-terminal half of the Nod protein contains two putative DNA-binding domains. The first of these domains, known as the HMGN domain, consists of three tandemly repeated high-mobility group N motifs. This domain was previously shown to be both necessary and sufficient for binding of the C-terminal half of Nod to mitotic chromosomes in embryos. The second putative DNA-binding domain, denoted HhH(2)/NDD, is a helix-hairpin-helix(2)/Nod-like DNA-binding domain. Although the HhH(2)/NDD domain is not required or sufficient for chromosome binding in embryos, several well-characterized nod mutations have been mapped in this domain. To characterize the role of the HhH(2)/NDD domain in mediating Nod function, we created a series of UAS-driven transgene constructs capable of expressing either a wild-type Nod-GFP fusion protein or proteins in which the HhH(2)/NDD domain had been altered by site-directed mutagenesis. Although wild-type Nod-GFP localizes to the oocyte chromosomes and rescues the segregation defect in nod mutant oocytes, two of three proteins carrying mutants in the HhH(2)/NDD domain fail to either rescue the nod mutant phenotype or bind to oocyte chromosomes. However, these mutant proteins do bind to the polytene chromosomes in nurse-cell nuclei and enter the oocyte nucleus. Thus, even though the HhH(2)/NDD domain is not essential for chromosome binding in other cell types, it is required for chromosome binding in the oocyte. These HhH(2)/NDD mutants also block the localization of Nod to the posterior pole of stage 9-10A oocytes, a process that is thought to facilitate the interaction of Nod with the plus ends of microtubules (Cui et al. 2005). This observation suggests that the Nod HhH2/NDD domain may play other roles in addition to binding Nod to meiotic chromosomes.

Influence of cumulative convergence on lithospheric thrust fault development and topography along the Australian‐Pacific plate boundary south of New Zealand

The development of faulting and topography resulting from initial convergence within oceanic lithosphere is largely unknown. We explore relationships among convergence, structural development, and topography along ∼1500 km of the submarine Australian‐Pacific plate boundary south of New Zealand, the Macquarie Ridge Complex (MRC). Due to the variable orientation of the boundary and close proximity of the Australian‐Pacific poles of rotation, individual segments of the plate boundary have experienced different convergence histories since ∼10.95 Ma (Chron 5o). Because interaction along the oceanic extent of the boundary involves oceanic lithosphere of broadly the same age and therefore thermal structure, structural and morphologic differences can be attributed primarily to variations in angles and rates of convergence with respect to the plate boundary orientation since 10.95 Ma. We relate plate boundary‐normal convergence determined from stage pole rotations to structural development, focusing on transitions from purely strike‐slip faulting to partitioned thrust and strike‐slip faulting between 47°S and 60°S since 10.95 Ma. Our results indicate that boundary‐normal convergence of ∼100 km marks the transition from strike‐slip dominated faulting to partitioned underthrusting and strike‐slip faulting (incipient subduction). Establishment of subduction at the Puysegur Trench and incipient subduction at the Hjort Trench corresponds to convergence at rates between ∼2 and 4 cm/yr, angles &gt;30°, and with durations of at least 10 m.y., resulting in &gt;100 km of boundary‐normal convergence. Anomalous topographic volumes resulting from tectonic deformation are quantified from swath bathymetry and compared to convergence history. Results from analysis of the central MRC suggest that prior to lithospheric‐scale thrust faulting, ∼100 km of boundary‐normal convergence can be accommodated by crustal deformation (6 km relief from ridge crest to adjacent trough) and strike‐slip faulting. Our research supports induced/forced intraoceanic initiation of subduction.

Motion between the Indian, Capricorn and Somalian plates since 20 Ma: implications for the timing and magnitude of distributed lithospheric deformation in the equatorial Indian ocean

