Moving hotspots: Evidence from paleomagnetism and modeling

The assumption that stationary hotspots underlie the Earth’s lithospheric plates has been most important in the development of the theory of plate tectonics. According to the fixed hotspot hypothesis seamount trails are formed by volcanism penetrating the lithospheric plates whilst moving over ”hotspots”of upwelling mantle. In turn, the azimuths and age progressions of seamount trails can be used to quantify plate motions with respect to an independent reference frame of hotspots in the mantle. Also, assuming fixed hotspots, the direction of characteristic remanent magnetization in the basalts acquired during cooling should always be the same. Even if due to plate motion the products of the hotspot are located far away from the position of the hotspot itself, paleomagnetic studies on the basalts must always provide the position of the hotspot itself. Recently the question arose, why a hotspot with its origin deep in the mantle would not get advected in the convecting mantle of the Earth. - In this thesis a possible motion of the Kerguelen hotspot in the southern Indian Ocean and of the Louisville hotspot in the Pacific has been studied. The Kerguelen hotspot is active since approximately 117 Ma. Since then it formed the Kerguelen Plateau and the Broken Ridge in the southern Indian Ocean as well as the Ninetyeast Ridge, which is the hotspot track going north up to India, and the Ramajal Traps in India. Drilling into basement rocks of Broken Ridge and the Kerguelen Plateau was aim of the Ocean Drilling Program, Leg 183, from December 1998 to February 1999. Eight sites have been drilled. In seven of the sites also the sediments have been recovered. In this thesis, a possible motion of the Kerguelen hotspot has been studied by determining its paleolatitudes. First, basalts from the Kerguelen Plateau have been studied paleomagnetically to compare the paleolatitudes with the latitude of the hotspot itself. Basement from a drillsite on the central Kerguelen Plateau (Site 1138) and of a site on the northern Kerguelen Plateau (Site 1140) were suitable for a determination of paleolatitudes. A sufficient number of independent lavaflows has been penetrated and sampled there to properly average out paleosecular variation, an important requirement for determining paleolatitudes. The characteristic magnetization from the subaerial Site 1138 with AA- and Pahoehoe lava and of the submarine Site 1140 with its pillow basalts is carried by magnetite and titanomagnetites and -maghemites and consists of a single remanence component with sometimes a small viscous overprint, that could easily be removed during demagnetization. Stepwise demagnetization in an alternating field and stepwise heating of the specimens provided the inclination value of the characteristic magnetization very precisely with small error. Conversion of the mean-site inclination into the paleolatitude of a site provided a latitude of λ = 43.6◦S (max.: 47.8◦S; min.: 37.9◦S) for Site 1138 on the central Kerguelen Plateau and a latitude of λ = 35.8◦S (max: 43.0◦S; min.: 28.9◦S) for Site 1140 on the northern Kerguelen Plateau. In Site 1136 on the southern Kerguelen Plateau only two lava flows have been sampled. Therefore paleosecular variation could not be averaged out properly. Site 1142 on the Broken Ridge has been tilted and deformed tectonically after its formation, as was found from seismic explorations prior to drilling, and the inclination of the magnetization could therefore not be used for a determination of paleolatitudes. Compared to the latitude of the Kerguelen hotspot at 49◦S, the paleolatitudes of the central and northern Kerguelen Plateau are further north. This result agrees with previous paleomagnetic studies on the southern Kerguelen Plateau and the Ninetyeast Ridge, where paleolatitudes have been found that indicate also a formation north of the present-day hotspot position. This difference indicates a southward movement of the hotspot since the Cretaceous relative to the spin axis of the Earth. The motion can be explained with a rotation of the whole mantle of the Earth relative to the spin axis (true polar wander) or with a motion of the hotspot within the Earth’s mantle. Therefore, the possibility was studied whether true polar wander can be responsible for the difference between the paleomagnetic data and the present-day latitude of the hotspot. Three independently obtained true polar wander paths have been used, that describe the motion of the whole mantle (with the hotspots) relative to the rotation or dipole axis. All three curves point to a shift of the mantle at the time when the central and southern Kerguelen Plateau formed in such a way that higher southern paleolatitudes should be observed. This prediction is just the opposite to what was found in the paleomagnetic studies. The Cenozoic parts of the three experimentally obtained true polar wander paths roughly agree within their uncertainties with a numerically calculated path that accounts for changes of moments of inertia of the mantle. This means