REPORT on the First InterMARGINS WORKSHOP “MODELING THE

EXTENSIONAL DEFORMATION OF THE LITHOSPHERE (IMEDL 2004)”

Garry D. Karner (Lamont-Doherty Earth Observatory,

Palisades, New York, 10964, USA)

Introduction


Figure 1. “Sunny” downtown Pontresina on the first morning of the InterMARGINS workshop. By early afternoon the snow had been replaced by clear skies and a warm day.

During the week of July 11–16, 2004, in the Swiss Alps, interspersed with brilliant sunshine and snow, 46 researchers working on extensional systems met in Pontresina, Switzerland (Fig. 1). And so commenced the first workshop to be organized by InterMARGINS. The aim of the workshop was to bring together, and improve interactions between, observationalists and numerical modelers working on rifted continental margins. The workshop was an enormous success and the participants were appreciative for its open, frank, and positive discussions and for the dramatic outcrops viewed in the field. The admittedly intensive workshop had as its ambitious objectives: 1) summarize the latest developments in extensional basin and lithospheric deformation concepts, key results (both observational and modeling), and modeling codes developed over the last few years. 2) Benchmark basin and lithospheric codes in terms of temperature structure, strain rates, and subsidence/uplift patterns in space and time. 3) Identify and generate a set of fundamental lithospheric extension observations (geological, onshore and marine geophysical, petrophysical, and petrological data) for constraining and testing theoretical and conceptual models. The workshop was convened by Garry Karner (Lamont-Doherty Earth Observatory, USA), Gianreto Manatschal, (University of Strasbourg, France), and Luis Pinheiro (University of Aveiro, Portugal). The workshop attracted participants from Australia, Brazil, Canada, Egypt, France, Germany, India, Ireland, Norway, Portugal, Switzerland, UK, and USA - a truly international affair (Fig. 2).


Figure 2: Attendees at the first InterMARGINS Workshop, held July 10-16, 2004, in Pontresina. The workshop was convened by Garry Karner, Gianreto Manatschal and Luis Pinheiro, and was attended by 46 invited scientists from 13 countries (Australia, Brazil, Canada, Egypt, France, Germany, India, Ireland, Norway, Portugal, Switzerland, UK and USA).

The workshop was made possible with funding from the European Science Foundation, the UK National Environment Research Council, InterMARGINS, and the US National Science Foundation. Workshop logistics were the responsibility of the US MARGINS Office and the University of Strasbourg. Attendees were invited following reponse to an EOS advertisement – the workshop organizers and an international selection committee made a preliminary selection list, with final approval of the list being made by the InterMARGINS Steering Committee.

Workshop Format

The workshop consisted of three different but intimately related components:

1) A series of invited presentations, which summarized: 1) the latest research and results dealing with lithosphere extension modeling, 2) new algorithms and engineering applications that could be employed to model the deformation of the lithosphere, and 3) details of the pertinent observations and experiments crucial for constraining and testing models of lithospheric deformation and basin evolution.

2) A set of smaller presentations (or “mini-talks”) was presented allowing workshop attendees to present their own work and views (alternative ideas, results and concerns) relative to the themes outlined above.

3) Three field trips, which were an integral part of the workshop theme and which were intended to reinforce the theme of the invited and short presentations and to underscore the importance of including pertinent and critical observations not presently encapsulated in the modeling efforts. The field excursions were not just a “convenient break” during the workshop, but were an attempt to integrate at a fundamental level the field geology of extensional systems with basin and lithosphere deformation modeling.


Figure 3. A geological field trip adrenalin rush for many of the participants – the use of choppers to explore the Err nappe. The clear and bright day of the lower valley contrasts with the drop off point at 3000 m (Fig.5).

The keynote presentations set the foundation and tone for the workshop. Each keynote provided an objective overview of the geology and geophysics of a particular extensional system. In particular, presenters were asked to define: What are the pertinent observations and experiments crucial for defining, constraining, and testing models of lithospheric deformation? What fundamental observables does a model need to capture to be "successful"? How does the present research effort help constrain the mechanical, rheological, thermal, and structural development of extensional systems? Because the workshop consisted of two very different audiences, one dedicated to modeling extensional systems (both theoretical and applied) and another mapping extensional systems, both groups needed to learn to understand the challenges facing each other and to speak the same language – the keynote addresses did an excellent job in achieving this objective.

