Moving hotspots
The assumption that stationary hotspots underlie the Earth\u2019s lithospheric plates has been\nmost important in the development of the theory of plate tectonics. According to the \ufb01xed\nhotspot hypothesis seamount trails are formed by volcanism penetrating the lithospheric\nplates whilst moving over \u201dhotspots\u201dof upwelling mantle. In turn, the azimuths and age\nprogressions of seamount trails can be used to quantify plate motions with respect to an\nindependent reference frame of hotspots in the mantle. Also, assuming \ufb01xed hotspots, the\ndirection of characteristic remanent magnetization in the basalts acquired during cooling\nshould always be the same. Even if due to plate motion the products of the hotspot are\nlocated far away from the position of the hotspot itself, paleomagnetic studies on the\nbasalts must always provide the position of the hotspot itself. Recently the question\narose, why a hotspot with its origin deep in the mantle would not get advected in the\nconvecting mantle of the Earth. - In this thesis a possible motion of the Kerguelen hotspot\nin the southern Indian Ocean and of the Louisville hotspot in the Paci\ufb01c has been studied.\nThe Kerguelen hotspot is active since approximately 117 Ma. Since then it formed\nthe Kerguelen Plateau and the Broken Ridge in the southern Indian Ocean as well as the\nNinetyeast Ridge, which is the hotspot track going north up to India, and the Ramajal\nTraps in India. Drilling into basement rocks of Broken Ridge and the Kerguelen Plateau\nwas aim of the Ocean Drilling Program, Leg 183, from December 1998 to February 1999.\nEight sites have been drilled. In seven of the sites also the sediments have been recovered.\nIn this thesis, a possible motion of the Kerguelen hotspot has been studied by\ndetermining its paleolatitudes. First, basalts from the Kerguelen Plateau have been\nstudied paleomagnetically to compare the paleolatitudes with the latitude of the hotspot\nitself. Basement from a drillsite on the central Kerguelen Plateau (Site 1138) and of a\nsite on the northern Kerguelen Plateau (Site 1140) were suitable for a determination of\npaleolatitudes. A su\ufb03cient number of independent lava\ufb02ows has been penetrated and\nsampled there to properly average out paleosecular variation, an important requirement\nfor determining paleolatitudes. The characteristic magnetization from the subaerial Site\n1138 with AA- and Pahoehoe lava and of the submarine Site 1140 with its pillow basalts\nis carried by magnetite and titanomagnetites and -maghemites and consists of a single\nremanence component with sometimes a small viscous overprint, that could easily be\nremoved during demagnetization. Stepwise demagnetization in an alternating \ufb01eld and\nstepwise heating of the specimens provided the inclination value of the characteristic\nmagnetization very precisely with small error. Conversion of the mean-site inclination\ninto the paleolatitude of a site provided a latitude of \u03bb = 43.6\u25e6S (max.: 47.8\u25e6S; min.:\n37.9\u25e6S) for Site 1138 on the central Kerguelen Plateau and a latitude of \u03bb = 35.8\u25e6S (max:\n43.0\u25e6S; min.: 28.9\u25e6S) for Site 1140 on the northern Kerguelen Plateau. In Site 1136 on the southern Kerguelen Plateau only two lava \ufb02ows have been sampled. Therefore\npaleosecular variation could not be averaged out properly. Site 1142 on the Broken\nRidge has been tilted and deformed tectonically after its formation, as was found from\nseismic explorations prior to drilling, and the inclination of the magnetization could\ntherefore not be used for a determination of paleolatitudes. Compared to the latitude of\nthe Kerguelen hotspot at 49\u25e6S, the paleolatitudes of the central and northern Kerguelen\nPlateau are further north. This result agrees with previous paleomagnetic studies on the\nsouthern Kerguelen Plateau and the Ninetyeast Ridge, where paleolatitudes have been\nfound that indicate also a formation north of the present-day hotspot position. This\ndi\ufb00erence indicates a southward movement of the hotspot since the Cretaceous relative\nto the spin axis of the Earth. The motion can be explained with a rotation of the whole\nmantle of the Earth relative to the spin axis (true polar wander) or with a motion of the\nhotspot within the Earth\u2019s mantle.