|
Volcanic passive margins (VPM) and non-volcanic passive margins are the two forms of transitional crust that lie beneath passive continental margins that occur on Earth as the result of the formation of ocean basins via continental rifting. Initiation of igneous processes associated with volcanic passive margins occurs before and/or during the rifting process depending on the cause of rifting. There are two accepted models for VPM formation: hotspots/mantle plumes and slab pull. Both result in large, quick lava flows over a relatively short period of geologic time (i.e. a couple of million years). VPM’s progress further as cooling and subsidence begins as the margins give way to formation of normal oceanic crust from the widening rifts. ==Characteristics== Despite the differences in origin and formation, most VPMs share the same characteristics: *4 to 7 km thick basaltic and (frequently) silicic subaerial flows; dike swarms and sills running parallel to continent-facing normal faults.〔 *10 to 15 km thick bodies in the lower crust (HVLC) show high seismic P-Wave velocities, between 7.1 and 7.8 km/s which lie under the transitional crust (crust between continental crust and oceanic crust).〔 *Seaward Dipping Reflector (SDR) series: Inner SDR’s overlay transitional continental crust. They are composed of varying mixtures of subaerial volcanic flows, volcaniclastic and non-volcanic sediments which range from 50–150 km wide and are 5–10 km thick. Outer SDR’s overlay transitional oceanic crust are composed of submarine basaltic flows which range from 3 to 9 km thick.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Volcanic passive margin」の詳細全文を読む スポンサード リンク
|