Info

Details of tracks from seven localities

In this study we set out to define track anomalies for each area studied in which a particular geographical marker abruptly changes its rate and/or its direction of motion.

We very soon discovered that minor track anomalies could be found every million years or so. These extremely small track anomalies, which gave rise to a very small deflection of track and/or a minuscule change in speed, may possibly be attributed to relatively minor impact events which created craters with diameters of perhaps 20 km. Alternatively they possibly could be 'noise' inherent in the Atlas program.

We have attempted to distinguish the larger impact events, which we assume gave rise to craters of at least 100 km diameter, from this 'background noise'. However, as we have noted, there are problems in attributing a definitive 'magnitude' to a specific event. For example, it has been pointed out that an impact event, which we invoked to explain the generation of the Parana plateau basalt, caused a dramatic change in rate and direction of motion of what is now S America, 135 Ma ago. However, Africa exhibited no detectable disturbance to its track, as measured at the current E coast of that continent. Clearly, one can infer that the response to an impact is not only influenced by the energy of that event. In some circumstances, the effects of the impact may be masked by the resistance to movement of the mass, and by the boundary conditions of the plate in (or adjacent to) which the impact event took place. The estimated magnitude of a specific impact may appear to be very large in one of the areas under discussion, whereas in another area the track anomaly, of the same date, would lead one to suggest that the magnitude of the event was significantly smaller, while in a third area, evidence of a sudden change in track at this time may be entirely lacking.

Consequently, we have been forced to adopt a subjective method of assessment for the various specific impact events. We consider that all the major events in this class of impact were capable of causing a crater with a diameter of at least 100 km. Within this 'large class' we take large (L) to give rise to craters possibly in excess of 400 km in diameter, moderate (M) sized events, we suggest, result in craters in the range 200400 km in diameter, while small (S) events are likely to have craters with diameters in the 100-200 km range. The letters L, M and S are used in Table 8.3 to designate the inferred magnitude of the various events. In areas where the signal of a distant major event is extremely small or completely lacking this condition is represented by a bar (-).

The six new sets of tracks were prepared for specific, easily recognised, points in the different chosen areas. In order of presentation these areas are (a) Hawaii, (b) Borneo, (c) Madagascar, (d) Japan, (e) S America and (f) N America. To this set of six areas, we add the track obtained for a point adjacent to the Mariana Trench which we considered in Chapter 7. (Only the track for the period 0 to 100 Ma for this example is given in this chapter (see Figure 8.26). The reader should refer back to Chapter 7 for details of this track from 100 to 260 Ma.)

Of the seven tracks noted above only that for Hawaii is discussed in any detail. We have done this so that the reader will understand how we assessed the order of magnitude of small, medium and large impact structures.

Hawaii

As the overall track for Hawaii is relatively extensive, it was decided to present the generalised track for this area in two diagrams, namely Figure 8.6a for the period from 265 Ma to 100 Ma and Figure 8.6b for the period from 100 to 0 Ma. We have used an insert in Figure 8.6a to show the track for the period from 265 to 248 Ma. This is on a sufficient scale to show that from 265 to 250 Ma the track is linear, but at 250 Ma it exhibits a sharp angular change of 55° and an increase in velocity by a factor of 7.8. This can clearly be classified as a very major impact event (L), which coincides in time with the initiation of the Siberian plateau basalt and is also within 2 Ma of the Permo/Trias boundary.

The next events occurred at 238.24, 230, 220 and 208 Ma, all of which are represented in Figure 8.7a. At 238.24 Ma, the track is deflected by 28° and subsequently slows by a few per cent. We classified this as a moderately large event (M). The 230 Ma event is clearly a >small (S) event, while the 220 and 208 Ma event, because of the marked change in the direction of the track, we would also classify as (M). Three main events are shown in Figure 8.7b, namely at 200, 194.9 and 190 Ma. At 200 Ma, the track shows a 40° change in direction and a significant increase in rate of movement, so that we classify this event as at least an (M). The events of 195 and 190 Ma we classify as relatively low intensity (S) (but still major) impacts. The 175, 169.7 and 1643 Ma events are shown in Figure 8.8a (with details in the inset diagram). Because of the large changes (almost reversals) in the direction of track, we classify as large (L) events. Events at 150 and 140 Ma (Figure 8.8b) we would also classify as (L), while the 138 Ma event is obviously an (S). As regards the tracks shown in Figures 8.8c and 8.8d, the events at 124 Ma, we would classify as (S) while those at 119, 100 and 96.5 Ma we would put at the low end of the (M) range. As regards the track events in

Table 8.3 Impact and stratigraphic events inferred from tracks.

Was this article helpful?

0 0

Post a comment