C. G. A. Harrison

Noise in GPS coordinate time series

We assess the noise characteristics in time series of daily position estimates for 23 globally distributed Global Positioning System (GPS) stations with 3 years of data, using spectral analysis and Maximum Likelihood Estimation. A combination of white noise and flicker noise appears to be the best model for the noise characteristics of all three position components. Both white and flicker noise amplitudes are smallest in the north component and largest in the vertical component. The white noise part of the vertical component is higher for tropical stations (+23 o latitude) compared to midlatitude stations. Velocity error in a GPS coordinate time series may be underestimated by factors of 5-11 if a pure white noise model is assumed.

Decelerating Nazca-South America and Nazca-Pacific Plate motions

Space geodetic estimates of the rate of Nazca-South America convergence and Nazca-Pacific spreading averaging over several years show that present day rates are significantly slower than the 3 million year average NUVEL-1A model. The implied rates of deceleration are consistent with longer term trends extending back to at least 20 Ma, about the time of initiation of Andes growth, and may reflect consequences of ongoing subduction and construction of the Andes, e.g., increased friction and viscous drag on the subducted slab as the leading edge of South America thickens.

Neotectonics in central Mexico

Regional neotectonic activity in central Mexico was mapped through analysis of eleven Landsat Thematic Mapper images. The study area stretches from Mexico City on the east to the Gulf of California on the west, between 17° and 21° N latitude. Fault scarps of Plio-Quaternary age define several large scale tectonic features within the Mexican Volcanic Belt, dominated by rifting, transtension, and shear, with probable sinistral components. Three large crustal blocks south of the Mexican Volcanic Belt are separated from the North American plate and from each other by zones of neotectonic activity that express the relative motions between the adjacent blocks. A simple vector diagram for plates and crustal blocks in central Mexico has been constructed using information from the literature combined with the orientations of fault zones as mapped in this study. The relative motion between the blocks varies from about 2.5 to 8 mm year−1, which is comparable to motions of crustal blocks in areas of active rifting in other parts of the world. In the Mexican Volcanic Belt, there is a clear relationship between volcanism and neotectonic activity. Fault zones that traverse nearly the entire region have the same location and orientation as the volcanic belt. This may indicate that deformation within the crust of the overriding slab plays a significant role in determining the unusual orientation of the Mexican Volcanic Belt relative to the trench. The ultimate factor controlling the location of Plio-Quaternary volcanism and tectonism in central Mexico may be large zones of crustal weakness formed during major Mesozoic and Cenozoic tectonic events.

Deep-imaging seismic and gravity results from the offshore Cameroon Volcanic Line, and speculation of African hotlines☆

Deep-imaging multi-channel seismic reflection data show that volcanic centers along the offshore part of the Cameroon Volcanic Line (CVL) are composed of uplifted, Aptian to Late Cretaceous oceanic crust, >4 km of sedimentary overburden, and Neogene igneous rocks, with volcanic material forming a cap < 1.5 km thick over pre-uplift sedimentary deposits. At Principe Island, the underlying oceanic basement has been uplifted by as much as 3 km to form a crustal arch less than 200 km wide perpendicular to the CVL trend. Vertical faults having small offsets and dikes are common across this arch. Reflection Moho shallows parallel to the uplifted crust along margins of the arch, but is not observed directly below the arch axis where volcanism and faulting are pervasive. The episode of crustal uplift is marked by a prominent reflection unconformity. This unconformity occurs at other CVL islands and seamounts and represents a synchronous period of crustal uplift and volcanism. Reflectors from this unconformity have been correlated to offshore boreholes indicating a Miocene age. Gravity modelling indicates that an elongate wedge of relatively less dense lithospheric mantle (Δρ = −0.1 g/cm3) underlies Principe Island to a depth of 40 km. This interpreted zone of lighter mantle material may form by a combination of intruded mafic partial melt and reheating of the lithosphere. Dynamic support from asthenospheric upwelling may also have contributed to uplift. Other NE-trending volcanic chains and rises off West Africa (Canary Islands, Cape Verde Rise, Sierra Leone Rise and Walvis Ridge) display similar features to CVL islands. These volcanic chains exhibit crustal uplift unconformities and intraplate volcanism occurring during the Miocene and later; Miocene and older marine sediments crop out on most of the islands; there are no flexural depressions surrounding volcanic centers; their ocean island basalts (OIB) have similar geochemical characteristics; the OIB does not appear to be the main construction material of each chain; anomalously high modern heatflow occurs along their lengths; and hotspot-like age progression of volcanism is not clearly defined along their lengths. It is apparent that the CVL is not the product of a single mantle plume or hotspot, and we speculate that the CVL and possibly other NE-trending volcanic chains off West Africa (and perhaps linear belts of Neogene volcanism on the African continent) are the result of linear, mantle upwelling zones or ‘hotlines’. These hotlines are suggested to form above upwelling flow currents in between cylindrical Rayleigh-Bernard convection rolls in the upper mantle. Such convection may be driven by heat transfer across and/or shear along the 670 km discontinuity as a result of convection in the lower mantle.

