Hotspot Trace Research Articles

Rifting between India and Madagascar – mechanism and isostasy

The nature of isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar (ECMM) and their conjugate nature are examined. The first part of the study deals with the estimation of effective elastic thickness (Te) of the lithosphere for the two margins through cross-spectral analysis of gravity and bathymetry data. The results bring out comparable Te values of 8–15 km for the WCMI and 10–13 km for the ECMM, despite the WCMI having been traversed along strike by a hotspot trace. We have also compared topographic profiles across the two margins extending inland, to examine the rift-flank uplift experienced by the two landmasses abutting the margins, which bear a striking similarity. These results establish the symmetrical rifting mechanism resulting in the formation of the two margins, and indicate that despite the presence of a hotspot trace along one margin (i.e. the WCMI), the two margins have comparable isostatic compensation mechanisms with low Te values. In the second part of the study we have examined the possible mechanism for the rift-flank uplifts using a process-oriented approach of backstripping the sediments along a traverse across the WCMI and reconstructing the original rift flank topography by flexurally backstacking the amount of eroded material on the present-day topography of the Western Ghats. The backstack load configuration is taken from the amount of erosion that took place over the Western Ghats, obtained from AFTA (Apatite Fission Track Analysis) studies. The reconstructed rift time topography and bathymetry are used to get the Moho configuration, which was used to calculate the gravity anomaly, and is compared with the observed anomaly to derive a possible evolution model for the region. We have tried to examine the observed gravity anomaly through two different processes. In the first process, a Te=15 km for backstripping and backstacking, and a compensated underplating for the topography give a best fit model to the observed gravity. In the second process, a necking model with a deep level of necking (20 km) with Te=15 km and without any underplating explains the topography as well as the observed gravity. The nature and amount of the Western Ghat topography and cratonic age (Archaen) of the pre-rifted crust of western India and eastern Madagascar are more in favor of the rifting mechanism (necking model) rather than due to the passage of the Reunion hotspot (underplating model). This is evidenced by the observed increase in topography away from the influence of the Reunion hotspot trace as well as the presence of a similar topography in Madagascar.

Seismic stratigraphy of the Frontal Hawaiian Moat: implications for sedimentary processes at the leading edge of an oceanic hotspot trace

Multi-channel seismic imaging reveals the seismic stratigraphy and associated sedimentary processes of the Frontal Hawaiian Moat (FHM) to the southeast of the island of Hawaii. Two sedimentary units are defined: (1) a basal layer of pelagic sediment draping the oceanic basement and (2) a wedge of volcaniclastic material infilling the FHM and onlapping the Frontal Hawaiian Arch. Three distinct seismic facies within the volcaniclastic unit are recognized: (A) areas of chaotic or incoherent reflections interpreted as proximal debris avalanche or slump deposits; (B) groups of hummocky and distorted reflections interpreted as distal debris avalanche or debris flow deposits; and (C) regions of parallel and laterally continuous reflections interpreted as turbidite deposits. The distribution of these facies delineates slope apron, proximal basin, and distal basin depositional environments (respectively) within the FHM. The northwest drift of the Pacific Plate over the Hawaiian hot spot results in the apparent southeasterly migration of the Hawaiian chain. Advancement of the depositional environments within the FHM occurs as individual volcanoes evolve, erode, and are superseded by new volcanic centers. The interplay between depositional processes and tectonic forces (plate motion and lithospheric flexure) predicts a coarsening-upward stratigraphy within the FHM. The combined accumulation of pelagic and volcaniclastic sediment defines a heterogeneous, and potentially unstable, layer of low strength material beneath the volcanic edifice that may influence the mobility of the island flanks.