SUMMARY Approximately 2200 magnetic anomaly crossings and 800 fracture zone crossings flanking the Carlsberg ridge and Central Indian ridge are used to estimate the rotations of the Indian and Capricorn plates relative to the Somalian Plate for 20 distinct points in time since 20 Ma. The data are further used to place limits on the locations of the northern edge of the rigid Capricorn Plate and of the southern edge of the rigid Indian Plate along the Central Indian ridge. Data south of and including fracture zone N (the fracture zone immediately south of the Vema fracture zone), which intersects the Central Indian ridge near 10 ◦ S, are well fit assuming rigid Capricorn and Somalian plates, while data north of fracture zone N are not, in agreement with prior results. Data north of fracture zone H, which intersects the Central Indian ridge near 3.2 ◦ S, are well fit assuming rigid Indian and Somalian plates, while data south of and including fracture zone H are not, resulting in a smaller rigid Indian Plate and a wider diffuse oceanic plate boundary than found before. The data are consistent with Capricorn‐Somalia motion about a fixed pole since ≈8 Ma, but require rotation about a pole 15 ◦ farther away from the Central Indian ridge from 20 to ≈8 Ma. The post-8-Ma pole also indicates Capricorn‐ Somalia displacement directions that are 7 ◦ clockwise of those indicated by the pre-8-Ma stage pole. In contrast, India‐Somalia anomaly and fracture crossings are well fit by a single fixed pole of rotation for the past 20 Ma. India‐Somalia motion has changed little during the past 20 Myr. Nonetheless, astronomically calibrated ages for reversals younger than 12.9 Ma allow resolution of the following small but significant changes in spreading rate: India‐Somalia spreading slowed from 31 to 28 mm yr −1 near 7.9 Ma and later sped up to 31 mm yr −1 near 3.6 Ma; Capricorn‐Somalia spreading slowed from 40 to 36 mm yr −1 near 11.0 Ma, later sped up to 38 mm yr −1 near 5.1 Ma and further sped up to 40 mm yr −1 near 2.6 Ma. The motion between the Indian and Capricorn plates is estimated by differencing India‐Somalia and Capricorn‐Somalia rotations, which differ significantly for all 20 pairs of reconstructions. India has rotated relative to the Capricorn Plate since at least ≈20 Ma. If about a pole located near 4 ◦ S, 75 ◦ E, the rate of rotation was slow, 0.11 ◦ ± 0.01 ◦ Myr −1 (95 per cent confidence limits), from 20 to 8 Ma, but increased to 0.28 ◦ ± 0.01 ◦ Myr −1 (95 per cent confidence limits) at ≈8 Ma. The onset of more rapid rotation coincides, within uncertainty, with the inferred onset at 7‐8 Ma of widespread thrust faulting in the Central Indian basin, and with the hypothesized attainment of maximum elevation and initiation of collapse of the Tibetan plateau at ≈8 Ma. The plate kinematic data are consistent with steady India‐Capricorn motion since 8 Ma and provide no evidence for previously hypothesized episodic motions during that interval. The convergence since 8 Ma between the Indian and Capricorn plates significantly exceeds (by 13 to 20 km) the convergence estimated from three north‐south marine seismic profiles in the Central Indian basin. Where and how the additional convergence was accommodated is unclear.

Tectonic evolution of the Antarctic–Phoenix plate system since 15 Ma

Abstract Joint inversion of magnetic isochron and fracture zone data from the extinct Antarctic–Phoenix spreading system in SW Drake Passage yields seven new finite reconstruction poles. The inversion results are very well constrained for such a short length of plate boundary. Although this is partly because the finite poles are located close to the reconstructed region, the optimum use of fracture zone identifications from satellite-derived free-air gravity data is also important – as the stability of stage poles throughout the short intervals in the model affirms. The model results describe a well-organised spreading system since magnetic anomaly chron C5AD (∼15 Ma) in which the Phoenix plate rotated about stage poles nearby to the southwest. Stage pole locations are broadly consistent with a hypothesis of pivoting subduction as the driving force of Phoenix plate movement, and there is some evidence in the progression of stage poles for late stage movement of the subduction pivot in response to the changing azimuth of the subduction zone at which the Phoenix plate was being consumed. The model kinematics alone provide no unequivocal support for previous interpretations of disruption of the subducted part of the Phoenix plate. The very latest stages of spreading saw falling spreading rates between magnetic anomaly chrons C4 (∼8.1 Ma) and C2A (∼3.3 Ma) when the Antarctic–Phoenix Ridge became extinct. This is consistent with an increase in shear stress across the plate bounding Shackleton Fracture Zone due to a plate reorganisation in the neighbouring Scotia Sea following the cessation of spreading on the West Scotia Ridge.

Rab11 polarization of the Drosophila oocyte: a novel link between membrane trafficking, microtubule organization, and oskar mRNA localization and translation.

