Plates, models, and what a boundary line is

Fifty-two plates, and thirteen admissions

Model PB2002 comprises plate boundary curves in 229 segments, 52 plates as closed outlines, and 13 orogens — regions where deformation is distributed rather than confined to a boundary, so that the rigid-plate description fails.^[1,2]

Those thirteen regions are the most interesting thing in the model, because they are where it declares its own limits. Their outlines delimit where a model stops working. They are not orogenic belts, and Bird gives them spelled-out names specifically, in his words, "to help prevent confusion between orogens and plates".^[1]

A boundary is classified, and only sometimes

The model recognises seven classes of boundary step. Subduction requires convergent motion together with a Benioff zone of intermediate to deep earthquakes and/or a parallel volcanic arc with Quaternary activity — criteria which, the documentation notes, exclude the Alps, Zagros, Pamirs, Himalaya, Tien Shan and Taiwan. Convergence across the Himalaya is not in question. What the criteria exclude is the label.^[1]

The other six classes combine oceanic or continental setting with divergent, strike-slip or convergent relative velocity. Oceanic means sea floor younger than 180 million years by magnetic anomaly, or water deeper than 2,000 metres; everything else, including transitional crust, counts as continental.^[1]

Why Lithoen names the model on every line

A boundary’s position, its class and sometimes its existence depend on which model is being read. A plate that appears in one model may be part of a larger plate in another. Lithoen therefore names PB2002 on every plate geometry it publishes and draws no unattributed boundary anywhere.^[1]

The Somali Plate is the clearest case. PB2002 separates it from Africa along the East African Rift, which is why the rift appears as a plate boundary on Lithoen’s map. Whether the rift has yet made two plates is a question about how much relative motion a model needs before it draws a line — not a fact about the rock.^[1,2]

Commonly misunderstood