Cretaceous Superplume

Certain hotspots, as described in Section 5.5, are thought to be the surface manifestation of plumes of hot material ascending from the deep mantle. These are of mod erate size and can be considered to form part of the normal mantle convecting system. It has been proposed, however, that at least once during the history of the Earth there has been an episode of much more intense volcanic activity. The cause has been ascribed to a phenomena termed superplumes, large streams of overheated material rising buoyantly from the D" layer at the base of the mantle (Section 2.8.6), that derived their heat from the core. These spread laterally at the base of the lithosphere to affect an area ten times larger than more normal plume activity.

Larson (1991a, 1991b, 1995) proposed that a superplume was responsible for the widespread volcanic and intrusive igneous activity that affected abnormally large amounts of ocean floor during the mid-Cretaceous. One manifestation of this activity was the creation of numerous seamounts and ocean plateaux in the western Pacific (Fig. 7.15) at a rate some five times greater during this period than at other times. Similarly there were extrusions of thick, areally extensive flood basalts on the continents, such as the Paraná Basalts of Brazil.

Phenomena attributed to the mid-Cretaceous superplume episode are illustrated in Fig. 5.13. At 120-125 Ma the rate of formation of oceanic crust doubled over a period of 5 Ma, decreased within the next 40-50 Ma, and returned to previous levels about 80 Ma ago (Fig. 5.13d). The additional production of crust required increased subduction rates, and it is significant that major batholiths of the Andes and the Sierra Nevada were emplaced at this time.

Coupled to the increased crust production, and caused by the consequent general rise in the level of the sea floor, was a worldwide increase in sea level to an elevation some 250 m higher than at the present day (Fig. 5.13b). At high latitudes the surface temperature of the Earth increased by about 10°C, as shown by oxygen isotope measurements made on benthic fora-minifera from the North Pacific (Fig. 5.13a). This effect was probably caused by the release of large amounts of carbon dioxide during the volcanic eruptions, which created an enhanced "greenhouse" effect (Sections 13.1.1, 13.1.2). During the superplume episode the rates of carbon and carbonate sequestration in organisms increased due to the greater area of shallow seas and the increased temperature, which caused plankton to thrive. This is reflected in the presence of extensive black shale deposits at this time (Force, 1984) and in the estimated oil reserves of this period (Tissot, 1979;

Superplume

High-latitude surface

Superplume

High-latitude surface

200 m 100

5 25

15 0

Oil resources

Black

Oceanic crust production

Oceanic crust production

Reversal rate

Shales

Cenozoic

Cretaceous

Figure 5.13 Phenomena associated with the mid-Cretaceous superplume (after Larson, 1991a, 1991b, with permission from the Geological Society of America).

Fig. 5.13c), which may constitute about 50% of the world's supply. Also of economic significance is the placement of a large percentage of the world's diamond supply at this time, probably as a result of the diamonds' having been translated to the surface by the rising plumes. During the plume episode the rate of geomagnetic reversals (Section 4.1.4) was very low (Fig. 5.13e), with the field remaining in normal polarity for some 35 Ma. This indicates that activity in the core, where the geomagnetic field originates (Section 3.6.4), was low, perhaps related to the transfer of considerable quantities of heat to the mantle.

Acceptance of a mid-Cretaceous superplume episode is not universal. For example, Anderson (1994) suggests that the phenomena of this period were caused by a general reorganization of plates on a global scale associated with the break-up of Pangea and reorganization of the Pacific plate. The mantle upwelling in the latter may then have been a passive reaction to plates being pulled apart by their attached slabs. The episode would thus be viewed as a period when mantle ascended passively as a result of changing plate motions.

5.8 DIRECT MEASUREMENT OF RELATIVE PLATE MOTIONS

It is now possible to measure the relative motion between plates using methods of space geodesy (Gordon & Stein, 1992). Before about 1980 the only methods available for this type of investigation were the standard terrestrial geodetic methods of baseline measurement using optical techniques or laser ranging instruments such as the geodolite (Thatcher, 1979). These methods are certainly sufficiently precise to measure relative plate motions of a few tens of millimeters a year. However, as noted in Section 5.3, in some regions the strain between plates is not all dissipated across a narrow plate boundary, but may extend into the adjacent plates for great distances, particularly in continental areas (Fig. 5.5). In order to study these large-

scale problems it is necessary to be able to measure across very large distances to very great accuracy. Terrestrial methods are extremely time consuming on land, and impossible to use across major oceans. Since 1980, however, the measurement of very long baselines using extraterrestrial methods has become possible via the application of space technology.

Three independent methods of extraterrestrial surveying are available. These are very long baseline inter-ferometry, satellite laser ranging, and satellite radio positioning. The most common and best known example of the latter method is the Global Positioning System (GPS).

The technique of very long baseline interferometry (VLBI) makes use of the radio signals from extraga-lactic radio sources or quasars (Niell et al., 1979; Carter & Robertson, 1986; Clark et al., 1987). The signal from a particular quasar is recorded simultaneously by two or more radio telescopes at the ends of baselines which may be up to 10,000 km long. Because of their different locations on the Earth's surface, the signals received at the telescopes are delayed by different times, the magnitude of the delays between two stations being proportional to the distance between them and the direction from which the signals are coming. Typically, during a 24-hour experiment, 10-15 quasars are each observed 5-15 times. This scheme provides estimates of baseline length that are accurate to about 20 mm (Lyzenga et al., 1986). The usefulness of this system has been greatly enhanced by the development of mobile radio telescopes that frees the technique from the necessity of using fixed observatory installations.