Sea level variations, global sedimentation rates and the hypsographic curve

Sea level changes during the Neozoic have been estimated by two different methods. The first involves measuring the amount of present-day land area which was flooded during the past, and using the present-day hypsographic curve to estimate the amount of sea level rise necessary to produce this flooding. The second involves the estimation of the changing volume of mid-oceanic ridges through time, and estimating sea level changes after having allowed for isostatic adjustment. A difference in sea level of 170 m is obtained from the two methods for the Cretaceous (80-100 m.y.B.P.). This is equivalent to a difference in continental flooding of 24 Mm 2, using the present-day hypsographic curve. We attempt to explain this difference firstly by allowing for the fact that the present-day ocean basins have more sediment in them than did the Cretaceous ocean basins. This produces a sea level change in the opposite direction to that produced by the reduction in mid-ocean ridge volume since the Cretaceous. Secondly, we suggest another large factor in producing the difference is that the present-day hypsographic curve is not the correct one to use when studying sea level stands in the Cretaceous. Present-day average continental heights are closely related to continental areas. Accepting this principle, if continents are joined together in the past, their average height must be greater, and so their hypsographic curve must be steeper. A given sea level rise would produce less continental flooding during times of continental aggregation than it would today.

Rates of continental erosion and mountain building

The first objective of this work was to obtain values for the rates at which continental erosion can smooth out or remove the topographic expression produced by orogeny. The dominant part is played by mechanical erosion, which acts most strongly in regions of large topographic expression. Chemical erosion depends strongly on precipitation or runoff in individual river drainage basins, but because most continents have very similar average rainfall, chemical erosion is fairly uniform for continental sized areas, and will succeed in planing down all continents to a level peneplain if given enough time. The exception to this rule is Australia, which has a very low chemical erosion rate because of its dryness. The time constants for mechanical and chemical erosion so obtained vary between about 30 and 300 My depending on the continent and the assumptions made. Mountain building occurs throughout the geological time-scale, but at a non-uniform rate. Although there will not be a balance between erosion and mountain building over a short time-scale, due to the non-uniform rate of mountain building, the long-term situation must be that the two phenomena should balance out. It is shown that the freeboard of continents will respond to the long-term balance between mountain building and erosion. An expression has been derived for the average continental elevation in which the rate of mountain building depends on the rate of radiogenic heat production within the earth. It is shown that relatively small changes in average elevation above sea level of a few hundred metres are predicted to have occurred since the beginning of the Proterozoic. As mountain building is predicted to decrease on average with time, because of the reduction in internal heat generation, and as erosion is dependent on the average elevation, this average elevation will decrease slowly through time, the opposite of what some workers have predicted. A more complicated model of mountain building is then investigated, in which one component of mountain building has a sinusoidal signal. The oscillations in average elevation depend on the period of the sinusoid, being smaller for shorter periods. Finally, an average continental elevation is derived using a list of real orogenic events. Although this list of orogenies is incomplete, there is some indication that the actual continental elevation as seen in the flooding history of the continents is similar to that derived in this paper.