Basin Modelers: The benchmarking exercise

A main focus for the modelers in the Pontresina workshop was a benchmark comparison of efforts to model a combined seismic section across the Iberia and Newfoundland margins, including their respective ocean-continent transitions, augmented with seminal research manuscripts and links to ODP data sets. The benchmarking exercise consisted of an invitation to model a combined Iberian-Newfoundland transect, using ISE-1 and SCREECH-1 multichannel seismic reflection data, refraction inversions and tomography for crustal velocity structure and Moho topography, and ODP drilling constraints. It was already noted that in all likelihood, no one model would likely capture all of the geological and petrological nuances of the Iberian-Newfoundland transect - some models will be able to deal with the details but offer no insight into the large picture, while other modeling efforts will be in the opposite situation, no detail but important insights into the general lithospheric architecture of extensional systems. That is, the various codes and modeling packages will address various scales of the real-world problem.

It was crucial to define what the range of those nested scales are, where there is overlap, and where the modeling efforts are independent. The model predictions and simulations consisted of some combination of the following: 1) subsidence/uplift history, 2) sedimentation and erosion history, 3) crustal geometry (basin architecture and Moho topography), 4) crustal/lithospheric temperature structure as a function of time, 5) fault development in space and time and fault sequencing and offset history, 6) heat flow history, and 7) gravity and geoid across the modeled section. The benchmarking exercise goal was to test for conformity, differences, and most importantly, attempt to explain the differences between the predictions and simulation of the various modeling packages. While some modelers attempted the benchmarking exercise, other modelers took a more thematic approach and explored parameter sensitivity in terms of the geometries of large extensional systems. While the modeling results provided important contributions and insights into understanding extensional systems, it became clear that the Iberian-Newfoundland transect was a complex and difficult example to model.

Observationalists: Extensional Systems

“Non-modelers” were expected to prepare an objective overview of the fundamental constraints that their research can provide on the thermal and dynamical structure of extensional systems, to summarize how the models can further their science, and to participate in the discussion of the feasibility of the various proposed models. Many of these overviews formed the foundation of the keynote presentations.

During the meeting, we "stumbled" upon a very exciting second phase of the Iberia-Newfoundland transect modeling in which the key constraints required by the modelers were supplied by the experts working in this area. Through workshop consensus, it was determined that the fundamental modeling constraints considered essential for extensional modeling, in general, and the next phase of the benchmarking and modeling extensional systems, in particular, include:

·  Crustal thickness (error range)

·  Lithospheric thickness

·  Radioactive heat generation (surface heat production)

·  Initial temperature structure

·  Strain rates

·  Onset of rifting/cessation of rifting/rift history (i.e., multiple rifting events)

·  Magma volumes and when does magma production start

·  Crustal/mantle composition (for rheology)

·  Syn-rift/post-rift sediment history (form of syn-rift and post-rift subsidence)

·  Profile length (spatial scales of deformation)

·  Number and age of unconformities/discontinuities

·  Pre-rift geology (pre-existing weaknesses; large faults or large batholiths)

·  Paleobathymetry constraints (pre-, syn- and post-rift estimates of water depth)

Workshop Fieldtrips

The workshop field trips were an integral part of the workshop theme. The interspersing of presentations with field trips proved to be extremely important in relaxing the head while exercising the body and vice versa. The dedication and enthusiasm of Gianreto Manatschal and Othmar Müntener resulted in an incredibly dynamic, insightful and exciting field trip that reinforced the theme of the workshop. The three field trips were:

Excursion 1: A Preserved Ocean-Continent Transition (Tasna nappe)

·  Preserved structure of an Ocean-Continent Transition.

·  Importance of detachment structures and their relationships to lower crustal rocks, mantle rocks, magmatic rocks, and post-rift sediments.

Excursion 2: Rift-Structures in the Distal Margin (Err-nappe)

·  Preserved low-angle detachment structures.

