Lessons in Tectonics, Climate, and Eustasy from the Stratigraphic Record in Arc Collision Zones
Price, Utah USA
10–14 October 2005
- Peter D. Clift
- Schools of Geosciences, Meston Building, University of Aberdeen, Aberdeen AB24 3UE, UK
- Amy E. Draut
- University of California at Santa Cruz and U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA
Active plate margins are of great research significance to the earth science community because they are the primary locations for the formation and destruction of the continental crust, and because they also appear to be important controls on the long-term evolution of global climate. Moreover, they are of great societal importance because of their potential for hydrocarbon exploration in the deep basins formed in these settings, as well as for the dangers associated with the earthquakes and tsunamis that characterize these regions. Although subduction margins are often active for long periods of geologic time, they are rarely steady-state in their tectonic evolution. These regions are strongly affected by episodic collisions between the subduction trench and topography on the subducting plate.
On 10–14 October 2005, 45 geoscientists from 10 countries met for a GSA Penrose Conference in Price, Utah, USA, to discuss how the sedimentary record in such areas can be used to reconstruct arc collisions in the past and to assess their effects on the tectonic, erosional, and climatic development of active plate margins. The conference included oral and poster presentations of recent research on arc collisional settings and a scientific planning phase to identify future topics for focused research.
A number of areas in which full understanding of arc collisional tectonic and sedimentary processes is lacking were identified. Some of the most pressing research goals in the modern oceans are being addressed by proposed Integrated Ocean Drilling Program (IODP) operations in Costa Rica, the Nankai Trough, the Okinawa Trough, the Izu-Bonin forearc, and the Yakutat-Alaska Collision Zone. Nonetheless, other goals remain unresolved, requiring new research efforts if we are to fully understand the role that arc collisions play in such first-order processes as the formation of the continental crust, control of global climate, and societally relevant issues such as seismogenesis and tsunamis. A combination of offshore and onshore work in modern settings, together with informed exploitation of ancient examples from the geologic record, hold the best chances for major progress in the next five to ten years.
Although simple climate-tectonic models have been developed over the past few years, the detailed role that evolving climate plays in collisional tectonism remains obscure. There is general consensus that precipitation can govern orogenic structure at least to some extent, but the connection between orogenic-trench tectonics and sediment flux is less well defined. The formation of the Western Pacific Warm Pool appears to have been triggered by an arc collision, yet its presence affects the local climate and oceanography, most notably the Asian monsoon, and as such it may in turn affect the subduction and collisional tectonics of the western Pacific, including Taiwan. Defining in detail how such linkages work, here or in other settings, was highlighted as target for future research.
This topic is connected to another identified goal of understanding how increased sediment flux to the trench can influence subduction tectonism, either causing more or less tectonic erosion of the overriding plate and at the same time enhancing or reducing seismic coupling across the plate boundary. This is important because it is such coupling that can result in catastrophic earthquakes and their associated tsunamis. The sedimentary record at arc collisions has yet to be fully exploited to help understand these events (such as the 2004 Indian Ocean Tsunami) and to help plan responses to future events. Techniques have been developed to look at storm deposits in coastal settings, yet in only a few locations have they been applied to active margins. Use of the deep-sea record to understand seismogenesis is in its infancy, but this could potentially provide a readily dated, more complete history of seismicity than is available onshore.
Similarly, it is still conjecture whether arc collisions can change oceanic circulation patterns beyond the cutting of oceanic gateways. Collision of the Yakutat Terrane with North America must have increased orographic precipitation following uplift and thus driven increased run-off into the Bering Sea. The freshening of these waters reduced their ability to sink and in turn drive thermohaline circulation in the North Pacific. The cessation of deep-water flow from the Bering Sea into the North Pacific seems to have resulted in a significant reorganization of circulation and upwelling patterns over the region. Demonstrating that enhanced fluvial run-off from collisional orogens has changed oceanic circulation patterns should be a goal for describing a new type of climate-tectonic coupling.
If some of these higher goals are to be addressed, then it is crucial to better understand the processes that generate forearc sedimentary records in the first place, so that the evolution of arc collisions can be reconstructed in ancient examples. We rely on this record to date collisions and to trace rates of subduction erosion and/or accretion, yet the transport of material to forearcs is not well understood because of the complicated tectonic morphology often seen in these areas. The conference attendees agreed on the need to determine to what extent the stratigraphy is formed by steady-state processes and how much through catastrophic events, such as collapse of volcanic centers or tsunami generation. As part of this effort, we also need to identify ways to make better long-distance correlations in volcaniclastic forearc deposits, which are often hard to trace far across or along strike. Tephra and mass-wasting deposits from major explosive eruptions seem to offer the best chance of making such correlations, though determining how to make possible correlations convincing is not yet clear. Detailed radiometric dating would seem the most promising avenue to pursue, given recent analytical advances in inductively coupled plasma–mass spectrometry technology. Along with dating, we need to develop additional, more detailed methods to quantify tectonic and erosional processes in the source regions from the sedimentary record. Improved, less ambiguous provenance methods are required to date collisions and quantify the response to these events. Single-grain isotopic methods are needed as supplements to, but not replacements for, detailed classic petrographic studies. While methods based on rare mineral groups can be informative, they can be hard to apply in some settings, such as in drill cores, where sample size is an issue. As well as being of great academic interest, the development of new techniques for studying provenance and exhumation would be of use to petroleum exploration in forearc regions.
Such analytical methods will, however, not result in better understanding until the best locations in the forearc for the study of arc evolution can be identified. This is because trenchward basins are usually too tectonically disrupted by the collision itself to form continuous records and are often separated from the source volcanoes by forearc topography, instead being filled by reworking of older arc sediments. Conversely, those parts of the forearc closest to the arc are subject to episodic slumping and mass wasting, preventing formation of continuous sequences. Further surveying and drilling of modern forearcs are needed if the optimal location of continuous forearc records is to be better defined for the general case.
Conference attendees also recognized the need to better define rates of mass gain and loss in forearc regions. Although some progress has been made in quantifying rates of trench retreat or growth, it has become apparent that in many settings the simple assumption of steady-state forearc geometry cannot be made. This is especially true given the increased sediment flux to trenches since the Pliocene that favors subduction accretion compared to the conditions that prevailed over longer periods of geologic time. The conclusion appears to be that mass loss cannot be reliably estimated based on one forearc drill site, but requires transects, including onshore basins where those exist. Identifying ways to reconstruct subaerial uplift as well as submarine subsidence would be a crucial part of this challenge. Resolution of this issue is fundamental to geochemical and geophysical models for planetary-scale crustal recycling because uncertainties in the present mass recycling budget are very high and make accurate predictions of crustal production rates impossible.
|Penrose Conference Participants
Click on image for larger photo.
Continuous sedimentary records are required to shed light on a number of issues related to arc–passive-margin collisions. Can the sedimentary record be used to identify and quantify lower crustal delamination of the colliding arc crust during collision? Such delamination is predicted by geophysical modeling but has yet to be convincingly identified in modern arc settings. Similarly, can the sedimentary record be used to quantify and date the nature of post-collisional extension and collapse? In the classic example of Taiwan, debate continues as to whether Okinawa Trough represents a basin formed by trench forces that is coincidentally propagating into the arc collision or whether Okinawa Trough is a product of that collision. If it is the latter, are there other examples of similar processes in modern and ancient records? Determining whether Okinawa Trough is truly a new type of marginal basin would be a first-order accomplishment for active-margin tectonics.