Faulting within the median valley

Abstract In order for the median valley to be maintained as a steady-state feature with time, certain constraints on relative motions of fault blocks are necessary. Three possible ways in which this can occur are either a combination of normal faulting and reverse faulting, or normal faulting alone, or rotations of blocks. These alternative constraints are discussed in the light of what is presently known about the detailed tectonic structure of the median valley. It is concluded that tectonic processes at ridge crests are probably a combination of all these models.

Magnetic anomaly patterns on Mid-Atlantic Ridge crest at 26°N

Magnetic anomalies over ocean crust with ages between 0 and 3.5 m.y.B.P. have been identified on the Mid-Atlantic Ridge crest at 26oN. These anomalies indicate a spreading rate of 1.1 cm/yr on the west flank of the ridge and 1.3 cm/yr on the east flank. Computer modeling of the anomalies has shown that simple blocks of uniform thickness, whose width and direction of magnetization are obtained from the reversal time scale and whose intensities are chosen for best visual fit of the anomaly, are not capable of explaining the details of the observed magnetic anomalies. Shorter-wavelength variation in the intensity of magnetization parallel to the ridge crest can, however, explain the observed anoma- lies. A possible degradation of the intensity of magnetization within the axial anomaly is associated with large faulted blocks in the wall of the rift, which is the site of the Trans-

The paleomagnetic record from deep-sea sediment cores

Abstract The paleomagnetic record of deep-sea sediment cores is compared with that which would be expected from our knowledge of the Earths magnetic field. It is found that some of the scatter in directions of magnetization obtained from deep-sea cores is removed in cores with very low sedimentation rate, the cause being that the secular variation of the Earths magnetic field is more completely averaged out over the finite size of each sample, when these samples comprise a longer time span of sedimentation. Corrections have been applied to the results from a series of cores in order to obtain the inclination of the average direction of magnetization from the average inclination and the scatter of inclination. These corrected inclination values confirm the hypothesis that the average Earths field over the past few million years has been similar to an axial dipole displaced towards the North Pole. The amount of displacement obtained was 168 km. The record of short-period polarity intervals within the Brunhes, the Matuyama and the Gauss epochs was studied. It was shown that these intervals are very scattered in position. It is thought that some hitherto undiscovered short-period polarity intervals may be responsible for part of the scatter, but it is also highly likely that many samples give spurious reversals (i.e., ones not caused by the Earths magnetic field). The possible correlation between climatic changes and the Earths magnetic field is examined. It is concluded that the cores which show correlations between direction and/or intensity of magnetization and climatic indicators, thus suggesting the possible correlation between climate and the Earths magnetic field, are not accurately recording the relevant parameters of the Earths magnetic field. The correlation must be caused by climatic effects which have a direct influence on the magnetization of the sediments. Very little is known about the mechanisms of magnetization in deep-sea sediment cores, and there are several unexplained phenomena, such as the fact that many cores have maximum susceptibilities which are vertical, and the fact that cores differ widely in their ability to record accurately the Earths magnetic field.

What is the true rate of reversals of the Earth's magnetic field?☆

Abstract Known reversals of the Earths magnetic field occur at a rate of five per million years. However, the methods whereby the reversal pattern has been established act as a high frequency cut-off filter and will not record two reversals as separate if they occur close to each other. It is thus probable that the true reversal rate is much greater than five per million years. By comparing the observed distribution of polarity interval lengths with a theoretical distribution it is shown that the most likely rate of reversal is close to ten per million years.