·  Study of pre-, syn- and post-rift sequences and their relationship with detachment faults.

Excursion 3: Basin Geometry and Stratigraphic Record in the Proximal Margin (Il Motto, Ortler-nappe)

·  Preserved rift-basin architecture.

Study of pre-, syn- and post-rift sequences.


Figure 4. Leaders of the very successful workshop excursions, Gianreto Manatschal and Othmar Müntener, give a passionate discourse on the local Swiss geology to eager workshop participants.

Figure 5. The 1st drop off point for Excursion 2 – the upper Err nappe at 3000 m.
Detailed list of observational and modeling issues for extensional systems

As a result for five days (and nights) of intensive discussions, a series of recommendations were produced relating to the next generation of data sets, observations, processes requiring research and modeling approaches, cross-verification and benchmarking.

Observational/process issues

/

Modeling issues

1. Timing and rates of deformation (require sediment age and thickness, paleowater depth information, radiometric data) / 1. Understanding/describing the kinematics of extension
2. Formation of “spreading” magnetic anomalies / 2. 3D dynamic modeling - testing conceptual models/processes
3. Mapping the rift-spreading transition / 3. Testing conceptual models/processes vs. simulating reality (the two end-members of the philosophy of modeling)
4. Petrology of syn-rift basalts and early post-rift basalts / 4. Quantify/understand new physical and chemical processes in extensional systems
5. Physical properties vs. seismic data (e.g. velocity, rock properties, Q, composition) / 5. Feedbacks between kinematics and dynamic models (cross-validation)
6. Along-strike variations on margins / 6. Fundamental driving processes (diff. kinematic situations)
7. Changing mode of extensional deformation – why? (e.g., magma production, rate changes) / 7. What is the first order behavior of extensional systems:
·  From observations?
·  From kinematic models (bridge with geodynamic models)?
·  From geodynamic models?
8. More quality 2D and 3D seismics – better penetration (both data sets are extremely important) / 8. Utilizing new rheology lab results in models
9. Scales of processes vs. scales of observations. Characterization of crust/rocks – ambiguities? / 9. What do the models provide?
·  strain history
·  heat flow history
·  rheology?
·  fault weakening parameters?
·  PTt of upper crust/exposed mantle
10. 3D geometry and rate constraints required (e.g. seismics and GPS) / 10. Can models be consistent with a total data set?
11. Recognition of pertinent data sets to constrain models and simulations / 11. Under what conditions does the lower/middle crust flow? Rheology, mineralogy, heat/°C, strain rate and water content
12. “Missed” processes in modeling extensional systems:
·  water/convection in mantle
·  magma in extensional systems
·  radioactivity of crust (initial conditions) / 12. Decompression melting and serpentinization included in extension modeling
13. Depth-dependent extension/partitioning – exactly how is it balanced and by what mechanism(s) does it occur? / 13. Modeling of gravity and geoid; constrained by isostasy (local or flexural)
14. Seismic character/properties, velocity, etc., plus new techniques (MT and EM) / 14. Modeling faults - what are detachments and how are they generated (low-angle normal faults; the S-reflector; decoupling surfaces)?
15. Timing of fault movement – related to syn-rift history, fault growth, fault migration (again, rates and timing)
16. Melting processes/ magmatism, the window to mantle convection:
·  tomography
·  geochemistry of melts
·  post-rift subsidence
·  gravity/geoid of active rifts
·  seismic anisotropy & attenuation
·  mantle/magma interactions
Post-workshop educational outreach

To ensure that the workshop results are accessible for the whole scientific community, all keynote presentations, poster presentations, and field guides will be available on the US MARGINS web site (http://www.imedl.wustl.edu/). The keynote presentations and the second iteration of the benchmarking modeling exercise will form the basis of an InterMARGINS-sponsored publication. The workshop conveners will serve as editors of the book and workshop attendees will review all contributions. We also hope that this first Intermargins workshop produced new ideas on how modelers, observationalists, and experimentalists can strengthen their collaborative efforts to address common problems and identify what type of observations and experiments are necessary to better understand and model local and regional extensional systems.

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