Morphometric analysis and its relation to tectonics in Macaronesia

For an investigation of the relation between the morphology and the tectonics in the Macaronesian Islands (i.e. the geographical group of middle Atlantic Islands between 10°N and 40°N), orientation studies have been made of steeply dipping joints and, where possible, of the trends of valleys and ridges. A coincidence of the “preferred directions” of these features indicates a probable genetic relation between them. The results are: (1) Joint strikes, stream directions and ridge trends (where available) generally agree with each other on each individual island, indicating a common genetic (tectonic) origin of these features. Thus, the main result of this study is that the morphology of all Macaronesian Islands, although surficially of volcanic origin, has been significantly codesigend by tectonic processes. (2) The direction data for corresponding geomorphic features are essentially the same across each individual archipelago, certainly with regard to the joints, somewhat less so with regard to the links and ridges. (3) The Canary and the Madeira Islands Archipelago yield the same values for the direction maxima of joints and streams. (4) The compression ( σ 1)-axis indicated by the fault plane solution of the Canary earthquake of 9 May 1989 lines up with one of the bisectrices and not with one of the strike-maxima of the surrounding joints; this indicates that the joint directions may be indicative of the shear lines rather than of the principal stress directions of the neotectonic stress field in the area. (5) There is a steady anticlockwise rotation of corresponding morphotectonic directions as one moves from Africa westward from the Canary Islands through Madeira and the Cape Verde Islands to the Azores. This may be due to a general rotation of the area: whilst drifting N, Africa has been subject to an anticlockwise rotation (pole west of Madeira) owing to the drag caused by the eastward-spreading Atlantic Ocean floor. (6) The mid-Atlantic ridge seems to have no influence on the orientation of the joints/streams/ridges on the islands. In the plate-tectonic model, this lining up of joint orientations may be the consequence of the mirror reflection of the displacements away from the Ridge on its two sides. (7) The situation at the triple junction of the Eurasian, African and American plates is not clear; additional microplates (Azorean, Iberian) might even be present. The Azores might (i) have arisen around the triple junction from basaltic lava upwelling from a mantle hot spot to be carried away as islands on the moving plates; or (ii) they may represent a hot-spot trace on moving plates. Since trachytic/andesitic lava flows have been found on the islands, (iii) a tectonic control model has been proposed in the literature without recurrence to basaltic plates. The results of our morphometric studies would be able to conform to all three models.

Lithospheric structure below the eastern Arabian Sea and adjoining West Coast of India based on integrated analysis of gravity and seismic data

Four uniformly spaced regional gravity traverses and the available seismic data across the western continental margin of India, starting from the western Indian shield extending into the deep oceanic areas of the eastern Arabian Sea, have been utilized to delineate the lithospheric structure. The seismically constrained gravity models along these four traverses suggest that the crustal structure below the northern part of the margin within the Deccan Volcanic Province (DVP) is significantly different from the margin outside the DVP. The lithosphere thickness, in general, varies from 110–120 km in the central and southern part of the margin to as much as 85–90 km below the Deccan Plateau and Cambay rift basin in the north. The Eastern basin is characterised by thinned rift stage continental crust which extends as far as Laxmi basin in the north and the Laccadive ridge in the south. At the ocean–continent transition (OCT), crustal density differences between the Laxmi ridge and the Laxmi basin are not sufficient to distinguish continental as against an oceanic crust through gravity modeling. However, 5-6 km thick oceanic crust below the Laxmi basin is a consistent gravity option. Significantly, the models indicate the presence of a high density layer of 3.0 g/cm3 in the lower crust in almost whole of the northern part of the region between the Laxmi ridge and the pericontinental northwest shield region in the DVP, and also below Laccadive ridge in the southern part. The Laxmi ridge is underlain by continental crust upto a depth of 11 km and a thick high density material (3.0 g/cm3) between 11–26 km. The Pratap ridge is indicated as a shallow basement high in the upper part of the crust formed during rifting. The 15 –17 km thick oceanic crust below Laccadive ridge is seen further thickened by high density underplated material down to Moho depths of 24–25 km which indicate formation of the ridge along Reunion hotspot trace.