that the difference between paleomagnetic data and the present-day position of the hotspot can not be explained by true polar wander. The next starting point to explain the discrepancy is hotspot motion. For the determination of hotspot drift, geodynamic modeling has been carried out. Assuming that a mantle plume rising from the core-mantle boundary is advected in an convecting mantle, a hotspot sould move relative to the surface of the Earth. Seismic tomography models were converted into density models of the Earth’s mantle. Then a velocity field derived from the mass motion due to the density heterogeneities is calculated. The rising mantle plume is then inserted into the model and becomes advected in the velocity field. Seven different tomographic models have been used to obtain velocity fields. All seven models result in a southward motion for the Kerguelen hotspot since its first appearance approximately 117 Ma ago. The motion is in a similar direction for the different models, and its magnitude varies from 5 to over 10 degrees. So far, the program to model the hotspot drift assumed a constant viscosity within the rising plume. More realistic is the assumption of a depth-dependent plume radius, based on estimates of temperature- and hence viscosity variations within the plume. This has been integrated as a subroutine into the program. The plume radius affects the buoyancy of the plume. A plume with larger radius rises faster through the mantle, and will hence have a stronger tendency to straighten up. In contrast, a plume with smaller radius rises slowly and will be influenced more strongly by the velocity field of the mantle. Allowing for the variation of viscosity within the plume, the hotspot motion was calculated again. A comparison of the resulting hotspot motion for various input parameters showed that the result is rather independent of the parameters. The calculations also yield a southward motion of 5 to 10 degrees, only the shape of the hotspot path is somewhat changed. This southward motion of the Kerguelen hotspot by 5 to 10 degrees can explain the difference between the paleomagnetic data and the present-day position of the hotspot. Even combined with true polar wander it fits the paleomagnetic results, although true polar wander, taken by itself, even increases the difference that has to be explained. The consistency of paleomagnetic results with the model calculations allows the conclusion that the Kerguelen hotspot indeed moved southward by some degrees since its first occurence 117 Ma ago. A magnetostratigraphy has been made using the sediments of ODP Leg 183. It yielded a contribution to the age dating of the basalts prior to 40Ar/39Ar dating. Paleomagnetic studies on the sediments contributed to a combined Bio/Magnetostratigraphy. The stratigraphy helps to determine the minimal age of the underlying basalts. Using the reversals found in the magnetization and a correlation with the paleontological data, the lowermost sediments of Site 1136 (southern Kerguelen Plateau) are dated to have an age in the Early Cretaceous, Site 1138 (central Kerguelen Plateau) in the Late Cretaceous, and Site 1140 (northern Kerguelen Plateau) in the Oligocene. These results are meanwhile confirmed by precise 40Ar/39Ar age dating of the basement yielding an age of 100 Ma for Site 1138 and of 35 Ma for Site 1140. The Ontong Java Plateau, a Large Igneous Province in the western Pacific, was thought to be formed by the rising mantle plume of the Louisville hotspot approximately 120 Ma ago. However, according to a recent plate reconstruction, the plateau has been formed well to the north of the location of this hotspot. In this thesis it could be shown that the formation of the Ontong Java Plateau by the Louisville hotspot is possible if hotspot motion in the convecting mantle is allowed. For this purpose, the motion of the Louisville hotspot for the last 120 Ma years has been modeled, using the same method as already applied for the Kerguelen hotspot. The calculations indicate, that the Louisville hotspot has probably shifted by some degrees to the south since its first occurence approximately 120 Ma ago. There is a considerable variation between different model results, though. The Louisville hotspot is now located too far south to be responsible for the formation of the Plateau. However, it could have been in the right place at the time of the formation 120 Ma ago if hotspot motion is considered. This is an example that the drift of hotspots can affect plate tectonics and tectonic reconstructions and that it should be considered.

@phdthesis{id368,
  author = {Antretter, Maria},
  language = {en},
  school = {LMU Munich: Faculty of Geosciences},
  title = {Moving hotspots: Evidence from paleomagnetism and modeling},
  url = {http://edoc.ub.uni-muenchen.de/archive/00000427/01/Antretter\_Maria.pdf},
  year = {2001},
}

%O Thesis
%A Antretter, Maria
%G en
%C LMU Munich: Faculty of Geosciences
%T Moving hotspots: Evidence from paleomagnetism and modeling
%U http://edoc.ub.uni-muenchen.de/archive/00000427/01/Antretter_Maria.pdf
%D 2001