Chikyu first leg sailed 9/07. $500,000,000 riser drillship to drill 500-2500 m bathtub ring. First stop, Nankai.

1. Chikyu first leg sailed 9/07. $500,000,000 riser drillship to drill 500-2500 m bathtub ring. First stop, Nankai.

3. Marine Geology 9/26/2008 • Divergent (Passive) Continental Margins • Grow, J., and Sheridan, R.E., U.S. Atlantic continental margin; A typical Atlantic-type or passive continental margin, In Sheridan, R.E., and Grow, J., The Atlantic Continental Margin: U.S., DNAG vol. I-2, Geol. Society America, p. 1-7, 1988. • Kennett Chapt 11. • three basic types • of continental • margin: • divergent, • convergent, • translational

4. Divergent (= passive = Atlantic type) margins • places where continental crust meets oceanic crust without a plate boundary • formed through rifting of continent • little seismic or volcanic activity • thick sediments • smooth relief • regions of general extension, not compression • subsidence: • simple thermal subsidence, • loading, • compaction increase in flexural rigidity • result of continental crust (lower ) • meeting ocean crust • Where continents meet oceans • Where is the edge of the continent? Off NJ it is beneath the outer continental shelf

5. Offshore NJ: Baltimore Canyon Trough, a typical passive margin

6. basement hinge zone: rapid increase from 2-4 km to 16 km may mark boundary between unstretched & stretched continental crust maximum stretching (thinning) of crust b= 3-4 near ocean-continental crust boundary 16+ km of Middle Jurassic-Recent sediments History of Deposition in BCT Buried rift basins, overlain by post-rift unconformity deposition of thick shallow marine Middle Jurassic to lower Cretaceous f(rapid thermal subsidence after rifting + loading) combination of clastic and carbonate margin reef- Callovian to Berriasian (late Neocomian) similar to modern Great Barrier Reef why did the reef die? changes in paleogeography and climate? mid-Cretaceous-onlap of present coastal plain: sea level controls

7. SEISMIC VELOCITIES USED TO RECOGNIZE CRUSTAL STRUCTURE AND OCEAN-CONTINENT TRANSITION structure of ocean crust revealed by seismic refraction studies Rait (1963) LAYER SEISMIC VEL THICKNESS water 1.5 km/s 3.8 km layer 1, sediments: 1.5-3+ km/s < 500 m thick layer 2, basalts 5 km/s 1.7 km thick layer 3, gabbro 6.7 km/s 5 km total 7-8 km + 3.8 km water Mantle, ultramafics 8.1 km/s layer 2 thought to be pillow basalts layer 3 thought to be gabbros and sheeted dikes Structure of Continental Crust Granitic composition 5-6 km/s >30 km Transitional crust underneath middle to outer shelf off NJ Normal layer 3a 6.3-6.9 km/s Expanded layer 3b 7.2-7.4 km/s evidence of underplating? i.e., crust was thinned during rifting and during cooling, plated by higher density mantlematerial that cooled below solidus

8. East Coast Magnetic Anomaly: a magnetic anomaly resulting from an edge effect highly magnetized oceanic crust meets less magnetized continental crust

9. Transitional crust is absent beneath non-volcanic margins like Bay of Biscay p. 157 in Bond et al.

10. often block faulted with listric, normal faults Example from west of Galicia Bank (Bay of Biscay) by Charpal et al. (1978) showing tilted crustal blocks and hints of listric normal faults

11. Rifting spreading apart, though not neccesarily seafloor spreading which associated with formation of ocean crust rifting begins with extension generally continental, but submarine rifts occur (little sedimentary record) 3 models of extension a) broad (100's km) thermal doming of lithosphere ~ 1 km thermal uplift Sleep's idea. Not consistent with data b) lithospheric stretching (thinning) subsidence: then rapid mechanical subsidence, then thermal subsidence Mckenzie stretching c) dike swarms into thinned crust Rift onset unconformity e.g., no Permian to lower Triassic in NJ

12. thinning by stretching yields rapid "mechanical" subsidence, thermal anomaly, and exponential cooling McKenzie stretching parameter  Thickness 1/  Return to original thickness by underplating

13. pure shear extension models: causes listric faulting in brittle upper crust and ductile flow in lower lithosphere. They work in predicting post-rift subsidence, but fail for initial tectonic subsidence. Predicts symmetrical subsidence (grabens, 1/2 grabens)

14. Simple shear model (Wernicke) studies from extensional regime of basin and range asymmetric failure along low angle detachment faults or low-angle shear zones divide into upper and lower plate margins east coast: compatible w/ this model Paleozoic thrust fault provide remnant structures for detachment

15. Formation of rift basin SYN-RIFT SEDIMENTATION Accumulation when active plate boundary within basin rift stage involved rapid stretching of crust accompanied by faulting + igneous activity deposition in deep elongate fault governed troughs i.e., not a passive margin at all active crustal stretching often faulted numerous inward-facing 1/2 grabens, intruded (e.g., Palisades sill) block faulting: listric normal faults thick deposits (e.g., Newark basin, a rift basin, >6 km sediments) graben formation (really 1/2 grabens)

18. Example: Structure of Newark Basin Follows regional grain of Appalachian structures 30-50 NE NW border: continuous line of faults, up to 18,000 ft vertical displacement Border fault: Ramapo listric normal fault controlled by reactivation of Paleozoic faults movement on these may include strike-slip motion within the basin: Hopewell and Flemington Faults both up to 10,000' vertical displacement (Van Houten, 69) and 20 km strike slip movement (Saunders, '63) constrained on SE side by onlap of coastal plain some folding

