It's always good to have a plan, even though they are seldom followed. I usually keep my cruise plans secret so no-one notices when they fail. This one is written down (a) so that participating students can be better prepared (b) so interested crew members have a better idea of our intentions and (c) to remind myself after the cruise of what we were supposed to have done.

Alternative plans B through F (not written down) accommodate such possible nuisances as equipment problems, bad weather, and failure to obtain necessary diplomatic clearances for research in Exclusive Economic Zones.

Leg 9 is basically a 15-day transit to which 3.7 days have been added to allow a scientific program. The three objectives of the leg are: 1. To return Melville and her crew to San Diego before the end of the Padres` winning season, specifically by 0800 June 29. 2. Scientific exploration of several patches of seafloor that can be reached with minor diversions from the direct Samoa-San Diego route. 3. The education of professor and students.

Overall Route

As shown in map 1, we will initially head south from Pago Pago toward the island of Niue, and spend 2 - 3 days of our added time working on objectives A-D in the Samoan region. There will then be a long (10 day) straight transit across the equator to the Molokai fracture zone/Guadalupe area (objectives E and F) where we will use our remaining time surveying before a 12-hour transit into San Diego. The transit from D to E will cross several enigmatic and important structures that we will not have time to investigate, but which our single Seabeam 2000 swath may help clarify. These include the northwest side of Manihiki Plateau, where the southern part of the East Pacific Rise originated; the Line Islands Ridge and some of its "cross-trend ridges"; Clipperton and Clarion fracture zones and several smaller normal and oblique fracture zones revealed by recent satellite altimetry; and the zone of widespread erosional topography and manganese nodule fields north of Clipperton fracture zone. Much of the seafloor structure on this "transit" part of the leg will be obscured by the thick equatorial sediments. (We will not have a seismic profiler to look through these sediments.) I will offer a couple of lectures about the equatorial current systems responsible for the enhanced pelagic sedimentation; otherwise the students will be busy processing data and samples from A-D. (We may not maintain a close 24 hour scientific watch during this segment.)

Equipment to be used

Primary survey tool will be the Seabeam 2000 sonar, with its bathymetric and side-scan displays, supplemented by a towed magnetometer and 3.5Khz acoustic profiler. A rock dredge and pinger will be used at site A. XBT (bathythermograph) probes that measure the upper-water velocity structure (needed for Seabeam 2000 operation) will also be used to identify water masses and current system.

Scientific Objectives

A) Drowned atoll geology. At the request of Profs. Hawkins and Winterer, 16 hours have been added to this leg for studying a large (unnamed?) seamount with a submerged coral cap, about 40 miles south of Pago Pago. We will survey the volcanic side-slopes and the perimeter of the coral cap with Seabeam 2000 circuits, and try to recover rock samples by dredging both environments (aiming for 2 dredge samples from 2000-3000m deep sides, and 2-3 samples from the 25-50m deep cap). Questions to be addressed by the samples and survey include: (i) how old is the volcano and how thick is its coral cap (ii) how is its origin and petrology related to the hotspot and/or propagating tear origin of the Samoan Islands and (iii) when and why did its coral cap die. There are several other similarly shallow drowned atolls clustered west of Samoa (in the "Melanesian Border Plateau" region), and one still emergent example (Rose Atoll) at the eastern end of the chain.

B) Structure and age of Niue Trough and related fractures. The boundary of the Pacific plate has a sharp bend at the north end of the Tonga Trench (map 2). The plate south of this bend gets subducted at the trench, the plate to the north, around the Samoan Islands, continues northwest on the surface. A large hinge fault appears to separate these two parts of the plate; we will cross and examine its eastern end, where it cuts sediments of the Samoan apron. Further south there is a large subparallel structural trough, Niue Trough, which bisects an unusually shallow trench-outer-rise and then turns to intersect the lower volcanic slopes of Niue. Although this has a less obvious origin than the hinge fault, Niue Trough also appears to be a tectonically active response of the approach of the Pacific plate to the bend of the Tonga Trench. We will augment Seabeam 2000 swaths collected across this structure in 1994 and 1995 to examine its relationship to the pre-existing plate fabric and prepare a genetic model that can be tested with satellite altimetry.

