New version page

sr143_28

This preview shows page 1-2-3-4 out of 11 pages.

View Full Document
View Full Document

End of preview. Want to read all 11 pages?

Upload your study docs or become a GradeBuddy member to access this document.

View Full Document
Unformatted text preview:

Winterer, E.L., Sager, W.W., Firth, J.V., and Sinton, J.M. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 14328. STRUCTURAL IMPLICATIONS OF GRAVITY ANOMALIES, RESOLUTION AND HEEZENGUYOTS, MID-PACIFIC MOUNTAINS1William W. Sager2ABSTRACTDrilling showed that carbonate rocks make up most of Resolution Guyot, located in the western Mid-Pacific Mountains.Density data from Hole 866A, in the top of the guyot, were used to calculate a forward model of the gravity anomaly caused bythe guyot's topography. After this anomaly was subtracted from the observed free-air anomaly, a significant positive residual, 35mGal in amplitude, remained. The same densities were used for nearby Heezen Guyot, which yielded a similar, 45 mGal residual.Inverse models of the Resolution Guyot residual indicate that most of the mass excess can be attributed to the contrast betweensurrounding sediments and the dolomites at the bottom of the guyot's limestone section and the basalt pedestal beneath the guyot.Nevertheless, models with a central mass concentration fit the residual significantly better than those without, suggesting that theremay be either a buried, conical, seamount remanent in the center of the guyot or a central conduit with dense intrusive rocks. Thelatter seems more plausible because seismic reflection profiles show no evidence of a buried conical structure. In addition, modelswith bottoms below the predicted top of the underlying basaltic plateau give more plausible density contrasts, suggesting that eithera dense zone exists within the plateau beneath the guyot or that the extrapolated depth to the plateau top is incorrect by 500 m to1000 m. The Resolution Guyot models imply that the carbonate bank buried a small seamount or igneous pedestal and retainednearly the same shape and width. Although the Heezen Guyot residual anomaly was not explicitly modeled, it is similar to that ofResolution Guyot and implies an analogous subsurface structure. In contrast, the Heezen Guyot residual is located to the west sideof that edifice and does not have the same elongated shape as the guyot. Thus, it appears that the carbonate bank of Heezen Guyotexpanded eastward from its pedestal.INTRODUCTIONOcean Drilling Program (ODP) Hole 866A was drilled into the topof Resolution Guyot in the Mid-Pacific Mountains. A surprise resultwas that all of the buildup above the surrounding seafloor consists ofcarbonate rock. The guyot sits atop a basaltic plateau, approximately1 km in height, that underlies many of the Mid-Pacific Mountainsseamounts (Winterer and Metzler, 1984). Consequently, if a volcanicconstruct rises from the plateau beneath the guyot, it must be small,perhaps only 500 m in height (Sager, Winterer, Firth, et al., 1993).Both magnetic and gravity data were collected over ResolutionGuyot and nearby Heezen Guyot (Fig. 1) during pre-drilling sitesurveys. Their magnetic anomalies imply a magnetic contrast beneatheach guyot, as might be expected from a basalt cone buried bynonmagnetic carbonate rock; however, their gravity anomalies likelyresult primarily from the contrast between carbonate rock and seawa-ter. Drilling at Site 866, during Leg 143, and nearby Site 463, duringDSDP Leg 63, provided a detailed profile of density through the topof Resolution Guyot and the sediments that surround it. With thesedata it is possible to use gravity modeling techniques to calculate theeffects of known density contrasts and elucidate the deeper structure.In this chapter, I describe the results and implications of gravitymodeling for Resolution and nearby Heezen guyots.Geologic SettingResolution and Heezen guyots are located in the western Mid-Pacific Mountains province, approximately 100 km apart, and aresimilar to many other guyots in the region. Resolution Guyot is sub-circular in plan view and has a flat summit platform about 25 km indiameter at a depth of approximately 1300 m (Fig. 2). Heezen Guyothas a similar platform at nearly the same depth, but the edifice isWinterer, E. L., Sager, W. W., Firth, J. V., and Sinton, J.M. (Eds.), 1995. Proc. ODP,Sci. Results, 143: College Station, TX, (Ocean Drilling Program).2 Departments of Oceanography and Geophysics, Texas A&M University, CollegeStation, TX 77843-3146, U.S.A.kidney-shaped, approximately 12 km in width by 20 km in length(Fig. 3). Both guyots are located in an area of shallow seafloor, witha depth of about 2500 to 3000 m, as compared to the abyssal seafloordepths (5100-5500 m) of the Mesozoic seafloor in the region sur-rounding the Mid-Pacific Mountains (Fig. 1). The regions of shal-lower seafloor appear to be a low basaltic plateau, about 1 km inheight, which is topped by guyots and as much as about 1 km ofsediment (Winterer and Metzler, 1984). Resolution Guyot is locatedin the middle of an area of shallower seafloor and the deepest depthson its flanks are only about 3000 m (Figs. 1,2). Heezen Guyot islocated near the edge of this platform, so depths on its east side aremuch shallower than those on its west (Figs. 1, 3).The age of the lithosphere beneath the Mid-Pacific Mountainsin the vicinity of Resolution and Heezen guyots is somewhat uncer-tain, but magnetic anomaly lineations extrapolated from the sur-rounding area suggest an Early Cretaceous age of about 130 Ma(Sager, Winterer, Firth, et al., 1993). 40Ar-39Ar radiometric dates ofbasalts drilled from Hole 866A average 126 Ma (Pringle, this vol-ume), indicating that the igneous basement beneath the guyot formedsoon after the lithosphere. It is thought that the age of the plateaubeneath the guyots is approximately the same age (Sager, Winterer,Firth, et al., 1993).Constraints from DrillingDSDP Site 463 is located 44 km to the northeast of ResolutionGuyot at a depth of 2525 m (Fig. 1; Thiede, Valuer, et al., 1981). Thehole penetrated 823 m of sediments, but did not strike basement.From a single-channel seismic reflection line over Site 463 collectedduring the site-survey cruise, basement has been extrapolated to beanother 150 to 200 m deeper, at about 1000 meters below the seafloor(mbsf) or 3525 meters below sea level (mbsl) (Sager, Winterer, Firth,et al., 1993). This may be the level of the top of the plateau beneaththe guyots.Hole 463 penetrated a succession (proceeding downward) of nan-nofossil ooze overlying nannofossil chalk overlying limestones andlimestones interbedded with volcaniclastics (Fig. 4). Discrete sample419W.W. SAGER18170° E172C174C176CFigure 1. Generalized


Loading Unlocking...
Login

Join to view sr143_28 and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view sr143_28 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?