ABSTRACT Jurassic northward migration of Mexico, which lay on the southern part of the North America plate, resulted in temporal evolution of climate-sensitive depositional environments. Lower–Middle Jurassic rocks in central Mexico contain a record of warm-humid conditions, indicated by coal, plant fossils, and compositionally mature sandstone deposited in continental environments. Paleomagnetic data for central Oaxaca and other regions of central and eastern Mexico indicate that Lower and Middle Jurassic rocks were deposited at near-equatorial paleolatitudes. In the Late Jurassic, the Gulf of Mexico formed as a subsidiary basin of the Atlantic Ocean when the Pangea supercontinent ruptured. Upper Jurassic strata across Mexico, including eolianite and widespread evaporite deposits, indicate dry-arid conditions. Available paleomagnetic data (compaction-corrected) from southern and northeast Mexico for Upper Jurassic strata indicate deposition at ~15°N–20°N. As North America moved northward during Jurassic opening of the Atlantic Ocean, different latitudinal regions experienced coeval Middle–Late Jurassic climatic shifts. Climate transitions have been widely recognized in the Colorado Plateau region. The plateau left the horse latitudes in the late Middle Jurassic to reach temperate humid climates at ~40°N in the latest Jurassic. Affected by the same northward drift, the southern end of the North America plate represented by central Mexico gradually reached the arid horse latitudes in the late Middle Jurassic as the Colorado Plateau was leaving them. As a result, Late Jurassic epeiric platforms developed in the circum–Gulf of Mexico region after a long period of margin extension and were surrounded by arid land masses. We propose that hydrocarbon source-rock deposition was facilitated by arid conditions and wind-induced coastal upwelling.

Abstract Determining the timescales of magma degassing is essential for understanding the mechanisms controlling the eruption style and the dynamics of magmatic systems. Towards this end, we measured 210 Pb– 226 Ra disequilibria in andesite lavas erupted from Volcán de Colima between 1998 and 2010. ( 210 Pb/ 226 Ra) 0 activity ratios range from 0.86 to 1.09, and are best explained in terms of 222 Rn degassing and accumulation. The range in 210 Pb deficits indicates that the timescales of 222 Rn degassing did not exceed 11 years. 210 Pb excesses are rare and small (<10%), which signifies that 222 Rn degassing is more effective than 210 Pb accumulation in this intermediate system despite the relatively low gas output at the surface. The absence of significant 210 Pb excesses strongly suggests that the volcanic activity results from episodic ascent of small magma batches through the vapour-saturated section of the magmatic system. Overall, the degassing models based on 210 Pb– 226 Ra disequilibrium suggest an open and complex subvolcanic magmatic system comprising several conduits in which multiple magma batches reside for up to 10 years. Shifts from effusive to explosive Vulcanian eruptive phases are not related to changes in degassing mode on timescales resolvable using 210 Pb– 226 Ra disequilibria.