Gary Gray, Ph.D.
Adjunct Professor, Rice University
The structural geology of the northern Sierra Madre Oriental, Mexico is characterized by a wide region of detached, folded, late Mesozoic carbonate platform strata. This folding occurred during the Mexican Orogeny between 80 and 47 Ma. A line-length and area-balanced interpretation of the surface geology indicates that total contraction of the cover sequence is about 41 km. Folding of the cover strata occurred mostly from 80-60 Ma. The upper crust contracted slightly later, from 70 to 47 Ma, and total contraction was less than 12 km.
The crust is about 8 km thicker on the west than the east, and this thickening apparently also occurred during the orogeny. Previous work has demonstrated that the region was subsiding and accumulating up to 7 km of overburden from at least 70-50 Ma. This work documents that subsidence occurred during the main period of contraction. The rate of subsidence began to wane around 50 Ma and became erosive around 35 Ma. The rate of erosion increased until around 20 Ma, then began to decrease. These results are most compatible with flat-slab subduction of oceanic lithosphere beneath the Mexican continent, and with strong coupling between the over-and under-riding plates. Our analogue is the current flat slab beneath central Mexico, which exerts an estimated dynamic pull-down of approximately 1.5 km on the continental crust today. This is focused where the oceanic plate sinks into the mantle beneath the continent. The contraction must have been caused by continued westward drift of the Guerrero Terrane. Once the coupling was broken, the slab rolled back westward, and the region began a sustained isostatic rebound and erosion stage. The dynamic pull-down phase led to the storage of a huge quantity of sediment that was being delivered from the Laramide and Mexican orogens to the north and west. This cache of sediment was then recycled into the Gulf of Mexico during the early and middle Cenozoic as the crust isostatically rebounded.