"A "+" indicates that sea floor older than the anomaly number exists.
"A "+" indicates that sea floor older than the anomaly number exists.
isochron map has implications for continental drift and ocean evolution. The age of the last anomaly observed between previously adjacent continents gives the age at which the continents started to separate. Table 5.2 gives estimates of the age of separation of various continents determined from the age of the oldest anomalies observed in each case. In many cases sea floor still exists beyond the oldest anomaly that can be identified. In Table 5.2 this is indicated by a "+" after the anomaly number. In that case an arbitrary 5 Myr has been added to the age of the oldest identifiable anomaly to give the age of separation. In some cases the onset of sea-floor spreading anomalies is time transgressive, becoming progressively younger along the margin.
Polarity chrons older than 5 Ma are designated by numbers correlated with the marine magnetic anomalies. For example polarity chron C26r represents the time of reverse polarity between the normal polarity chrons correlated with magnetic anomalies 26 and 27. Using the prefix M for Mesozoic, polarity chrons for the pre-Aptian sequences are generally described in order of increasing age by the designation MO through M29. The situation is, however, somewhat confusing because these M sequence anomalies were mainly (but not exclusively) assigned to reverse polarity anomalies. For consistency, Opdyke and Channell (1996) used the prefix "C" (e.g., CM29) to distinguish polarity chrons as observed in magnetostratigraphy from the magnetic anomaly numbers.
As additional polarity chrons have been identified the nomenclature has had to evolve (LaBrecque et al., 1977; Harland et al., 1982, 1990; Cande and Kent, 1992a). The current nomenclature, following Cande and Kent (1992a), enables every chron and subchron to be identified uniquely. The longest intervals of predominantly one polarity are referred to by the corresponding anomaly number followed by the suffix n for normal polarity, or r for the preceding reverse polarity interval. When these chrons are subdivided into shorter polarity intervals, they are referred to as subchrons and are identified by appending, from youngest to oldest, .1, .2, etc., to the polarity chron identifier and adding an n or r as appropriate. For example, the three normal polarity intervals that make up anomaly 17 (chron C17n) are called subchrons C17n.ln, C17n.2n, and C17n.3n. Similarly, the reverse interval preceding (older than) C17n.ln is referred to as subchron C17n.lr. For more precise correlations, the fractional position within a chron or subchron can be identified by the equivalent decimal number appended within parentheses. For example, the younger end of chron C29n is C29n(0.0) and a level three-tenths from this younger end is designated C29n(0.3).
Cande and Kent (1992a) introduced the term cryptochron (see Table 4.3) to describe tiny wiggles in magnetic anomaly records that are clearly related to paleomagnetic field behavior (Cande and Kent, 1992b) but may not be short polarity subchrons or microchrons since they have not been confirmed in magnetostratigraphic section. They can be modeled either as short subchrons (or microchrons) or more likely as being due to longer period (50-200 kyr) global changes in the intensity of the Earth's magnetic field. This latter interpretation has been strongly supported by the similarity of ocean surface profiles of the central anomaly with synthetic profiles based on Brunhes age paleointensity records derived from deep-sea sediments (Gee et al., 1996).
Studies of marine magnetic anomalies have identified two quiet or smooth zones of Cretaceous and Jurassic age in which there appear to be either no anomalies or anomalies of extremely low amplitude with unclear interpretation. The Cretaceous Quiet Zone is the best defined and is bounded by magnetic anomalies M0 and M34 corresponding to the Cretaceous Normal (KN) Superchron originally identified by Helsley and Steiner (1969), as discussed in §4.2.3 and §4.3.6 and shown in Fig. 4.8. It seems clear that this quiet zone originated because the geomagnetic field remained in the normal polarity state from 118 to 84 Ma (Kent and Gradstein, 1986) and no magnetic anomalies were produced.
Larson and Pitman (1972) proposed that the KN Superchron represented a time during which there was a pulse of rapid spreading at all spreading centers in both the Atlantic and Pacific oceans. They related this pulse of rapid spreading to episodes of circum-Pacific intrusive and extrusive activity and orogenesis during this period. Plutonism on a large scale occurred in eastern Asia, western Antarctica, New Zealand, the southern Andes, and western North America during the mid-Cretaceous. This is best documented in western North America, where more than 50% of the exposed batholiths are dated between 115 and 85 Ma. If the granodiorites and granites that make up these batholiths are derived from underthrust oceanic lithosphere, then large-scale lithospheric subduction (a consequence of the rapid spreading in all the oceans during that time) is required. Although this pulse of rapid spreading was disputed by Berggren et al. (1975) on the grounds that the age limits of the quiet zone had been incorrectly assigned, the current state of the polarity time scale shown in Fig. 4.8 apparently still supports the original proposal (Larson, 1991). However, Heller et al. (1996) argue that there are sufficient uncertainties in reconstructing plate motions to question the reality of the rapid pulse in spreading rate.
Sclater et al. (1971) and Parsons and Sclater (1977) have shown that the depth of the ocean floor is primarily a function of its age, with the most recent age-depth curve being given by Stein and Stein (1992). This age-depth relation is a consequence of the fact that the lithosphere formed at a spreading ridge is hot and therefore elevated. As it moves away from the ridge axis it cools and subsides (Menard, 1969; McKenzie and Sclater, 1971). As a result there is an increase in the volume of spreading ridges with increase in spreading rate. Therefore the volume of any ridge is a function of its spreading rate history. Changes in the spreading rate cause changes in ridge volume together with associated rises and falls in sea level. Larson and Pitman (1972) proposed that a pulse of rapid spreading occurred during the time of the KN Superchron. Hays and Pitman (1973) demonstrated quantitatively that the worldwide great marine transgression and subsequent regression that occurred in the mid- to Late Cretaceous may have been caused by this contemporaneous pulse of rapid spreading deduced by Larson and Pitman (1972) to have occurred during the KN Superchron. However, Gurnis (1991) shows that there are problems with this linkage. Heller et al. (1996) point out that there are many other explanations for the timing and magnitude of long-term sea level changes, including plate reorganization with little or no change in spreading rates, such as the formation of a new ridge, a new ocean, a new continental rift, ridge jumps and reconfiguration of subduction zones. During the KN Superchron, plate reorganization in the Pacific shows evidence of ridge jumps, indicating that more of the ocean floor of this age may be preserved than has been presumed. Therefore, both the basis of and apparent need for the mid-Cretaceous pulse of sea-floor spreading is now in question (Heller et al., 1996).
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