Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Indian Ocean’s Atlantis Bank Yields Deep-Earth Insight Henry J.B. Dick Senior Scientist, Geology and Geophysics Department With the Ocean Drilling Program Leg 176 and RRS James Clark Ross Cruise 31 Scientific Parties -140 0 -130 0 30 0 -1200 0 20 -1 -1200 -1 -1500 Bathymetry of Atlantis Bank Atlantis II Fracture Zone: South West Indian Ridge -110 0 -1100 -1400 -120 0 -1300 -1000 -1000 -900 -1200 0 20 -1 -800 -100 -13 -1100 0 -900 -900 0 -100 00 0 -110 00 -11 0 -140 0 -800 00 0 -900 -1 30 -1 0 Headland -900 -1100 20 -800 -1 -800 13 00 -120 0 -900 -100 0 -1000 -1100 -800 -800 oon 00 -1100 -9 -800 -900 Lag -110 0 -100 0 -1000 -1 10 0 -1100 -900 0 -120 -110 0 -10 00 -1000 -800 00 0 -900 -900 -900 -1 -800 -1100 -1200 0 -8 00 -80 Headland -800 -900 0 -110 -1000 0 -700 -900 -1 20 0 0 -1000 -800 -700 -1 0 0 -130 -1 -1 10 00 0 -90 0 0 50 0 -140 -1 -130 0 0 -110 -800 -1500 00 -9 0 -110 00 -10 0 -1200 60 -1400 -1 0 30 -1 0 -170 -1200 -900 -1000 -800 0 -800 -120 0 -1400 40 -1300 -1100 -1100 0 -900 20 -1 -150 0 -1200 -900 00 -8 0 00 -9 00 -1000 -800 0 -1 -1200 -100 -900 -80 0 Headland 735B -1 10 0 -130 0 -1200 -7 00 30 -700 1105A -800 -900 -700 -700 -1 -1000 -1100 Lag -700 -900 0 00 -1 -1000 -1100 -800 oon 0 -800 Seastack 00 -7 -1000 0 -110 -120 -13 s 00 -1100 -9 -700 00 -8 -700 -130 0 00 -900 -1200 -1 -700 00 -11 *Hess, H.H., 1962, The History of the Ocean Basins, in Petrologic Studies: A volume in honor of A.F. Buddington, A.E.J. Engel, H.L. James, and B.F. Leonard, eds., Geological Society of America, pp. 599-620. upper temperature and pressure limit of peridotite and olivine as stable elements beneath the ocean ridges. Hess thought Layer 2 was the volcanic carapace formed as lavas erupted above the hydrothermally altered mantle rock. -130 0 0 -800 -100 I never imagined I would spend six weeks of my life “wandering around” the seafloor exploring an 11 million year old beach, and it never occurred to me to look for a fossil island. But that’s what I did, and that’s what we found on two research voyages separated by more than a decade. But, first, a bit of history: During World War II, Princeton mineralogist Harry Hess convinced US officials to keep Navy ships’ echo sounders running as they crisscrossed the Pacific. Hess, a captain and later admiral in the Navy, even managed to have his men bring back rock samples when they returned to the ship after Pacific Island landings. We not only won the war, but also identified the mid-Pacific mountain range now known as the East Pacific Rise—a discovery that proved fundamental to the then developing theory of plate tectonics. Thus began modern marine geology and geophysics. Prompted by Hess’s discoveries, when peace returned, American scientists initiated a systematic study of this little known region. One of the first results was the discovery that the ocean crust appeared to present a consistent, layered seismic structure composed of sediments (layer 1 at the seafloor) followed by two layers with rapidly increasing sound velocity, and then a reflector, dubbed the Moho after the its discoverer, a Czechoslovakian seismologist named Andrija Mohorovicic. What surprised scientists was the Moho was found at a very shallow, nearly constant depth of 6 to 7 kilometers, and that seismic layers 2 and 3 above also appeared uniform in the oceans, whereas this boundary ranges from 35 to 60 kilometers deep beneath continents. This led to a great debate about the nature of the Moho beneath the oceans and the composition of the layers above it. Hess proposed in his now famous 1962 paper The History of the Ocean Basins * that the Moho was an alteration front created by hydrothermal solutions converting the earth’s mantle to a lighter rock known as serpentine. Serpentine forms by hydration of the primary mantle rock peridotite, which is named for the gem stone peridote (the most common mineral, known to scientists as olivine). The uniform depth of the Moho, then, represented the Five-meter bathymetric map of Atlantis Bank created by WHOI scientist Maurice Tivey from the James Clark Ross echo sounding tracks in collaboration with the other scientists on the cruise. The extraordinary high-resolution map shows the very top of Atlantis Bank with the wave cut platform and the precipitous sea cliffs to either side. One can see old headlands and seastacks in the image, as well