Chapter 13: Divergent Boundaries & The Sea Floor

Models for early continental rifting

i. Horizontal tension ii. Uplift from mantle upwelling (plume?) iii. Passive vs. active upwelling

San Andreas fault system

right-lateral (strike-slip) transform fault boundary

Oceanic trench

the deepest parts of the sea floor - associated with subduction zones (convergent plate boundaries)

Abyssal plain

the flattest regions on Earth, abyssal plains make up the deep sea floor (~5 km deep) and are blanketed by sediments deposited by turbidity currents and pelagic sediments.

Seamounts

volcanoes that are found on the sea floor, they sometimes rise above sea level. They are often found in chains known as aseismic ridges that are associated with hot spots.

Seamounts, Guyots, and Aseismic Ridges

volcanoes that are found on the sea floor, they sometimes rise above sea level. They are often found in chains known as aseismic ridges that are associated with hot spots. Volcanoes that are eroded away and then subside, form flat topped seamounts known as guyots. Reef systems often develop around guyots in the tropics as they are forming producing atolls, when the volcano falls below sea level.

Continent-Continent type (Examples: Himalaya, Appalachians)

Structure: Suture Zone, Opposing Thrust Belt Zones (Mountain Belts), and basins on either side of high alpine zone

Mid-oceanic ridges and rift valleys

extensive global underwater "mountain range" extending ~80,000 km found in the centers of ocean basins. Mid-ocean ridges have topographic relief of ~1,000 to 3,000 m and some have central rift valleys ~1,000 km deep. The Mid-ocean ridge system is created 8 through sea floor spreading at divergent plate boundaries

Passive Margin

features include continental shelves, slopes, and rises: submarine canyons that cut across the shelves and slopes; and Abyssal plains at the base of the rise.

Active Margin

features include oceanic trenches; Waditi-Benioff Zones, where earthquakes occur at shallow depths near trenches and at increasing depths landward from trenches; and chains of volcanoes.

Pelagic sediments

fine grained deposits that settle through the water column. Consists of wind blown clay sized particles and the remains of siliceous and carbonaceous organisms. Pelagic sediments are thin near ridge crests where the oceanic crust is young and they have not had time to accumulate. They get thicker with increasing distance from the ridge crest

Consequences and sequences of rifting

1. East African Rift (early) 2. Red Sea Rift (intermediate) 3. Atlantic Ocean Basin (mature)

Transform faults and Fracture Zones

Basic distinction between these features - transform faults occur between offset mid-ocean ridge segments, are seismically active, and are boundaries between two plates. Fracture zones continue "outside" of the offsets, but are not seismically active, and separate different age parts of the same plate

Transform boundaries

Basic types: ridge-ridge (most common) with earthquakes occurring between ridge segments and aseismic fracture zones outside , ridge-trench, trench-trench

Ocean-Ocean type (Examples: Lesser Antilles, Marianas)

Main features: Oceanic crust caps both subducting and overriding plates, Deep Sea Trench, accretionary wedge and forearc basin, Volcanic Island Arc, Backarc basin

Ocean-Continent type (Examples: Andes, Cascades)

Main features: Oceanic crust caps subducting plate, continental crust caps overriding plate, Deep Sea Trench off coast, accretionary wedge and forearc basin, Magmatic Arc (Mountain Belt), backarc thrust belt, sedimentary Basin

Mid-ocean ridge crest features and processes

a. Creation of new oceanic crust by basalt eruptions b. High heat flow c. Shallow focus, small earthquakes d. Black Smokers and hydrothermal circulation e. Biological activity and mineralization

Active vs. Passive Continental Margins

active continental margins are located at plate boundaries (subduction zones); passive continental margins are found at the edges of continents where there isn't a plate boundary (the oceanic crust and continental crust are part of the same tectonic plate).

Continental shelves

are gently sloping (~0.5°) regions next to continents

continental rises

are gently sloping (~0.5°) wedge shaped regions at the base of the slopes.

continental slopes

are more steeply sloping (~4 to 5°) regions seaward of the shelves

Terrigenous sediments

are sourced from land - sand, silt, and clay sized sediments. Thick deposits near continents - thinner away from continents.

Oceanic crust structure and ophiolites

we believe that oceanic crust consists of three layers: Layer 1 consists of sediments that accumulate on the sea floor, Layer 2 consists of pillow basalts underlain by basaltic dikes and is ~ 1.5 to 2 km thick, Layer 3 is believed to consist of gabbro and is ~5 km thick. Layer 3 has not been directly sampled via deep sea drilling, but sequences of basalts and gabbros are found in some places on continents. These sequences known as ophiolites are believed to be parts of oceanic crust that have been emplaced on continents through tectonic processes.