19. ONSET OF SPREADING proto-oceanic basin; MORB, MMA basalts injected along faults (not the same as Palisades) two types of margins: volcanic: w/ thick (several km) piles seaward dipping basalts (SDB) w/ interbedded sediments extruded in short time (e.g., Leg 152, within C24r) Rockall, E. Greenland, Norwegian, Norfolk: Jurassic Volcanic Wedge seaward of rift basin non-volcanic: Newfoundland, Bay of Biscay no excess volcanics why volcanic types of margins: hot spot/mantle plume? note proximity to Iceland "Clear evidence of a genetic relationship between deep-seated mantle plumes and volcanic rifted margins is still lacking." Leg 152, 1994, EOS, p. 406

21. THE FOLLOWING MUST BE INFERRED FROM SEISMIC AND DEEP WELLS POST-RIFT UNCONFORMITY: controversial some regard it as fundamental result of the transition from rifting to sea-floor spreading (i.e., the rift/drift transition) Klitgord et al Deepest mappable reflector in BCT same in Norfolk basin separates sediments deposited in very different tectonic settings: rift from drift time transgressive surface marks end of rifting separates upper lo. Jurassic and mid Jurassic marine carbonates and anhydrite from lower lower Jurassic and up Tr evaporites and nonmarine beds PRU associated with or precedes development of thick salt deposits Argo salt in Scotian Basin, Luan salt in GOM; probably post-PRU some salt in BCT, salt diapir on Line 14 do thick (1 km or so) salt (halite, gypsum, anhydrite) form? if you desiccate the modern Mediterranean, you get deposit about 10 m thick

22. three processes which control subsidence at passive margins 1. simple thermal subsidence as on MOR; rate of subsidence decays exponentially; range of subsidence different than MOR; decay constant may also be a little less 2. isostatic loading both Airy and flexural Airy isostasy: mountains matched by deep "roots" in mantle; Airy assumes no crustal rigidity (point loading) relatively true for young, recently rifted crust; not for old, cold 3. compaction: dewatering and reduction of porosity from 60- 80% to 5% during lithification burial

23. SIMPLE THERMAL SUBSIDENCE the process of rifting of continental crust and subsequent sea floor spreading results in addition of heat to the crust. This heat decays through time All heat flow from the lithosphere (both continents and oceans) decays exponentially For "mid"-ocean ridges, this relationship has been empirically determined Paleodepth = 6400 - 3400*(e-t/) paleodepth at time t, t = time t; empirical decay constant of 62.8 for example, for crust of 63 Ma, the depth to basalt is 5,159 m This can be approximated as an square root of t relationship D = 2500 + 350t1/2 The subsidence of rifted continental crust behaves like MOR, with an exponential decrease in the rate of subsidence (rapid at first, slow later), although the constants differ.

25. However, the loading effects of sediment and water must be accounted. Loading effects involve the concept of isostasy, either Airy or flexural. ISOSTASY Principle derived from geodetic surveys in India in mid-1800's Himalayas deflected vertical far less than expected Explanation : rigid outer skin of earth "floats" on deformable interior competing models: mountains composed of unusually light material (PRATT) mountains matched by unusually deep "roots" into mantle (AIRY) both occur, Airy is usually assumed "level of compensation" = asthenosphere ~ 120-150 km +

26. how much subsidence of the ocean crust occurs during a 100 m rise in sea level? m*x = 1.033*watert + m*(x +e-watert) m*x = watertm*x + m*e- m*watert watert + m*e- m*watert m*watert - s*watert = m*e watert = m)/m-w)*e for m = 3.3 g/cc, w= 1.033 g/cc watert = 1.44*e Watert*.69 = e answer = 69 m (31 m of subsidence) in general, seafloor subsides 30% amount of SL rise, rebounds 30% amount of SL fall i.e., 100 m eustatic rise = observed 70 m change relative how much sediment does it take to displace 1 km of seawater? m-w/m-s*Z = S where Z is the amount of water displaced, and S is the amount of sediment required to fill the hole to zero water depth answer = 2.3 (using s = 2.3 gm/cc) to 2.55 km (using s = 2.4 gm/cc) in general, need 2.3 times WD of sediments to build to reach sealevel Loading alone cannot explain thick shallow-water sediments

27. Subsidence History Initial phase: rifting, isostatic response to lithospheric thinning during rifting; includes fault related subsidence in rift basins (e.g., Newark Basin) post-rift: main effects are loading and thermal subsidence; explains thick (16 km) shallow-water deposits in BCT. Determine the effects of each by backstripping (geohistory) Backstripping stepwise removal of the effects of compaction (decompaction) then sediment loading (generally Airy is assumed) shows predictable decay through time: effect of simple thermal subsidence to account for thermal subsidence, fit an exponential to the data remove the effects of paleowater depth changes the residual should be the effects of global sea level

28. Backstripping answers the question what was tectonic subsidence TS • what would basement have done in the absence of a sediment load (S*) & eustatic variations (DSL) • backstripping is an inverse model (backward) because it uses physical principles and observed • stratigraphy to obtain a subsidence history • Physical principles: • 1) isostatic compensation for sediment and water load (Airy or Flexural) • 2) compaction as a result of exponential decrease in porosity through time

30. TS

32. simple Airy isostasy complicated by flexural rigidity of lithosphere flexure: includes effects of vertical shear stress; result of rigidity/strength of crust point load support in part by strength of lithosphere width of supporting area function of thermal history hot = narrow cool = wide max. 70 Ma after rifting under "rapid" load (1000 to 10,000 yrs) behaves elastically the increased flexural rigidity of the lithosphere off NJ resulted in the shifting of the hinge zone from offshore in the Jurassic-Early Cretaceous to onshore in the mid Cretaceous. flexural effects: explain why the basin becomes progressively wider through time

34. is the flexural factor varies from 1 in pure Airy to less than 1 in flexural High elastic thickness Te yields Low values for 