GEOL 3400 Geologic Hazards Plate Tectonics Triple Junctions

GEOL 3400: Geologic Hazards Plate Tectonics Triple Junctions

Plate Tectonics - Triple Junctions Background GEOL 3400: Geologic Hazards • mostly we have dealt with plate boundaries between two plates – by far the most common types • there is, however, a third possibility, i. e. a boundary between three plates – triple junction • looking at a map of tectonic plates you can identify several prominent triple junctions 20 -May-21 2

Plate Tectonics - Triple Junctions GEOL 3400: Geologic Hazards Background 20 -May-21 3

Plate Tectonics - Triple Junctions GEOL 3400: Geologic Hazards Naming • a triple junction is where three plate boundaries meet in a point • obviously, three types of boundaries are: – ridge (R) – trench, i. e. convergent margin (T) – transform fault (F) • triple junctions are identified or named by using indicating the three boundaries that meet – RRR – three ridges – RFT – ridge-transform-trench • order of boundaries is unimportant • geometrically there are only ten possible triple junction combinations 20 -May-21 4

Plate Tectonics - Triple Junctions GEOL 3400: Geologic Hazards Junction Stability • not all triple junctions are created equally • two types based on stability are: junctions: maintain their geometric form over time – stable triple junctions • exist for long time junctions: can exist only for short (geologically) time – unstable triple junctions before they transition into another geometric form 20 -May-21 5

Plate Tectonics - Triple Junctions Junction Stability - Stable GEOL 3400: Geologic Hazards triple junction: RRR (ridge-ridge) 20 -May-21 • ideally stable – always • magnetic strips form Y-shaped pattern around junction • rates of spread may be different all different ridges 6

Plate Tectonics - Triple Junctions Junction Stability - Unstable GEOL 3400: Geologic Hazards triple junction: TTT (trench-trench) 20 -May-21 • always unstable • will change to another shape • crust is consumed here • only one stable configuration: if relative motions of plates A and B are parallel to plate boundary between B and C 7

Plate Tectonics - Triple Junctions Junction Stability - Unstable GEOL 3400: Geologic Hazards triple junction: RRF (ridge-transform) 20 -May-21 • always unstable • exists only for a geologic instant • transitions to two stable FFR triple junctions 8

Plate Tectonics - Triple Junctions GEOL 3400: Geologic Hazards Existing Triple Junctions • of the ten possible triple junction combinations, only seven exist given today’s plate tectonic system • they are: – RRR: South Atlantic, Indian Ocean, west of Galapagos Islands – TTT: central Japan – TTF: off coast of Chile – TTR: off Moresby Island, western North America – FFR, FFT: junction of San Andreas fault and Mendocino transform fault – RFT: southern end of Gulf of California 20 -May-21 9

Plate Tectonics - Triple Junctions GEOL 3400: Geologic Hazards Background FFR TTT RRR TTF 20 -May-21 RRR 10

Plate Tectonics - Triple Junctions GEOL 3400: Geologic Hazards Summary 20 -May-21 11