Seamounts as recorders of hot-spot epeirogeny S. THOMAS CROUGH* Department of Geosciences, Purdue University, West Lafayette, Indiana 47907 ABSTRACT In this case also, seamounts accurately reflect the vertical motion of the lithosphere beneath them. Observations in the central Pacific suggest that older seamounts are The subsidence of seamounts is fairly well understood, and so it is uplifted if plate motion carries them over mantle hot spots. A depth- now possible to use seamount depths as indicators of epeirogenic uplift. anomaly map of the area reveals several volcanic chains now located on Given the ages of both a seamount and the sea floor it sits on, the expected younger hot-spot swells. Uplift is estimated by comparing the observed depth of the seamount's top can be calculated as the amount of subsidence depth of a seamount summit or the observed stratigraphic section in an expected of standard sea floor in the time between its present age and its atoll with that predicted assuming standard subsidence of the sea floor age when the seamount formed. For seamounts that originally developed after the seamount formed. Cretaceous seamounts on the margin of the on older crust, the effect of lithospheric reheating can be accounted for. Hawaiian Swell have standard summit depths, whereas those on the crest Therefore, one can compare observed seamount depths with expected of the swell are at least 1,100 m shallower. Summit depth correlates with depths based on known ages and make inferences about postformational sea-floor depth anomaly, implying that the Hawaiian Swell has caused the episodes of uplift. Of course, an expected depth calculation assumes that a uplift. Eocene fossils are exposed at the surface in the Tuamotu Atolls, seamount was planed at sea level soon after its origin; if it never reached whereas, according to the standard subsidence relation, Eocene deposits sea level, it will at present be deeper than otherwise predicted. should be covered by 1,000 to 2,000 m of more recent material. The inferred uplift is consistent with the atoll's present position atop the Society HOT-SPOT EPEIROGENY IN THE CENTRAL PACIFIC Swell. Possible evidence for 600 m of uplift in the Southern Cook Islands, however, is unexplained by swell height. The observed uplifts of the Cre- Although many forms of mid-plate uplift may be recorded by sea- taceous seamounts near Hawaii and of the Tuamotu Atolls put a limit on mounts, the central Pacific region indicates that hot-spot swells are likely when the Hawaiian and Society Swells formed at their present locations of to cause the largest and most frequent effects. Figure 1 is a depth-anomaly maximum elevation. The swells must have developed within the past map of the area made by combining previous maps of the Society Islands several million years and probably formed at the same time as did the region (Crough, 1978) and the Marquesas-Line Islands region (Crough younger volcanic edifices on their summits. and Jarrard, 1981) with more recent work north of 15°N. The details of map construction, including all the input data, will be published separately INTRODUCTION in a Pacific-wide study of depth and age (P. A. Schroeder and S. T. Crough, unpub. data). Four of the five recently active hot-spot volcanic It is well established that seamounts are affected by the subsidence of chains—Hawaii (Dairymple and others, 1981), Marquesas (Duncan and their underlying sea floor, thereby providing a record of epeirogenic sink- McDougall, 1974), Society (Dymond, 1975), and Tubuai (Dalrymple and ing. The flat guyot tops of the Pacific become progressively deeper with others, 1975)—all cap rises that are more than a thousand kilometres increasing age; these gave one of the first indications that the sea floor itself across and several hundred metres high. The fifth hot spot, Pitcairn (Dun- subsides with age (Menard, 1964). The subaerially eroded crests of can and others, 1974), has little or no swell, as is typical for hot spots on aseismic ridges drilled by the Deep Sea Drilling Project (DSDP) also sea floor less than 30 Ma old (Crough, 1978). It should be noted that there become deeper with age and their age-depth relation closely parallels the are no depth anomalies of similar amplitude that are not associated with age-depth curve estimated for standard sea floor (Detrick and others, hot spots. 