The technique of satellite laser ranging (SLR) calculates the distance to an orbiting artificial satellite or a reflector on the Moon by measuring the two-way travel time of a pulse of laser light reflected from the satellite (Cohen & Smith, 1985). The travel time is subsequently converted to range using the speed of light. If two laser systems at different sites simultaneously track the same satellite, the relative location of the sites can be computed by using a dynamic model of satellite motion, and repeated measurements provide an accuracy of about 80 mm. Periodic repetition of the observations can then be used to observe relative plate motions (Christodou-lidis et al., 1985).

The technique of satellite radio positioning makes use of radio interferometry from the GPS satellites (Dixon, 1991). It is a three-dimensional method by which the relative positions of instruments at the ends of baselines are determined from the signals received at the instruments from several satellites. The simultaneous observation of multiple satellites makes extremely accurate measurements possible with small portable receivers. This is now the most efficient and accurate method of establishing geodetic control on both local and regional surveys (e.g. Sections 8.5.2, 10.4.3).

Gordon & Stein (1992) summarized the early determinations of relative plate motions by these methods. Generally, plate velocities averaged over a few years of observation agree remarkably well with those averaged over millions of years. The methods were first applied to the measurement of the rate of movement across the San Andreas Fault in California. Smith et al. (1985), using SLR, reported that a 900 km baseline that crossed the fault at an angle of 25° had been shortened at an average rate of 30 mm a-1. Lyzenga et al. (1986) have used VLBI to measure the length of several baselines in the southwestern USA and have found that over a period of 4 years movement on the fault was 25 ± 4 mm a-1. These direct measurements of the rate of displacement across the San Andreas Fault are lower than the 48-50 mm a-1 predicted from global models of plate movements (DeMets et al., 1990). However, during the period of observation, no major earthquakes occurred. Over longer time intervals, the discrete jumps in fault movement associated with the elastic rebound mechanism of large earthquakes (Section 2.1.5) would contribute to the total displacement and provide a somewhat higher figure for the average rate of movement. Alternatively, motion between the Pacific and North American plates may be occurring along other major faults located adjacent to the San Andreas Fault (Fig. 8.1, Section 8.5.2).

Tapley et al. (1985), using SLR, measured changes in length of four baselines between Australia and the North American and Pacific plates, and found that the rates differ by no more than 3 mm a-1 from average rates over the last 2 Ma. Similarly Christodoulidis et al. (1985) and Carter & Robertson (1986) measured the relative motion between pairs of plates and found a strong correlation with the kinematic plate model of Minster & Jordan (1978). Herring et al. (1986) made VLBI measurements between various telescopes in the USA and Europe and determined that the present rate of movement across the Atlantic Ocean is 19 ± 10 mm a-1. This agrees well with the rate of 23 mm a-1 averaged over the past 1 Ma.

Sella et al. (2002) provided a comprehensive review of the determinations of relative plate velocities, using the techniques of space geodesy, up to the year 2000. Most of the data summarized were obtained by the GPS method after 1992, when the system was upgraded and the accuracy greatly improved. They presented a model for recent relative plate velocities (REVEL-2000), based on this data, that involves 19 plates. The velocities obtained for numerous plate pairs within this model were then compared with those predicted by the "geologic" model for current plate motions (NUVEL-1A) that averages plate velocities over the past 3 Ma (DeMets et al., 1990, 1994). The velocities for two-thirds of the plate pairs tested were in very close agreement. An example of the comparison between the two models, for the Australian-Antarctica boundary, is shown in Fig. 5.14. Some of the exceptions are thought to be due to inaccuracies in the NUVEL-1A model, for example the motion of the Caribbean plate relative to North and South America; others could well be due to real changes in relative velocities over the past few million years. Examples of the latter include Arabia-Eurasia and India-Eurasia, which may well reflect long term deceleration associated with continental collision.

Most of the space geodetic data points in stable plate interiors confirm the rigidity of plates and hence the rigid plate assumption of plate tectonics. Of the major plates the only exception to this generalization is the Australian plate.

These techniques of direct measurement are clearly extremely important in that they provide estimates of relative plate movements that are independent of plate tectonic models. It is probable that their accuracy will continue to improve, and that observations will become more widely distributed over the globe. The determination of intra-plate deformation and its relationship to intra-plate stress fields, earthquakes, and magmatic activity should also become possible. Important new findings are anticipated over the next few decades.

Au-An

Au-An

REVEL-2000 NUVEL-1A Seafloor spreading rate

REVEL-2000 NUVEL-1A Seafloor spreading rate

100 120 140 Longitude (°E)

Au-An

Au-An

--REVEL-2000

O Transform fault (Altimetry)

■ Transform fault (Bathymetry)

--REVEL-2000

O Transform fault (Altimetry)

■ Transform fault (Bathymetry)

100 120 140 Longitude (°E)

Figure 5.14 Measured sea floor spreading rates and transform fault azimuths for the Australian-Antarctic plate boundary, compared to predicted rates and azimuths from REVEL 2000 and NUVEL-IA. Details of NUVEL-IA, measured spreading rates,and transform azimuths obtained from bathymetry, from DeMets et al., 1990,1994. Transform azimuths from altimetry from Spitzak & DeMets, 1996 (redrawn from Sella et al., 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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