The Drosophila embryonic body plan is specified by asymmetries that arise in the oocyte during oogenesis. These asymmetries are apparent in the subcellular distribution of key mRNAs and proteins and in the organization of the microtubule cytoskeleton. We present evidence that the Drosophila oocyte also contains important asymmetries in its membrane trafficking pathways. Specifically, we show that alpha-adaptin and Rab11, which function critically in the endocytic pathways of all previously examined animal cells, are localized to neighboring compartments at the posterior pole of stage 8-10 oocytes. Rab11 and alpha-adaptin localization occurs in the absence of a polarized microtubule cytoskeleton, i.e. in grk null mutants, but is later reinforced and/or refined by Osk, the localization of which is microtubule dependent. Analyses of germline clones of a rab11 partial loss-of-function mutation reveal a requirement for Rab11 in endocytic recycling and in the organization of posterior membrane compartments. Such analyses also reveal a requirement for Rab11 in the organization of microtubule plus ends and osk mRNA localization and translation. We propose that microtubule plus ends and, possibly, translation factors for osk mRNA are anchored to posterior membrane compartments that are defined by Rab11-mediated trafficking and reinforced by Rab11-Osk interactions.

Revised tectonic implications for the magnetic anomalies of the western Weddell Sea

Abstract Ten years after the USAC ( U.S. – A rgentina– C hile) Project, which was the most comprehensive aeromagnetic effort in the Antarctic Peninsula and surrounding ocean basins, questions remain regarding the kinematics of the early opening history of the Weddell Sea. Key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the Weddell Sea and merging the USAC data set with the other magnetic data sets in the region. For this purpose we reprocessed the USAC data set using a continuation between arbitrary surfaces and equivalent magnetic sources. The equivalent sources are located at a smooth crustal surface derived from the existing bathymetry/topography and depths estimated by magnetic inversions. The most critical area processed was the transition between the high altitude survey over the Antarctic Peninsula and the low altitude survey over the Weddell Sea that required downward continuation to equalize the distance to the magnetic source. This procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion. The enhancement of the Mesozoic sequence permits refining the interpretation of the seafloor-spreading anomalies. In particular, the change in shape and wavelength of an elongated positive in the central Weddell Sea suggests that it was formed during the Cretaceous Normal Polarity Interval. The older lineations in the southwestern Weddell Sea are tentatively attributed to susceptibility contrasts modeled as fracture zones. Numerical experimentation to adjust synthetic isochrons to seafloor-spreading lineations and flow lines to fracture zones yields stage poles for the opening of the Weddell Sea since 160 Ma to anomaly 34 time. The corresponding reconstructions look reasonable within the known constraints for the motions of the Antarctic and South America plates. However, closure is not attained between 160 and 118 Ma if independent published East Antarctica–Africa–South America rotations are considered. The lack of closure may be overcome by considering relative motion between the Antarctic Peninsula and East Antarctica until 118 Ma time, an important component of convergence.

Discrepant Cretaceous paleomagnetic poles between Eastern China and Indochina: a consequence of the extrusion of Indochina

Abstract A paleomagnetic study of Cretaceous and Paleocene red-beds has been carried out in the Yunlong area (Yunnan province of China) of the Northern Indochina block. A high-temperature component with dual polarities was isolated from both Lower Cretaceous and Paleocene rocks, and of solely polarity for upper Early Cretaceous and lower Late Cretaceous rocks. Thirteen magnetic polarity zones are found along the Pijiang section. The primary nature of Cretaceous magnetic remanence is ascertained by positive and reversal fold tests. These results, combined with previous Cretaceous results from the Yunlong and Yongping areas of the Lanping basin, indicate that these areas behaved as a relatively rigid block, and rotated 37.2±17° clockwise relative to Eurasia since the Paleocene. Comparing available Early Cretaceous paleomagnetic results obtained both from Indochina and Eastern China, a statistically significant paleolatitude difference of 9.6±5.7° is inferred. This difference is consistent with an estimated 1000±400 km extrusion of Indochina relative to South China, through a counter-clockwise rotation of 14° of Indochina around a finite rotation pole inferred from the stage poles of rotation derived from the magnetic isochrons in the South China Sea. The reconstruction is fully consistent between, the Cretaceous pole obtained from the Khorat basin of the stable core of the Indochina block and the coeval pole of Eastern China.