\nTherefore, the possibility was studied whether true polar wander can be responsible\nfor the di\ufb00erence between the paleomagnetic data and the present-day latitude of the\nhotspot. Three independently obtained true polar wander paths have been used, that\ndescribe the motion of the whole mantle (with the hotspots) relative to the rotation\nor dipole axis. All three curves point to a shift of the mantle at the time when the\ncentral and southern Kerguelen Plateau formed in such a way that higher southern\npaleolatitudes should be observed. This prediction is just the opposite to what was found\nin the paleomagnetic studies. The Cenozoic parts of the three experimentally obtained\ntrue polar wander paths roughly agree within their uncertainties with a numerically\ncalculated path that accounts for changes of moments of inertia of the mantle. This\nmeans that the di\ufb00erence between paleomagnetic data and the present-day position of\nthe hotspot can not be explained by true polar wander. The next starting point to\nexplain the discrepancy is hotspot motion.\nFor the determination of hotspot drift, geodynamic modeling has been carried out.\nAssuming that a mantle plume rising from the core-mantle boundary is advected in an\nconvecting mantle, a hotspot sould move relative to the surface of the Earth. Seismic\ntomography models were converted into density models of the Earth\u2019s mantle. Then\na velocity \ufb01eld derived from the mass motion due to the density heterogeneities is\ncalculated. The rising mantle plume is then inserted into the model and becomes\nadvected in the velocity \ufb01eld. Seven di\ufb00erent tomographic models have been used to\nobtain velocity \ufb01elds. All seven models result in a southward motion for the Kerguelen\nhotspot since its \ufb01rst appearance approximately 117 Ma ago. The motion is in a similar\ndirection for the di\ufb00erent 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\nthe rising plume. More realistic is the assumption of a depth-dependent plume radius,\nbased on estimates of temperature- and hence viscosity variations within the plume.\nThis has been integrated as a subroutine into the program. The plume radius a\ufb00ects\nthe buoyancy of the plume. A plume with larger radius rises faster through the mantle,\nand will hence have a stronger tendency to straighten up. In contrast, a plume with\nsmaller radius rises slowly and will be in\ufb02uenced more strongly by the velocity \ufb01eld\nof the mantle. Allowing for the variation of viscosity within the plume, the hotspot\nmotion was calculated again. A comparison of the resulting hotspot motion for various\ninput parameters showed that the result is rather independent of the parameters. The\ncalculations also yield a southward motion of 5 to 10 degrees, only the shape of the\nhotspot path is somewhat changed.\nThis southward motion of the Kerguelen hotspot by 5 to 10 degrees can explain the\ndi\ufb00erence between the paleomagnetic data and the present-day position of the hotspot.\nEven combined with true polar wander it \ufb01ts the paleomagnetic results, although true\npolar wander, taken by itself, even increases the di\ufb00erence that has to be explained. The\nconsistency of paleomagnetic results with the model calculations allows the conclusion\nthat the Kerguelen hotspot indeed moved southward by some degrees since its \ufb01rst\noccurence 117 Ma ago.\nA magnetostratigraphy has been made using the sediments of ODP Leg 183. It yielded\na contribution to the age dating of the basalts prior to 40Ar/39Ar dating. Paleomagnetic\nstudies on the sediments contributed to a combined Bio/Magnetostratigraphy. The\nstratigraphy helps to determine the minimal age of the underlying basalts. Using the\nreversals found in the magnetization and a correlation with the paleontological data,\nthe lowermost sediments of Site 1136 (southern Kerguelen Plateau) are dated to have\nan age in the Early Cretaceous, Site 1138 (central Kerguelen Plateau) in the Late\nCretaceous, and Site 1140 (northern Kerguelen Plateau) in the Oligocene. These results\nare meanwhile con\ufb01rmed by precise 40Ar/39Ar age dating of the basement yielding an\nage of 100 Ma for Site 1138 and of 35 Ma for Site 1140.\nThe Ontong Java Plateau, a Large Igneous Province in the western Paci\ufb01c, was thought\nto be formed by the rising mantle plume of the Louisville hotspot approximately 120 Ma\nago. However, according to a recent plate reconstruction, the plateau has been formed\nwell to the north of the location of this hotspot. In this thesis it could be shown that\nthe formation of the Ontong Java Plateau by the Louisville hotspot is possible if hotspot\nmotion in the convecting mantle is allowed. For this purpose, the motion of the Louisville\nhotspot 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\nhas probably shifted by some degrees to the south since its \ufb01rst occurence approximately\n120 Ma ago. There is a considerable variation between di\ufb00erent model results, though.\nThe Louisville hotspot is now located too far south to be responsible for the formation of\nthe Plateau. However, it could have been in the right place at the time of the formation\n120 Ma ago if hotspot motion is considered. This is an example that the drift of hotspots\ncan a\ufb00ect plate tectonics and tectonic reconstructions and that it should be considered.