Current plate velocities relative to hotspots: implications for hotspot motion, mantle viscosity and global reference frame

The HS2-NUVEL1 model being used for current plate velocities relative to hotspots is shown to be generally inconsistent with the observed hotspot data sampled from non-Pacific regions. Instead, we determine the T22A model, which provides a good and consistent fit to the trends of globally distributed hotspot traces, but predicts plate velocities at hotspots systematically lower than the observed rates of hotspot volcanic migrations. As a result, it implies that the return flow in the lower mantle has a velocity about 1/4 of the plate velocity, and that the lower mantle is about 20 times more viscous than the upper mantle. Although hotspots are not relatively fixed, they do define a global reference frame for plate motion and mantle convection.

Geochemical evidence for the Trindade hotspot trace: Columbia seamount ankaramite

The Columbia seamount ∼825 km offshore from Brazil at ∼20°S lies on the east–west ‘trace’ of the Trindade hotspot. Continental and oceanic magmatism believed to have originated with this hotspot is alkalic and SiO 2-undersaturated, and dates from ∼85 Ma in southern Brazil to <3 Ma on the islands of Trindade and Martin Vaz ∼1100 km offshore. An ankaramite (clinopyroxene ∼16 vol%) dredged from Columbia seamount (est. 10 Ma) conforms to this geochemistry with SiO 2-undersaturated Al-rich clinopyroxene (8–13 wt.% Al 2O 3) and rhönite. Clinopyroxene isotopic compositions are 87Sr/ 86Sr=0.703900, 143Nd/ 144Nd=0.512786, 206Pb/ 204Pb=19.190, 207Pb/ 204Pb=15.045, and 208Pb/ 204Pb=39.242 — resembling those for Trindade, except for slightly higher 207Pb/ 204Pb. The isotopic composition and abundance ratios among weathering-resistant Nb, La, and Yb suggest that Columbia seamount magmatism represents the present-day Trindade plume, but ∼10 million years earlier and perhaps when the plume manifested a signature of ‘contamination’ from subducted sediments. The Columbia seamount analyses provide the first quantitative assessment for the Trindade hotspot trace existing between the Brazil margin and Trindade, strengthening the case for a continuum of magmatism extending from the ∼85 Ma Brazilian igneous provinces of Poxoréu and Alto Paranaiba.

Vertical movements and material transport during hotspot activity: Seismic reflection profiling offshore La Réunion

The structure of the submerged part of the La Réunion hotspot island is determined by a grid of multichannel seismic reflection profiles. The submarine part of the edifice appears as a poorly stratified wedge of material lying above a significant thickness of preexisting sediments and the oceanic basement. The dense data coverage has allowed us to derive contour maps of the top of the basement and of the base of the volcaniclastic edifice, further constrained by coincident wide‐angle profiles. The resulting isobath maps reveal new, unsuspected features that could not be deduced from observation along a single seismic line since the geometry of these horizons varies significantly from one radial profile to the next. Both maps show a large degree of heterogeneity in the topography, with no axial nor cylindrical symmetry, indicating that plate flexure is not dominant. A slight depression toward the island is observed only in the southern area, ahead of the hotspot trace. The lack of angular unconformity in the volcano‐sedimentary pile that covers the oceanic basement firmly establishes the lack of significant vertical movement and flexure. The base of the edifice is roughly domed, centered on the island, with several topographic highs or lows superimposed. The submarine apron appears as a composite constructional body, spreading by slumping of its flanks. Superficial lenses of laterally transported material are observed on the seismic data south of the island, not only to the east of the active Piton de la Fournaise volcano. Oceanic sediments trapped beneath the apron seem undeformed.

Perturbation to the lithosphere along the hotspot track of La Réunion from an offshore‐onshore seismic transect

A 250 km long NE‐SW lithospheric transect spanning the 40 km wide island of La Réunion and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute high‐velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island‐crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material.

