• Tectonic and stratigraphic response to flexure across a continent-ocean transform boundary: Falkland/Malvinas Plateau, South Atlantic. 

• Juan M. Lorenzo; Department of Geology and Geophysics, Louisiana State Univ., Baton Rouge, LA 70803-4101; ph. 504-388-3353; fax 504-388-2302; e-mail: juan@tellus.geol.lsu.edu, Paul Wessel, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, U.S.A. 


We examine existent bathymetric, gravity and multichannel seismic reflection data across the northern Falkland/Malvinas Plateau continent-ocean transform boundary to determine the degree of mechanical coupling across the fracture zone after ridge-transform intersection time. Previous geodynamic models highlight the importance of continent-continent and ocean-continent shearing along the active transform. However, in contrast to oceanic fracture zone settings, the influence of coupled flexure on the stratigraphy and tectonics of these margins remains undocumented.

The fossil transform basement rim of the Falkland Plateau comprises a micro-continental block (Maurice Ewing Bank) in the east and a continental marginal fracture ridge 50-to-100 km wide elsewhere. Strike-slip faults and transpressional folds are the dominant structures, developed during the active transform stage. The central Plateau (Falkland Basin) is underlain by up to 7 km of Middle Jurassic-to-Recent sediments and partly floored by overthickened oceanic crust. Depositional sequences can dip either south from the marginal fracture ridge, or north from the Maurice Ewing Bank.

Gravimetric isostatic anomalies at two representative transects across the margin (42deg. W and 52.5deg.W) indicate that vertical shear stresses are consistent with differential thermal subsidence models in a locked fault regime. By forward modeling free-air gravity anomalies we interpret that the marginal fracture ridge is coupled mechanically to oceanic elastic lithosphere both north (Early-Middle Cretaceous Argentine Basin) and south (Middle Jurassic Falkland Basin). Flexural subsidence may offset effects of thermal uplift induced by heating from the oceanic lithosphere.



Fig. 2 Satellite-derived free-air gravity anomaly map (Sandwell et al. 1995) over the Falkland Plateau study area; bathymetry contours are superposed. Study area is located west of the South America. A negative gravity anomaly lineament in the Argentine Basin, running along the 6000 m isobath and parallel to the Falkland Escarpment corresponds to a linear negative magnetic anomaly modeled by Rabinowitz and LaBrecque (1979) as an oceanic fracture zoneBy extension, parallel lineations further north may also overlie fracture zones. Two thick black lines locate the extent of north-south sections used for modeling and seen in other Figures in paper (5A, 5E). 



Fig. 1 (A) Schematic plan view of continent-ocean active and passive transform boundaries. RTI: ridge-transform intersection, FZ: fracture zone, TF: transform fault. (B) Profile through the fracture zone, landward of the RTI shows effect of assuming a mechanical model where lithosphere on either side of fracture zone subsides at different rates but remains mechanically coupled. Only crustal blocks used for gravity modeling are shown. Total amount of subsidence implied by the length of the arrows is greater on the oceanic side. A dashed line marks the initial top of the crust at RTI time. Predicted bathymetry has a thermally younger (oceanic) side that bows up whereas the thermally older (continental) side bows down.

Published versions of figures above are available in the following issue of Geo-Marine Letters in the article by Lorenzo and Wessel

• Geo-Marine Letters 
An International Journal of Marine Geology
• Vol 17, 1997 
Sheared continent-ocean margins
• Juan M. Lorenzo, Special Editor 

Figure Idealized two-stage development of continent-ocean fracture zones across continental margins during rifting and drifting. During early drifting, the young oceanic block slides against the older continental block possibly inducing thermal uplift. After the passing of a ridge when the transform becomes inactive, mechanical coupling across the fracture zone is possible. The faster subsiding oceanic lithosphere drags down the continental block. Vertical black arrow implies thermally induced uplift on continental block. With subsequent cooling and mechanical coupling to faster subsiding oceanic block, the continental block may bow down on approaching the fracture zone. 

In journal order, manuscripts froma Special publication of Geo- Marine Letters on Sheared Continent-Ocean Margins Vol 17, 1997

1. J.M. Lorenzo Sheared continent-ocean margins: An overview. p. 1-3

2. J. Mascle, G.P. Lohmann et al.. Development of a passive transform margin: Cote d'Ivoire-Ghana transform margin; ODP Leg 159 preliminary results. p. 4-11

3. R.E. Edwards, R.B. Whitmarsh, R.A. Scrutton, Geophysical features and geological development of the Ghana transform continental margin. p. 12-20

4. S.A. Gadd and R.A. Scrutton. An integrated thermomechanical model for transform continental margin evolution. p. 21-30

5. F. Sage, B. Pontoise, J. Mascle and Ch. Basile Structure of oceanic crust adjacent to a transform margin segment: The Côte d'Ivoire-Ghana transform margin. p. 31-39

6. F. Sage, B. Pontoise, J. Mascle and Ch. Basile, L. Arnould Crustal structure and ocean-continent transition at the Cote d'Ivoire-Ghana marginal ridge. p. 40-48.

7. J. Benkhelil, J. Mascle, M. Guiraud, Ch. Basile, and the Equanaute scientific team Submersible observations of Cretaceous deformation along the Cote d'Ivoire-Ghana Transform margin. p. 49-54.

8. J.-P Bouillin, G. Poupeau, E. Labrin, Ch. Basile, N. Sabil, J. Mascle, G. Mascle, F. Gillot, L. Riou. Fission track study of the marginal ridge of the Ivory Coast-Ghana transform margin. p.55-61

9. G. Lamarche, C. Basile, J. Mascle & F. Sage. The Cote d'Ivoire-Ghana transform margin: sedimentary and tectonic structure from MCS data. p. 62-69.

10. M. Guiraud, J. Benkhelil, J. Mascle, C. Basile, G. Mascle, J.P. Bouillin and M. Cousin: Syn-rift to syn-transform deformation: Evidence from deep sea dives along the Cote-d'Ivoire-Ghana transform margin. p. 70-78.

11. M. Guiraud, J. Mascle, J. Berkhelil, C. Basile, G. Mascle and M. Durand: Early Cretaceous deltaic sedimentary environment of the Cote-d'Ivoire-Ghana transform margin as deduced from deep dive data. p. 79-86.

12. I.D. Reid and H.R. Jackson Transform margins of eastern Canada. p. 87-93.

13. J. Prims, K.P. Furlong, K.M.M. Rohr, R. Govers. Lithospheric structure along the Queen Charlotte margin in western Canada: Constraints from flexural modeling. p. 94-99.

14. E. Vagnes: Uplift at Thermo-mechanically coupled ocean-continent transforms; modelled at the Senja Fracture Zone, southwestern Barents Sea. p. 100-109

15. J.M. Lorenzo and P. Wessel Flexure across a continent-ocean fracture zone: The northern Falkland/Malvinas Plateau, South Atlantic. p. 101-110.