as the great arcuate fossil lagoon on the west side where oolitic limestones were dredged. OCEANUS • 29 This theory was rejected, however, because laboratory experiments on unit 0 I serpentine failed to produce the exact acoustic velocity characteristics inII ferred for seismic layer 3. Instead a model where the Moho was the crustIII mantle boundary, with layer 3 comIV posed of the rock known as gabbro V overlying the unaltered mantle peridotite has become the generally acVI cepted paradigm. Gabbro is a coarse 500 grained rock similar in composition to VII the basaltic lava that erupts from VIII submarine volcanoes. This rock is IX found exposed on land where deep layers of the earth have been uplifted onto mountain ranges and exposed by X faulting and erosion. It is believed to represent the frozen remains of 1000 magma chambers where lava rising out of the earth’s interior pooled deep in the crust. Gabbro is also found in XI “ophiolite” complexes, bits of fossil ocean crust and mantle commonly found on land in tectonic plate collision zones and in island arcs (see map on page 24). There it underlies XII extrusive pillow lava and sheeted 1500 dikes and overlies mantle peridotite; it is also believed to represent the reGabbronorite Iroctolitic Gabbro mains of magma chambers. The Olivine Gabbro Disseminated FeTi Oxide Gabbro Gabbro FeTi Oxide Gabbronorite ophiolite sequence of pillow lavas– FeTi Oxide Gabbro dikes–gabbros–periodite provides a Lithologic column convincing match to the seismic layering. compiled by JOIDES However, ophiolite crust is generally much thinResolution scientists ner than the seismic crust in the oceans, and the showing the varialavas there often have quite different chemical bility of lithologies compositions than those on the seafloor. Do downhole and the gradation from ophiolites represent broader ocean crust characterpredominantly oxideistics? Hess and geophysicist Walter Munk (Scripps rich gabbros at the Institution of Oceanography, University of Califortop to olivine-rich nia, San Diego) cooked up the idea of drilling a hole gabbros at the botinto the earth’s mantle and sold it to a group of their tom. The figure was constructed by using colleagues, known as the “American Miscellaneous a running average of Society,” who in turn sold it to Washington: Drill a the different lithohole into the mantle all the way through the ocean logic units drilled crust. Dubbed “Project Mohole,” the idea eventually (almost 1,000 in all). foundered on the rocks of the congressional budget office in cost overruns and ballooning budgets. It was later reborn, however, as the successful Deep Sea Drilling Project and its successor, the Ocean Drilling Program. (See Oceanus, Vol. 36, No. 4 for “An Abridged History of Deep Ocean Drilling.”) These programs have incrementally sought to drill deeper, first through the sediments, then through layer 2, which proved to be, as thought, pillow basalts and sheeted dikes, the remains of the lava that erupted onto the seafloor and its feeder channels. But there the drilling stopped. The material 0 Lithologic Variability 20 40 60 80 100 30 • Vol. 41, No. 1 • 1998 Author Henry Dick standing next to the Canadian ROV ROPOS on site in the Indian Ocean. being recovered was too brittle, too broken, and it seemed too difficult to go deeper. In 1984 my colleague Jim Natland (Scripps) and I proposed using tectonic windows in the ocean crust to get to the deeper rocks. Simply go someplace where Mother Earth had done a lot of work for us by tectonically stripping off the shallow layers, and start deep. In 1986, we surveyed the seafloor south of the island of Mauritius at a great oceanic transform fault named Atlantis II Fracture Zone, looking for a place to drill. The survey produced a map of a 200-kilometer long, mid-sea mountain range flanking the fault. In the middle of these mountains was a remarkable flat-topped peak rising up some 5 kilometers from the seafloor. On the walls of Atlantis Bank, as we named the feature, we sampled gabbro and peridotite, proof that drilling here should be productive. Then, in 1987, Jim and I returned to Atlantis Bank aboard the drill ship JOIDES Resolution along with several WHOI colleagues including co-chief scientist Dick Von Herzen, who had backed the project from the beginning. Over 16 days of drilling we recovered 437 meters of gabbro from a 500 meter hole, making