Mantle processes during Gondwana break-up and dispersal

This paper reviews the Mesozoic continental flood basalts (CFBs) associated with the break-up and dispersal of Gondwana from 185-60 Ma, the conditions for melt generation in mantle plumes and within the continental mantle lithosphere, and possible causes for lithospheric extension. The number of CFB provinces within Gondwana is much less than the number of mantle plumes that are likely to have been emplaced beneath it in the 300 Ma prior to its initial break-up. Also, the difference between the age of the peak of CFB volcanism and that of the oldest adjacent ocean crust decreases with the age of volcanism during the break-up and dispersal of Gondwana. The older CFBs of Karoo and Ferrar appear to have been derived largely from source regions within the mantle lithosphere. It is only in the younger Paranâ-Etendeka and Deccan CFBs that there are igneous rocks with major, trace element and radiogenic isotope ratios indicative of melting within a mantle plume. These younger CFBs are also clearly associated with hot spot traces on the adjacent ocean floor. The widespread 180 Ma magmatic event is attributed to partial melting within the lithosphere in response to thermal incubation over 300 Ma. In the case of the Ferrar (Antarctica) this was focussed by regional plate margin forces. The implication is that supercontinents effectively self-destruct in response to the build up of heat and resultant magmatism, since these effects significantly weaken the lithosphere and make it more susceptible to break-up in response to regional tectonics. The younger CFB of Paranâ-Etendeka was generated, at least in part, because the continental lithosphere had been thinned in response to regional tectonics. While magmatism in the Deccan was triggered by the emplacement of the plume, that too may have been beneath slightly thinned lithosphere.

Structure and morphology of submarine volcanism in the hotspot region around Réunion Island, western Indian Ocean

The form of the Deccan-Maldives-Mascarene-Réunion hotspot trace suggests that it has, at least in part, been strongly controlled by crustal structures, especially fracture zones. This makes it difficult to assess the present-day or past location of the hotspot, and thus complicates the interpretation of African plate motion reconstruction. We present here results of a cruise to the Réunion area of which the aims were: (a) to determine the extent of present-day volcanism associated with the Réunion hotspot in the region; and (b) to examine the rôle of pre-existing oceanic crustal structures in controlling the location of present-day volcanism. Additionally, we examined the morphology and geology of the important extinct spreading centre southwest of Réunion abandoned when spreading jumped to separate Seychelles from India during the Deccan flood basalt episode some 60–65 Ma ago. The extensive bathymetrie, seismic and geological investigation shows that significant present-day hotspot volcanism is confined to the Piton de la Fournaise edifice on Réunion Island itself. Apparently, the location of recent Réunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits. For plate motion reconstruction and plume flux calculation purposes, Piton de la Fournaise must be taken as the present location of the Réunion hotspot. Accretion at the extinct spreading centre progressively ceased at the time of anomaly A27 (63 Ma), and was associated with marked propagation of the rift tips.

Chemical and Os isotopic study of Cretaceous potassic rocks from Southern Brazil

During the late Mesozoic, an unusually broad range of alkalic magma compositions was erupted along the southern border of the Sao Francisco craton of Brazil. This magmatic activity includes carbonatite, kimberlite, lamprophyre, lamproite, syenite and the largest known example of extrusive kamafugite, the Mata da Corda formation. To determine the nature of the sources of this magmatism, and their geochemical history, an Os isotope study along with major and trace element and Sr, Nd and Pb isotope analyses of kimberlitic, lamproitic and kamafugitic rocks from the Alto Paranaiba province of Brazil was undertaken. This complements recent geochemical and isotopic studies of these magmas. The Os isotope data for Alto Paranaiba samples point to a peridotitic lithospheric mantle source for the kimberlites and lamproites that was variably depleted in Re, presumably by melt removal at some time between the late Archean and mid-Proterozoic. These lithospheric peridotites experienced LIL-element enrichment by fluid/melt metasomatism at roughly 1 Ga, most likely during mobile belt formation along the western border of the Sao Francisco craton. Kamafugitic samples have very radiogenic Os, suggestive of mafic (e.g. pyroxenite, websterite, eclogite) source materials that again appear to have been stabilized in the lithospheric mantle of Brazil in the mid to late Proterozoic. The Os isotope evidence for lithospheric sources for the Alto Paranaiba activity, coupled with Sr, Nd and Pb isotopic characteristics that overlap those of the Walvis Ridge hot-spot trace indicate that the EM1 component in South Atlantic ocean island basalts most likely represents the influence of delaminated Brazilian lithospheric mantle mixed into mantle circulation beneath the South Atlantic and is not related to the plume(s) commonly associated with this ocean island magmatism.