C) Age and origin of oceanic crest in the Samoan region. The seafloor of the Samoan Basin north and east of the Samoan Islands has lineated magnetic anomalies, but they have never been convincingly identified to determine the age of the crust, and which plate boundary it accreted at. New altimetry data has delineated fracture zones in this crust (eg. a major north-south one just east of Rose Atoll) and recent Seabeam 2000 swaths have revealed the direction of abyssal hill lineations (basically east-west, veering to NW-SE towards the north). We may be able to unravel the magnetic pattern (and thus the accretion history) by collecting a Seabeam swath and magnetic profile to the west of the Rose Atoll fracture zone (i.e., during our track to and across Niue Trough) and, especially, on a northbound track east of the fracture zone to the margins of Manihiki Plateau. We will sidestep seamounts (located by satellite altimetry) that would otherwise perturb the magnetic profiles.

D) Erosional topography of Manihiki Plateau. This plateau is an area of thickened oceanic crust created by voluminous Lower Cretaceous volcanism and with a thick blanket of pelagic sediment. As on most Pacific plateaus, part of this blanket has been dissected by a change from a depositional to an erosional regime that may have been caused by changes in bottom currents, deep-sea tides, or water chemistry. We will visit an eroded site identified on earlier profiler crossings, to see from swath bathymetry whether the erosional terrain is a honeycomb of enclosed depressions (as we have found or several other plateaus), or is a dendritic canyonlike system. The geomorphology will help distinguish which processes may have been responsible for the erosion.

E) The demise of Molokai transform. Molokai fracture zone, one of the great North Pacific traces of Pacific-Farallon transform faults, has an abrupt eastern end near 26N, 125W. My previous Seabeam and magnetic surveys of this region indicate that this marks the conversion of a complex, multi-strand transform system into a microplate-bounding propagating rift system, at 29-28 Ma. Augmenting the existing surveys with a few strategically located swaths and magnetic profiles will clarify how and why this happened, with both topical and regional implications (how transforms change their offsets, how microplates are created and captured, how spreading axes become extinct, how the "Murray-Molokai confused zone" was formed, etc).

F) The abandonment of the Guadalupe spreading centers. Previous mapping has discovered a pair of abandoned spreading centers, north and south of Guadalupe Island. This is another place where a carefully located Seabeam and magnetic swath, threaded through gaps in the existing data coverage, should provide definitive evidence on the events which led to this abandonment.

Some references (I will bring all these to the ship.)

Natland (1980). The progression of volcanism in the Samoan linear volcanic chain. Am. J. Sci., 280A,709-733 [Objective A]

Winterer et al (1995). Karst morphology and diagenesis of the top of Albian limestone platforms, Mid-Pacific Mountains. Proc. ODP, v. 143,433-470. [A]

Fairbridge and Stewart (1960). Alexa Bank, a drowned atoll on the Melanesian border plateau. Deep-Sea Res., 7100-116, [A].

Lonsdale (1975). Sedimentation and tectonic modification of Samoan archipelagic apron, AAPG Bull,780-798 [B].

Wright (1992). Convergence and strike-slip motion at the northern terminus of the Tonga Trench. CRC Handbook35-79, [B].

Winterer et al (1974). Structure and acoustic stratigraphy of the Manihiki Plateau, Deep-Sea Res,21,793-814, [D].

Searle et al (1993). The Molokai fracture zone near Hawaii. Geophys. Monograph 77155-169, [E].

Smith and Menard (1965). The Molokai fracture zone. Prog. in Ocean, 3[E] 333-345.

Lonsdale (1991) Structural patterns of the Pacific floor off peninsular California, AAPG Memoir 47,87-125 [F