Evolution of the Malvinas Plate South of Africa

We confirm that a Malvinas Plate is required in the Agulhas Basin during the Late Cretaceous because: (1) oblique Mercator plots of marine gravity show that fracture zones generated on the Agulhas rift, as well as the Agulhas Fracture Zone, do not lie on small circles about the 33o-28y South America-Africa stage pole and were therefore not formed by South America-Africa spreading, (2) the 33o-28y South America-Africa stage rotation does not bring 33o magnetic anomalies on the Malvinas Plate into alignment with their conjugates on the African Plate, and (3) errors in the 33o-28y South America-Africa stage rotation cannot account for the misalignment. We present improved Malvinas-Africa finite rotations determined by interpreting magnetic anomaly data in light of fracture zones and extinct spreading rift segments (the Agulhas rift) that are clearly revealed in satellite-derived marine gravity fields covering the Agulhas Basin. The tectonic history of the Malvinas Plate is chronicled through gravity field reconstructions that use the improved Malvinas-Africa finite rotations and more recent South America-Africa and Antarctica-Africa finite rotations. Newly-mapped triple junction traces on the Antarctic, South American, Malvinas, and African Plates, combined with geometric and magnetic constraints observed in the reconstructions, enable us to investigate the locations of the elusive western and southern boundaries of the Malvinas Plate.

How many Pacific hotspots are fed by deep-mantle plumes?

The Pacific plate is usually considered to host 14 hotspots, but most of this volcanism does not seem to originate from deep-mantle plumes. To reach this conclusion, we tried to establish how many of the seamount alignments on the Pacific plate correspond to classic hotspots, i.e., long-lived hotspots linked to oceanic basaltic plateaus. We retraced the tracks of the 14 Pacific hotspots by using (1) the absolute stage poles representing the Pacific plate absolute motion since 145 Ma, (2) an updated compilation of radiometric ages of seamounts and oceanic plateaus totaling 266 seamounts or islands, (3) the detailed bathymetry of the Pacific Ocean, and (4) the present locations of the hotspots. This analysis allowed us to correlate only three hotspots with the beginning of their tracks possibly corresponding in space and time to an oceanic plateau: Easter to the eastern Mid-Pacific Mountains, Louisville to the Ontong Java plateau, and, with less confidence, Marquesas to Hess Rise and Shatsky Ridge. In addition, the Hawaii hotspot has produced long-lived volcanism. These four are the only classic hotspots on the Pacific plate. However, seven hotspots present short tracks (,35 m.y.) that cannot be traced to an oceanic plateau and thus are not related to any deep-mantle phenomena: Foundation, Macdonald, Pitcairn, Rarotonga, Rurutu, Samoa, and Society. The two northeast Pacific hotspots, Kodiak-Bowie and Cobb, and the Caroline hotspot are unclassifiable because of close proximity to a subduction zone where the prior history of volcanism has been lost.

Logistic regression models for wind and snow damage in northern Finland based on the National Forest Inventory data

The susceptibility of forest stands to wind and snow damage was predicted with basic logistic regression models from a large systematic sample and with conditional logistic regression from a smaller matched study. Data was from the 8th National Forest Inventory, recorded in northern Finland during 1992–1994. The current investigation adds to previous studies by considering the role of individual explanatory variables more closely, instead of only predicting the absolute risk. From both, the basic logistic and the matched models, we obtain odds ratios for explanatory variables, which estimate the so called relative risk; i.e. increase in the damage risk by a particular factor. In our study, matching by cluster or municipality was used to control the possible effects of stochastic local factors that were not quantified, such as wind and snow conditions, on the odds ratios. According to the basic logistic regression model, the susceptibility of a stand to wind damage was increased by: large mean diameter, high stand age, seed-tree cutting, special cutting (cutting for ditches, roads or power lines, or sanitation cutting after damage), and decreasing temperature sum. Significant exposure factors for snow damage were: decreasing temperature sums, elevation >200 m a.s.l., conifer-dominance, mineral soil, undrained, unthinned, development stage pole stand and no proximity to stand edge. The distribution of damaged stands at the landscape level could not be predicted well with the basic logistic regression model, since it is only concerned with the susceptibility component of the probability, and not the occurrence of the damaging agents; i.e. unfavourable weather. Odds ratios can be used as an alternative to probabilities for comparing the damage risks related to combinations of exposure factors from site, stand and forest management. The matched approach showed promise as an alternative method for modelling the odds ratios. The results suggest that matching was useful for wind damage, but less so for snow damage. If the incidence of winds can be taken into account in some way, as with the matched model, the effect of exposure factors on damage risk can be estimated more precisely. Wind damage is a more stochastic phenomenon than snow damage: at least in northern Finland, snow damage may be more tied to more regularly occurring climatic factors at the site than wind damage. Consequently, the basic logistic regression model where these can be included as explanatory variables was better for snow damage than the matched model.