Volcanic fluidization and the Heart Mountain detachment, Wyoming

The presence along the Heart Mountain detachment in Wyoming of microbreccia containing volcanic glass grains with primary shapes and accreted grains equivalent to accretionary and armored lapilli supports the concept that injection of volcanic gases along the fault produced fluidization. The probable source of the volcanic contribution was a fixed feeder pipe, now beneath the Crandall intrusive complex, which left a trail of intrusives akin to a hotspot trace in the moving allochthon. Volcanic gas carrying glass and fluidized microbreccia was injected in sill-like fashion along a bedding horizon near the base of the Ordovician Bighorn Dolomite, resulting in gravitative collapse and spreading, probably catastrophic, of the overlying carbonate and volcanic massif.

Dike complexes in American Samoa

Coherent dike complexes in which dikes make up more than 50% of the total rock occur in American Samoa in two volcanoes on Tutuila and in Ofu-Olosega volcano in the Manu'a Islands. The dikes are remarkably narrow (median width 0.24–0.36 m; average width 0.46–0.56 m, based on 860 measured dikes), significantly narrower than dikes in the Koolau volcano (Oahu, Hawaii), the most closely comparable volcano for which quantitative data are available. The vesicularity of these narrow dikes indicates that they were injected at shallow depths below the land surface: depths of 200–900 m are inferred from the topography. Many dikes have prominent bands of vesicles parallel with their margins. They tend to form clusters where the weakness of the vesicular rock favored center injection of one dike up the center of another. Examples are found of center injection repeated up to eight times. Local breccias, often composed largely of dike fragments evidently brecciated in situ, occur where irregular dikes were accommodated by deforming the host rocks. We see evidence for two dike facies: (1) narrow and vesicle banded, early formed and shallow-injected “clustered” dikes in the dike complexes, which are cut by (2) scattered broad and almost nonvesicular deep-injected “solitary” dikes. Surface eruptions were fed by solitary dikes. The ENE-trending dikes and elongation of Tutuila Island contrast with the WNW trend of rift zones and island elongation elsewhere in Samoa. Tutuila is nearly in line with the North Fiji transform fracture zone suggesting that at 1.5-1.0 Ma, when Tutuila was formed, the volcanism of Tutuila was guided by a temporary extension of this fault zone into the Pacific Plate. Fault movement offset the Samoan hotspot trace by 45 km. The en-échelon arrangement of dike exposures on Tutuila is consistent with left-lateral transcurrent motion above this concealed fracture zone.

Gravity anomalies and deep structure of the Ninetyeast Ridge north of the equator, eastern Indian Ocean ? a hot spot trace model

The Ninetyeast Ridge north of the equator in the eastern Indian Ocean is actively deforming as evidenced by seismicity and its eastward subduction below the Andaman Trench. Basement of the ridge is elevated nearly 2 km with respect to the Bengal Fan; seismic surveys demonstrate continuity of the ridge beneath sediment for 700 km north of 10° N where the ridge plunges below the Fan sediment. The ridge is characterised by a free-air gravity high of 50 mgal amplitude and 350 km wavelength, and along-strike continuity of 1500 km in a north-south direction, closely fringing (locally, even abutting) the Andaman arc-trench bipolar gravity field. Regression analysis between gravity and bathymetry indicates that the ridge gravity field cannot be explained solely by its elevation. The ridge gravity field becomes gradually subdued northwards where overlying Bengal Fan sediments have a smaller density contrast with the ridge material. Our gravity interpretation, partly constrained by seismic data, infers that the ridge overlies significant crustal mass anomalies consistent with the hot spot model for the ridge. The anomalous mass is less dense by about 0.27 g cm−3 than the surrounding oceanic upper mantle, and acts as a ‘cushion’ for isostatic compensation of the ridge at the base of the crust. This ‘cushion’ is up to 8 km thick and 400–600 km wide. Additional complexities are created by partial subduction of the ridge below the Andaman Trench that locally modifies the arc-trench gravity field.

