Why is it so difficult for
scientists to determine how the Jalisco block is
moving

Extensional faulting observed in southwestern Mexico has been related to the incipient rifting of the Jalisco block from the Mexican mainland since the Pliocene. On the basis of new structural and geophysical data, we propose that (1) the continental boundaries of the Jalisco block are ancient structures reactivated since the Pliocene at a low (<1 mm/yr) rate of deformation, and (2) Pliocene-Quaternary extensional faulting at the edges of Jalisco block is a basement-controlled intraplate deformation related to plate boundary forces rather than to active continental rifting. The Jalisco block boundaries first developed in response to the uplift of the Puerto Vallarta batholith in pre-Neogene time and underwent a complex contractile deformation before the Pliocene. During Pliocene-Quaternary times north-northeast extension reactivated the northern boundary, forming the Tepic-Zacoalco rift, whereas east-southeast extension formed the northern Colima rift. South of the Colima volcano, active extension is found only west of the so-called southern Colima rift and partly reactivates old northeast-trending basement faults. The parallelism between the subducted Rivera-Cocos plate boundary zone and the eastern neotectonic boundary of the Jalisco block supports eastsoutheastward motion of the southern Mexican blocks induced by the differential motion and oblique subduction of the Cocos and Rivera plates. On the other hand, we envisage Pliocene-Quaternary extension along the northern boundary as an upper-plate response to the low convergence rate and the steep subduction angle of the Rivera plate.