The Magellan seamount trail: implications for Cretaceous hotspot volcanism and absolute Pacific plate motion

The Magellan Seamount Trail (MST) delineates a northwest trending chain of four Cretaceous guyots in the West Pacific Seamount Province (WPSP). Seamount morphology, 40Ar/39Ar geochronology and Sr–Nd–Pb geochemistry of the MST provides evidence for a hotspot origin between the Samoa, Rarotonga and Society hotspots of the South Pacific Isotopic and Thermal Anomaly (SOPITA). The MST yields an excellent linear age progression of 47.6±1.6 mm/yr (r2=1.000; MSWD = 0.23; 1σ SE) including Vlinder guyot (95.1±0.5 Ma, n=5; 2σ SD), Pako guyot (91.3±0.3 Ma, n=3) and Ioah guyot (87.1±0.3 Ma, n=2). The MST also exhibits a small range in Sr–Nd–Pb isotopic compositions indicating enriched mantle sources with an affinity of EMI. Nevertheless, three volcanic events are found out of sequence with linear MST hotspot volcanism: (1) an independent volcanic pedestal was formed 4–7 Myr before shield-volcanism started at Vlinder guyot, (2) a post-erosional volcanic cone was formed at least 20–30 Myr after drowning of Vlinder guyot, and (3) Ita Mai Tai guyot (118.1±0.5 Ma, n=3) was formed 34–36 Myr before the MST hotspot arrived at the predicted location of this guyot. By identifying and ruling out discordant volcanic events, we can use the age progression in MST to test the fixity of its hotspot. When presuming the fixed hotspot hypothesis, the local age progressions of the MST (47.6±1.6 mm/yr) and the copolar Musicians seamount trail (55.8±6.4 mm/yr) are not compatible with their 100–80 Ma Euler pole. We investigate two options: (1) acceptance of a `forced' Euler pole obeying the hotspot hypothesis by using both the age progressions and the azimuths of the studied seamount trails, or (2) acceptance of a `best-fit' Euler pole by using the azimuths of the studied seamount trail exclusively. In the first option, the angular speed of the Pacific plate during the 100–80 Ma stage pole is calculated at 0.502±0.017°/Myr. In the second option, the `best-fit' Euler pole is found approximately 35° different from the `forced' Euler pole. We argue that the observed age progressions can only be reconciled with the `best-fit' pole when allowing for the relative movement of the MST and Musicians mantle plumes with respect to one another. The calculated maximum velocity component parallel to the line of age progression could then be as much as 23 mm/yr for the mantle plumes — when assuming one fixed hotspot in this alternate model.

Induction of primitive streak and Hensen's node by the posterior marginal zone in the early chick embryo.

In the preprimitive streak chick embryo, the search for a region capable of inducing the organizer, equivalent to the Nieuwkoop Center of the amphibian embryo, has focused on Koller's sickle, the hypoblast and the posterior marginal zone. However, no clear evidence for induction of an organizer without contribution from the inducing tissue has been provided for any of these structures. We have used DiI/DiO labeling to establish the fate of midline cells in and around Koller's sickle in the normal embryo. In the epiblast, the boundary between cells that contribute to the streak and those that do not lies at the posterior edge of Koller's sickle, except at stage X when it lies slightly more posteriorly in the epiblast. Hypoblast and endoblast (a second lower layer formed under the streak) have distinct origins in the lower layer, and goosecoid expression distinguishes between them. We then used anterior halves of chick prestreak embryos as recipients for grafts of quail posterior marginal zone; quail cells can be identified subsequently with a quail-specific antibody. Anterior halves alone usually formed a streak, most often from the posterior edge. Quail posterior marginal zones without Koller's sickle were grafted to the anterior side of anterior halves. These grafts were able to increase significantly the frequency of streaks arising from the anterior pole of stage X-XI anterior halves without contributing to the streak or node. Stage XII anterior halves no longer responded. A goosecoid-expressing hypoblast did not form under the induced streak, indicating that it is not required for streak formation. We conclude that the marginal zone posterior to Koller's sickle can induce a streak and node, without contributing cells to the induced streak.

Factors influencing the locations of hot spots determined by the hot‐spotting technique

Seafloor beneath hot‐spot‐produced seamounts have crustal flow lines that intersect at the hot spot locations, provided there is more than one stage rotation and that hot spots are stationary. Convolving seamount shapes with their associated flow lines yields an image of cumulative volcano amplitude (CVA). We call the technique of correlating local CVA maxima with hot spot locations “hot‐spotting”. This technique is geometrical and requires no age information for seamounts or seafloor. We analyze synthetic and actual seamount data and demonstrate that factors such as across‐trail scatter in seamount locations, inaccurate stage poles, and migration of hot spots all exert strong influences on the locations and appearances of CVA maxima. The interpretation of raw CVA images is therefore not straightforward.