Seismic imaging of hotspot-related crustal underplating beneath the Marquesas Islands

VOLCANISM in active continental rift zones1,2or on rifted continental margins3is frequently associated with unusually high lower-crustal seismic velocities, indicating the presence of large igneous intrusions at the base of the crust. The only comprehensive investigation of crustal underplating at an oceanic hotspot, beneath Hawaii4–6, has yielded controversial results7. Here we report the results of seismic refraction experiments across the Marquesas Islands hotspot trace, which show that the island chain is underlain by crust 15–17 km thick, and that a large lower-crustal region (275 km wide and 2–8 km thick) has seismic velocities (7.3–7.75 km s−1) indicative of crustal underplating. Wide-angle reflections occur near the base of the normal lower-crustal velocities and at the base of the underplating complex; we interpret these reflectors as the relict pre-hotspot Moho and the current post-hotspot Moho, respectively. The high-velocity material may be purely intrusive or may consist of a mixture of intrusive and preexisting rocks. The volume of the high-velocity body is impressive, amounting to nearly twice the volume of volcanics erupted at the surface.

Mantle hotspots, plumes and regional tectonics as causes of intraplate magmatism

In continental areas it is often difficult to determine the cause of intraplate magmatism. Large volumes of magma, high eruption rates, and the presence of a hotspot trace on the adjacent ocean floor, are all evidence for the presence of an anomalously hot mantle. However, in many continental magmas there are chemical variations with time which are inferred to reflect changes from asthenospheric to predominantly lithospheric source regions, or vice versa. It is argued that these chemical characteristics constrain whether magmatism was triggered by the emplacement of a mantle plume, or by lithospheric extension.

Evidence for mantle heterogeneity from platinum-group-element abundances in Indian Ocean basalts

Oceanic-island tholeiitic basalts recovered from four sunken oceanic islands along the Reunion hot-spot trace show trace-element and mineralogical characteristics ranging from typical oceanic-island tholeiites to incompatible-element-depleted tholeiites resembling mid-ocean-ridge basalts. There are also variable degrees of magma evolution at each island. Noble metal (Au, Pd, Pt, Rh, Ru, Ir) abundances tend to decrease with magma evolution and with magma "alkalinity", indicating that the metals behave as compatible elements during crystal fractionation processes and during mantle melting processes. Palladium-to-iridium ratios also decrease with increasing alkalinity. Absolute abundances of elements such as Pd are higher than those in typical mid-ocean-ridge basalts, by factors up to 30, despite many major-element similarities with the latter. Comparison with other types of mafic rocks shows that Pd/Ir ratios increase with decreasing alkalinity in basaltic rocks but plunge to alkali-basalt values in komatiites. A model involving retention of low-melting-point Au, Pd, and Rh in mantle sulphides, which completely dissolve by intermediate percentages of melting, and the high-melting-point metals Ir and Ru in late-melting mantle alloys explains increasing Pd/Ir ratios with decreasing alkalinity (increasing melting percentages) in oceanic basalts and the low Pd/Ir ratios of high-percentage melt komatiites.The high noble metal concentrations in Indian Ocean basalts compared with basalts from many other ocean basins are most easily explained by higher concentrations in their source regions. This may be related to incomplete mixing of a post-core-formation meteoritic component of the upper mantle, or deep mantle plume-derived blebs of core material that either failed to reach the core, during core–mantle differentiation, or were plucked from the core by a convecting lower mantle. The latter is tentatively favoured due to the apparently higher noble metal concentrations in oceanic-island (plume) basalts.