Hole 735B the most successful hard Drilling & Penetration History Holes 735B & 504B 0 Leg 69 500 Hole 504B Recovery 10/19/79 Leg 118 12/4/87 Hole 735BB Recovery Leg 70 12/13/79 Leg 83 1000 1/1/82 Leg 176 Leg 111 Leg137 1500 11/27/97 Hole 10/9/86 4/25/91 Leg 140 735B Hole 504B 11/6/91 Leg 2/9/93 148 2000 0 4 8 16 20 23 Drilling Time in Weeks Drilling results for the two most successful hard-rock holes in the ocean crust: Hole 735B penetrated the lower ocean crust formed beneath the Southwest Indian Ridge and Hole 504B the upper ocean crust formed at the Costa Rica Rift in the eastern Pacific. Note that while Hole 504B is deeper, it took nearly four times as long to drill, and only 18 percent of the rock drilled was recovered while some 87 percent of the rock drilled was recovered from Hole 735B. rock hole ever drilled in the oceans. Although we boundary. That’s what expected to find gabbro in the lower ocean crust, we set out to find in these rocks still produced a lot of surprises. Scienspring 1998. To do Arabian tists had long debated whether the lower crust was this, Paul Robinson Basin crystallized from one great steady-state magma (Dalhousie Univerchamber many kilometers across lying beneath the sity), Chris MacLeod ocean ridge. There was no proof for that here— (Cardiff University), rather there were many small intrusions, hundreds Simon Allerton (Uniof meters in dimension, not kilometers. Crystallizaversity of Edinburgh), Madagascar tion here was ephemeral and short-lived. And, a and I organized an Basin great surprise—the magmas deep in the earth were international expediAtlantis Bank moved around as the rocks deformed when the tion on the British ocean crust stretched and ripped apart during Antarctic Survey ship Crozet Basin n tI es w seafloor spreading. The crust simply looked a lot James Clark Ross. th u So different than we expected. Using British Geologic The Indian Ocean is a tough place to get to. It Survey rock drills and took 10 years to get back. Finally, Jim (now at the the Canadian remotely operated vehicle ROPOS University of Miami) and I returned last fall aboard (Remotely Operated Platform for Ocean Science), JOIDES Resolution as co-chief scientists and we went back to see if we could find the cruststarted drilling again to deepen Hole 735B. We mantle boundary on the wall of the transform, and drilled another kilometer, and surprise—everyto map out the great gabbro massif through which thing changed again. we were attempting to drill. What we found was After careful study, the results showed that the more surprises. We did find the crust-mantle deep ocean crust is compositionally zoned. Its upboundary—outcropping on the side of Atlantis per layers are enriched in iron and titanium. In the Bank—only a few hundred meters deeper than lefthand figure on page 30, this is where we stopped drilling. So it appears the Moho reflected in a change in the amounts ~50 Kilometers of different rock types. Many large Basalt Lavas A (11.7 million years ago) & Dikes gabbro intrusions on land are also Rift Mountains Rift Valley compositionally layered, but not this way. In this piece of ocean crust, the Gabbros layers are created as tectonic forces drive the last liquid, with iron and Mantle Flow Mantle Upwelling titanium in solution, out of numerous small magma intrusions when they B (11.5 million years ago) solidify—pushing it from the lowerRift Valley most layers upward to “freeze” at the top of the section. Our discovery of Gabbros this “synekinematic igneous differentiation” reveals a whole new process Detachment Fault never before dreamed of—unique to ocean ridges. In the 10 years we had been absent, C (10.5 million years ago) British colleagues led by Tim Minshull Rift Mountains Block Faulting Rift Valley (Cambridge University) had been by to conduct a seismic experiment, which showed that the Moho beneath Gabbros Atlantis Bank was some 5 to 6 kilometers down. This was much deeper than expected or seemed reasonable in an area where the ocean crust was D (9.5 million years ago) Atlantis Bank believed considerably thinner than Water the usual 6 or 7 kilometers, and where shallow seismic layer 2 (1.5 to 2 kiloBasalt Rift Valley Altered meters) was already gone. Peridotite Gabbros Well, where was the mantle here anyway? If it were on the walls of Atlantis Bank, then the Moho beneath wasn’t the crust-mantle Africa Antarctica G