We investigate the thermal and seismic implications of two extreme configurations of lithospheric extension. One extreme treats the extension as being approximately uniform with depth, the upper crust extending by the rotation of fault-bounded blocks above a diffusely deforming lower crust; in the other extreme, crustal extension occurs only above a low-angle fault (dip less than about 10°). Very low-angle shear zones that initially move aseismically within a uniformly extending lithosphere may be raised through the depth range that is normally seismically active without cooling appreciably; the condition for this to occur is that extension should take place faster than about 10−14/s. In contrast, extension accommodated entirely above a very low-angle shear zone rapidly brings that zone to temperatures at which it should be seismically active, even if initial movement on it were aseismic. The different thermal histories implied by the two configurations of extension offer the possibility of distinguishing between modes of extension by observations of temperature-time and pressure-temperature paths on rocks close to low-angle faults.

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