Earthquakes along the Northwestern Boundary of the Labrador Sea

Abstract As in much of northeastern Canada, earthquakes in the Labrador Sea occur predominantly along the passive margin. Geologically and geophysically, this region is complex and consists of an extinct spreading ridge and transform faults, both oceanic and continental crust, a possible hot spot trace, and magnetic and gravity anomalies. We have studied five recent earthquakes in the magnitude 4.5 to 5.5 range to determine their source properties and to better understand how they fit into the seismotectonic framework of the region. A combination of body and surface wave analysis techniques were used to determine the source parameters. LS69, LS89 and PB89 were well recorded teleseismically, and thus their source properties are better constrained than those for LS86 and LS87, for which only a few teleseismic records were available and whose source parameters were determined more from first motions than by modeling. The two events (LS69, LS89) that occurred near the intersection of the extinct spreading ridge (and associated transform) and Mesozoic rifted margin are noteworthy in that the former is of the thrust-fault and the latter of the normal-fault type. Local structure and possibly post-glacial rebound could be the causative factors for the occurrence of normal and thrust faulting in the same area. The two events (LS86, LS87) that occurred along the continent ocean transition zone have fairly similar fault-plane solutions, and are both thrust-faulting events. The fifth event (PB89), which occurred in Payne Bay, is also a thrust-faulting event and could be associated with the Ungava Transform fault to the northeast, or the Cape Smith fold belt to the northwest. The focal depths of all five events lie between 10 and 15 km, and may be dominated by thermal effects. The observation that the deviatoric compression axis of all five earthquakes lies in the northwest (or equivalently in the southeast) quadrant is consistent with recent modeling efforts of the tectonic stress field in this region.

Seismic reflection character of the Cameroon volcanic line: Evidence for uplifted oceanic crust

Deep-imaging multifold seismic fines across submarine parts of the Cameroon volcanic line (west Africa-Gulf of Guinea) show asymmetric uplift of oceanic crust associated with extensive magmatism. The main pulse of uplift occurred after creation of a regional sequence boundary believed to be Miocene in age. The apparent synchroneity of uplift argues against the Cameroon line being a simple hotspot trace, as previously inferred. One plausible theory of origin for the seaward part of the Cameroon volcanic line and its asymmetric uplift geometry combines regional asthenospheric upwelling with restriction of magmatic egress to regularly spaced weak spots, corresponding to fracture-zone crossings. Horizontal motion and buckling also may have occurred along the Cameroon volcanic line.

Bermuda and Appalachian-Labrador rises: Common non-hotspot processes?

Other than the Corner Rise-New England seamounts and associated White Mountains, most postbreakup intraplate igneous activity and topographic uplift in the western North Atlantic and eastern North America do not readily conform to simple hotspot models. For example, the Bermuda Rise trends normal to its predicted hotspot trace. On continental crust, Cretaceous-Eocene igneous activity is scattered along a northeast-trending belt ∼500-1000 km west of and paralleling the continent-ocean boundary. Corresponding activity in the western Atlantic generated seamounts preferentially clustered in a belt ∼1000 km east of the boundary. The Eocene volcanism on Bermuda is paired with coeval magmatism of the Shenandoah igneous province, and both magmatic belts are associated with northeast-trending topographic bulges—the Appalachian-Labrador Rise to the west and the Bermuda Rise (Eocene?) to the east. The above observations suggest the existence of paired asthenosphere upwelling, paralleling and controlled by the deep thermal contrast across the northeast-trending continental margin. Such convection geometry, apparently fixed to the North American plate rather than to hotspots, is consistent with recent convection models by B. Hager. The additional importance of plate-kinematic reorganizations (causing midplate stress enhancement) is suggested by episodic igneous activity ca. 90-100 Ma and 40-45 Ma.