u lf of A Bay of Bengal de n ccadiv eR idg rg Ri d ge ra l India n Rid g Ch nt ago Ce Mascarene Basin s-L a be e Ca rls Somali Basin Central Indian Basin West Australian Basin Madagascar Ridge dg Ri ut he di an So Moza mb Basinique e Perth Basin South Australia Basin a st In di an Ridg e K Highly productive drilling on Atlantis Bank in the Indian Ocean gave scientists a surprising new look at the lower ocean crust. Hypothetical model for the unroofing and uplift of Atlantis Bank to sea level from beneath the Southwest Indian Ridge from the Scientific Results Volume of the Ocean Drilling Program (Leg 118). Once uplifted, the island then submerged as the ocean crust on which it sat cooled and sank over the last 10 million years. OCEANUS • 31 Seafloor photograph of Atlantis Bank shows a wave-cut outcrop of gabbro smoothed and polished by the sands with complex layering emerging from the fossil beach some 700 meters down on the seafloor. A pothole on a wave cut ledge of Atlantis Bank contains a pebble. Fossil ripple marks indicate where Atlantis Bank beach sands have hardened into limestone with a light dusting of modern sediment. An outcrop of layered gabbros is exposed on a wave cut bench on the top of Atlantis Bank. is an alteration front, as Harry Hess once supposed, at least in this one spot in the ocean. And we did, with our British and Canadian colleagues, map out an enormous block of gabbro over the top of the bank—proving that the crust here is relatively intact and not a great tectonic jumble. But we also found a fossil island. And what about that fossil beach? About twothirds of the bank is covered by limestone, with ripple marks, just like those in the sand at a modern beach. However, these were “frozen,” lithified as what was once an island sank beneath the waves millions of years ago. There are little pot holes ground into gabbro rock, still partially filled with pebbles and sand, and headlands and fossil seastacks (isolated erosional remnants of the island). No beach umbrella, but an old rotting fishing net—the place had a very large population of lobsters, crabs, sharks, sea fans, siphonophores, sponges, and other critters. It seems that long ago, the whole massif popped up out of the earth some 6 or 7 kilometers at the intersection of the Southwest Indian Ridge and the Atlantis II Fracture Zone to rise above sea level. This great tectonic island with an area of at least 25 square kilometers then slowly subsided back beneath the waves to its present position some 700 meters below the sea surface. It is a remarkable place, both for its modern biologic community and its unique fossil island characteristics. Flat topped seamounts called guyots are well known in the Pacific. They are submerged volcanic islands whose tops are worn away by wave action as they sink beneath the sea surface. But this is the first tectonic guyot anyone has ever studied. And with it comes a unique paleontological record. Between the many seastacks, boulders, and ledges of gabbros, we deployed our short, over-the-side rock drills—and came back with a lot of limestone in addition to the gabbro we sought. There were fossil clams, snails, a limestone made of sea urchin spines, and a kind of limestone known as oolitic, something that forms in lagoons. But they’re all now 700 meters down. Jim, formerly a Moho chaser like myself, found it so fascinating that he has converted himself into a carbonate sedimentologist to explore the mysteries of that fossil island. This research was funded by the US National Science Foundation with additional support for the author from WHOI’s Van Allen Clark, Jr., Chair. International support was provided by the Canadian Natural Sciences and Engineering Research Council and the British Natural Environment Research Council for remotely operated vehicle and rock drilling operations at Atlantis Bank aboard RSS James Clark Ross. Henry Dick first came to Woods Hole Oceanographic Institution from Yale University in 1975 as a postdoc with Bill Bryan of the Geology and Geophysics Department. He has been a Senior Scientist since 1990. He has a remarkably dedicated wife named Winifred and three small children, Helene, Spencer, and Lydia, who think their daddy goes to sea too much. He’s promised them he won’t even look at the ocean for at least a year, much less get his feet wet—after his next cruise this fall! 32 • Vol. 41, No. 1 • 1998