1977). These volcanic ridges apparently form near oceanic spreading cen- The central Pacific region contains at least five locations where an ters, are quickly truncated, and then sink solely due to the cooling of the older volcanic chain migrated over a hot spot and was perhaps uplifted by upper mantle beneath them. Seamounts that are originally formed in a the formation of a new swell beneath it. As labeled by number in Figure 1, mid-plate setting usually subside faster than their underlying sea floor the current direction of Pacific plate motion (Turner and others, 1980) would be expected to sink on the basis of standard cooling (Detrick and conveyed a group of Cretaceous seamounts (1) to near Hawaii, passed the others, 1977). However, mid-plate volcanoes form on unusually shallow Line Islands (2) over a younger hot spot, probably the Marquesas hot spot swells that seem to be the result of lithospheric reheating over mantle hot (Crough and Jarrard, 1981), and carried the Tuamotus (3), the Pitcairn spots (Detrick and Crough, 1978; Crough, 1978). The swells subside trace (4), and the Tubuai group (5) close to the Society hot spot. anomalously fast for their crustal age because they have been reheated and The expected scenario of motion over a hot spot involves rapid initial the seamounts on their surfaces subside in parallel with this rapid sinking. uplift followed by gradual subsidence (Fig. 2). As can be seen in Figure 1, hot-spot swells such as the one beneath Hawaii are strongly asymmetric, *Deceased. Publication process was aided by W. J. Hinze and John G. with the side upstream of the hot spot (to the southeast in the Pacific) Sclater. being very steep compared to the downstream side. The characteristic Geological Society of America Bulletin, v. 95, p. 3-8, 5 figs., January 1984. 3 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/1/3/3434555/i0016-7606-95-1-3.pdf by guest on 28 September 2021 170' 150° 130° H 30c -i io< • MARQUESAS -M0C Figure 1. Depth anomalies (in metres, contour interval 250 m) of the central Pacific calculated using the age-depth curve of Par- sons & Sclater (1977), which includes a de- parture from root time behavior. Positive values signify anomalously shallow sea floor. Seamounts are indicated by dark shading. Numbers identify areas where older chains have moved onto younger swells. Makatea Island (M) and Pitcairn Island (P) are labeled by letters. The three Cretaceous seamounts near Hawaii that have been dated are identi- fied with their ages (in Ma). The velocity of the Pacific plate relative to the hot spots is shown by arrows (Turner and others, 1980). 9.5 cm7lyr~~ "1-30° X2b0 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/1/3/3434555/i0016-7606-95-1-3.pdf by guest on 28 September 2021 SEAMOUNTS AS RECORDERS OF HOT-SPOT EPEIROGENY 5 mediately south of Oahu never experienced mid-plate uplift, they should be no shallower than 2,100 m and perhaps might be slightly deeper. The summits of these seamounts are, in fact, much shallower than predicted by standard subsidence. On the bathymétrie map of Chase and others (1971), the flat summits of these seamounts are only 300 to 1,000 m deep. Although there is a possibility that these depths are slightly shallow due to sedimentation atop the seamounts, igneous rocks were dredged from a depth of only 500 m (Dymond and Windom, 1968), indicating that the sediment caps are thin. Using the 2,100-m estimate of standard subsidence, the seamount summits are anomalously shallow by 1,100 to 1,800 m. Note that these seamounts lie between the 1,000-m and 1,250-m SUBSIDENCE AND contours of depth anomaly on the Hawaiian Swell and that this rise in f REEF GROWTH sea-floor depth is sufficient to explain the minimum amount of uplift Time 3 TT UNCONFORMITY inferred. As a check on whether the swell has caused the uplift, the depths of seamounts on different parts of the swell have been compared (Fig. 3). Two of the Musicians Seamounts to the northwest of Hawaii (Fig. 1 ) have Figure 2. Sequential views of an atoll's passage over a fixed been dated as forming at 65 and 87 Ma B.P. (Clague and Dalrymple, mantle hot spot. Basement volcanic rock is ruled; the atoll's carbonate 1975), making it likely that most of the large seamounts on the eastern part section is unruled. The younger volcanics above the hot spot are not shown, so the cross section may be considered to be located to the side SEAFLOOR DEPTH ANOMALY (m) of the younger chain. O 1000 shape is explainable by fairly instantaneous reheating above the mantle hot spot followed by slower cooling of this thermal pulse. If the uplifted seamount is a guyot, its upper surface simply parallels the vertical motion of the sea floor. If the seamount is an atoll, as depicted in Figure 2, the effects are more interesting. Before uplift, the carbonate cap contains a stratigraphic record of the seamount's earlier subsidence history. During uplift, the younger deposits are removed, exposing progressively older material. With renewed subsidence, the uplift event is recorded as a large unconformity within the carbonate section (Menard, 1973).