Active Continental Margins
I. Classification
- A. Island Arc Subduction Zones
- B. Continental Subduction Zones
II. Island Arc Subduction Zones
- A. Morphology & Structure
III. Continental Subduction Zones
- A. Structure
IV. Forearc Processes
- A. Accretionary Wedges
- 1. Generally characterized by growth
- a. both vertically & horizontally by accretion of oceanic
sediment & deeper crustal material onto the overriding plate
- i. material is removed from the subducting plate by offscraping
- b. older units are structurally higher & closer to the
volcanic zone
- c. the accretionary wedge may build to sealevel & form
an outer arc
- 2. Sediments & crustal material are highly-deformed into
a chaotic mixture called melange
- a. includes oceanic crust, pelagic & hemipelagic sediment
deposited on the subducting plate when it was far from the trench,
trench turbidites, and turbidites from the trench slope (often
deposited in small slope basins
- 3. Water provided by dewatering of accretionary wedge sediments
apparently lubricates the major thrusts between imbricate thrust
sheets in the accretionary wedge
- a. thrusts are dry
- B. Tectonic Erosion & Non-Accretionary Margins
- 1. Shown by subsidence of portions of the trench slope that
previously were above sea level and by Paleozoic & Precambrian
metamorphic/igneous terrances within 10 to 20 km of the trench
- C. Forearc Basins
- 1. Area of subsidence between accretionary wedge & volcanic
arc
- 2. Filled with immature clastic (volcanic & metamorphic)
sediments eroded from arc
- a. environments of deposition range from turbidites to fluvio-deltaic
V. Backarc Processes
- A. Backarc, or Marginal, Basins (Island Arc Subduction Zones)
- 1. Characteristic of the western Pacific
- 2. Result from seafloor spreading
- B. Foreland Fold & Thrust Belts (Continental Subduction
Zones)
- 1. Partial "subduction" of colder lithosphere of
the continental craton beneath the hotter arc
- C. Related to Absolute Motion Between Converging Plates
- 1. Eurasia moving northerly to northwesterly, Pacific to
west
- a. Eurasia pulling away from trenches - opens marginal basins
- 2. South American plate moving to west
- a. actively overriding Nazca plate - no marginal basins
VI. Geophysical Anomalies Associated with Active Continental
Margins
- A. Magnetic Anomalies
- 1. Backarc Basin = seafloor spreading
- 2. Arc itself = high amplitude, high frequency anomalies
- B. Gravity Anomalies
- 1. Slight Free-air positive over outer high
- 2. Strong (100-250 mgal) Free-air negative over trench &
inner trench wall
- a. mass deficit from trench & low density sediments
- 3. Strong (100-250 mgal) Free-air positive over arc
- a. cold dense subducted slab
- C. Heat Flow
- 1. Low heat flow over trench & inner trench wall
- 2. High heat flow over the arc
- 3. Variable heat flow plus a factor related to basin age
VII. Geochemistry
- A. Andesites
- 1. Characterize continental subduction zone volcanics
- a. partial melting of subducted oceanic crust & sediments
- 2. Basalts are more prevalent at island arc subduction zones
- B. K2O
- 1. Increases with distance from trench & depth to the
subducted slab
- a. increase pressure, increase K2O
- b. similar to difference in K2O between
tholeiites & alkalic basalts