Earth history is punctuated by continents breaking apart and births of oceans, phenomena we are witnessing today in the high Arctic, East Africa/Red Sea, the Gulf of California, and the Western Pacific. The tectonic, magmatic, geodynamic, and sedimentary processes associated with continental rifting and breakup interact to produce a variety of margin styles of rifting and breakup, ranging from narrow to hyperextended margins, from weakly to strongly magmatic margins, and from sediment-starved to thickly-sedimented margins. The wide variety of styles has puzzled and inspired earth scientists for generations, who have employed an increasingly diverse and sophisticated set of observational tools to investigate and conceptual models to explain continental rifting and breakup.

A trio of Integrated Ocean Drilling Program (IODP; www.iodp.org) platforms and associated technologies available beginning in late 2007 will provide researchers world-wide with enhanced capabilities for sampling, logging, and monitoring the Earth's crust and uppermost mantle beneath the sea. Opportunities offered via the proposal-driven IODP by the new Japanese riser drill ship Chikyu, the new to-be-named U.S. non-riser drilling vessel, and shallow water and Arctic mission-specific platforms provided by the European Consortium for Ocean Research Drilling (ECORD) essentially lay bare the entire Earth beneath the sea for investigation.

Fifty-one scientists (Figure 1) from six continents met in Pontresina, Switzerland, in mid-September to discuss strategies for advancing understanding of continental rifting and breakup using the new drilling platforms and associated technologies. Over four days of plenary and breakout group meetings, including a one-day field trip, observational scientists and modelers educated one another about the world's continental margins, discussed the outstanding problems related to margin initiation and development, and outlined a global mission to address these problems via drilling and monitoring, in conjunction with complementary geophysical and geological studies.

Figure1: Workshop participants on the Tasna ocean-continent transition.

Setting the Stage

Rifted continental margins globally span a continuum from highly magmatic to magma-poor, or hyperextended, although they are typically characterized as volcanic or non-volcanic end-members, respectively. To set the stage for intensive discussions, workshop participants heard global overviews of volcanic (Sverre Planke, Volcanic Basin Petroleum Research, Norway) and non-volcanic (Timothy Reston, University of Birmingham, UK) rifted margins, keynote presentations that illuminated both magmatic and tectonic aspects, respectively, of continental rifting and breakup. Focusing on the two most-studied conjugate margin end-members, Norway-East Greenland and Iberia-Newfoundland, respectively, Planke and Reston summarized current knowledge, hypotheses, and outstanding problems. The next keynote presentation (Anthony Watts, University of Oxford, UK) homed in on the role of the IODP in addressing key themes in continental rifting and breakup. Critical outstanding problems highlighted in the three keynote addresses included (1) determining the geochemistry, ages, volumes, fluxes, and emplacement processes of magmatism during rifting and breakup, important for understanding both geodynamic processes and potential paleoenvironmental impacts; and (2) overall rifting history (including PTt paths), symmetry/asymmetry of extension and breakup, timing and geometry of detachment faulting, vertical crustal movement and strain rate histories, the extent of mantle unroofing, the role of pre-existing crust (e.g., in rift localization and segmentation), and evolution of the continental lithosphere during rifting. The fourth and final keynote presentation (Greg Myers, IODP Management International) introduced and compared capabilities of the three IODP platforms (riser, non-riser, and mission-specific), summarized drilling and coring techniques and tools, and detailed downhole logging and observatory opportunities for researchers.

Margins around the Globe

The depth and breadth of expertise among workshop participants enabled sharing of continental rifting and breakup knowledge from around the globe. Active rifting in the Gulf of California, Woodlark Basin (Western Pacific), Lena Trough (Arctic), and East Africa/Red Sea is literally "where the action is," offering contemporary examples for investigations into the tectonic processes controlling and accompanying rifting and breakup. However, it appears that no highly magmatic margins are forming today, so ancient examples such as the conjugate East Greenland-Norwegian (global type example), conjugate Kerguelen/Naturaliste-East Antarctic/Bruce Bank, conjugate South Atlantic, and the conjugate-less West Australian margins constitute areas for investigation of magmatic processes associated with rifting and breakup.

The conjugate Iberia-Newfoundland margins, although inactive, constitute the global type example of hyperextended rifting and breakup, offering an array of opportunities for tectonic hypothesis-testing using drill cores and downhole logs as ground truth. Dense industry data coverage of the South Atlantic margins north of the Sao Paulo Plateau (South America) and Walvis Ridge (Africa) provides complementary prospects for understanding another hyperextended conjugate margin pair. Tectonic understanding may also be advanced at relatively young conjugate rifted margins such as those flanking the Gulf of Aden, the Adare Trough (East Antarctica), as well as at the margins of the marginal or backarc basins of the Western Pacific (e.g., South China Sea, East Sea/Sea of Japan, Shikoku Basin, Parece Vela Basin), the Aegean Sea, and the Black Sea.

Alpine Ground Truth

Northern Italy and the Engadin region of southeast Switzerland feature well-exposed remains of continental rifting and breakup. Workshop participants devoted a day to viewing, examining, and discussing remnants of hyperextended ancient Tethyan continental margins, led by native Engadiner Gianreto Manatschal (Universite Louis Pasteur, Strasbourg, France). The first locality, Livigno/Il Motto in Italy, preserves structures of the proximal Adriatic margin, and was viewed from a distance. The second locality, the Tasna ocean-continent transition above Scuol in the Lower Engadin of Switzerland, afforded participants the opportunity to walk from the continent to the ocean side of the transition and examine changing rock types and deformational styles (Figure 2). Both locales are analogs for the hyperextended conjugate Iberia-Newfoundland margins.

Figure2: View of the ocean-continent transition exposed in the Tasna nappe in Graubunden, southeast Switzerland (modified after Florineth, D., and Froitzheim, N., 1994. Transition from continental to oceanic basement in the Tasna nappe (Engadine window, Graubunden, Switzerland): evidence for Early Cretaceous opening of the Valais ocean. Schweiz Mineral. Petrogr. Mitt. 74: 437-448.). UTD: upper Tasna detachment; LTD: lower Tasna detachment.

Nuts and Bolts of IODP Proposals and Drill Site Characterization

Practical matters necessary for scientific ocean drilling were the subject of two presentations: 1) the IODP proposal process, including preparation, submission, nurturing, evaluation, ranking, and scheduling via the IODP Science Advisory Structure, IODP Management International, and the implementing organizations (Mike Coffin, University of Tokyo, Japan), and 2) drill site characterization, primarily on the basis of geophysical (seismic reflection) data, but also including geological data, and electronic submission of digital site characterization data to the IODP Site Survey Data Bank (Dale Sawyer, Rice University, USA). A proposal benefits from a team of co-proponents whose expertise encompasses all aspects of the proposal, including drill site characterization. Following drilling expeditions, both operational and scientific reviews are conducted.

During the workshop, participants presented and examined many site characterization data, especially seismic reflection and refraction, and highlighted the importance of linking such data with drilling much more closely. They endorsed the principle of integrated site characterization and drilling within the IODP, and unanimously recommended that the IODP provide financial support for site characterization as well as drilling in addressing high-priority scientific themes and initiatives of the IODP.

Key Themes

Salient problems involving continental rifting and breakup discussed at the workshop fall into six themes: a) rift initiation, b) tectonic and dynamic aspects of rift evolution, c) magmatic aspects of rift evolution, d) sedimentary, paleoenvironmental, and oceanographic aspects of rift evolution, e) initiation of seafloor spreading, and f) consequences and impact. Rift initiation objectives include understanding rift dynamics, inherited lithospheric conditions, and early magmatism. With respect to tectonic and dynamic aspects of rift evolution, goals are to elucidate both strain distribution and variations in dynamics in space and time. Key targets in magmatic aspects of rift evolution are increased knowledge of melting, magmatic flux, magma transport, and emplacement. For sedimentary, paleoenvironmental, and oceanographic aspects of rift evolution, the aim is to increase comprehension of paleogeography, paleoenvironments, and the development of depositional systems in space and time. The purpose of investigating the initiation of seafloor spreading is to glean information on seafloor spreading ridge evolution as well as the tectonic and magmatic response on the preferably conjugate margins. Finally, studies of consequences and impact will target paleoenvironmental changes related to rifting and rift magmatism, and the prospects for eventual subduction or obduction.

An IODP Mission

The IODP is poised to consider a new type of proposal, that for a mission, which is an intellectually integrated and coordinated drilling strategy originating from the scientific community that: a) addresses a significant aspect of an IODP Science Plan theme on a global basis over an extended period of IODP, and b) merits urgent promotion in order to achieve overall IODP program goals. Workshop participants reached consensus to develop a mission proposal addressing continental rifting and breakup globally that provisionally will target active rifting and breakup in the Gulf of California and Woodlark Basin, highly magmatic margins (e.g., conjugate Norwegian-East Greenland and conjugate-less Western Australian), and the hyperextended conjugate margins of Iberia-Newfoundland and the South Atlantic. Spearheaded by John Hopper (Texas A&M University, USA), the mission proposal will be developed between now and an anticipated 1 April 2007 submission by a team of observational and modeling specialists working on mission component themes. Importantly, however, the mission proposal will not exclude individual proposals on any aspect of continental rifting and breakup from being developed and submitted to the IODP by interested proponent groups.

The 'Investigating Continental Breakup and Sedimentary Basin Formation' workshop, presented by IODP Management International and InterMARGINS, was held 15-18 September at the Rondo Convention Center in Pontresina, Graubunden, Switzerland. A scientific white paper on continental breakup and sedimentary basin formation is in preparation for publication in Scientific Drilling (ISSN 1816-8957), and the full workshop report is scheduled to be available in late 2006 at www.iodp.org, which is also the source of comprehensive information about the IODP.

MILLARD F. COFFIN, University of Tokyo, Tokyo, Japan;
DALE S. SAWYER, Rice University, Houston, Texas, USA;
TIMOTHY J. RESTON, University of Birmingham, Birmingham, England, UK;
JOANN M. STOCK, California Institute of Technology, Pasadena, California, USA.