Sierra Madre Oriental, México

This Landsat 5 image illustrates the curved axial traces of folds in 
the Monterrey salient, which marks the apex of the more regional 
SMO salient, only a portion of which is pictured here.  Physical modeling 
of salient development over discontinuous weak decollement is the 
thesis topic of Masters candidate Christine Fox.  The fold-dominated, 
thin-skinned deformation developed during Paleogene orogeny, 
contemporaneous with Laramide basement uplifts in Wyoming.

Oblique aerial photograph of an unfaulted, isoclinal anticline in 
Mesozoic carbonate strata of the Monterrey salient.  The narrow 
hinge zone of the fold is exposed in the near ground peaks.  However, 
the crest of the anticline is eroded away at the level of erosionally 
resistant Lower Cretaceous carbonates beyond the dog-tooth peak, 
where the characteristic pair of mountain ranges mark the anticline.

The folds of the SMO salient are detached in Upper Jurassic 
gypsum-anhydrite (halite has never been found in the fold belt, 
although it crops out farther north), which is locally exposed in 
the cores of anticlines.  Extensive outcrops of the decollement 
layer and siliciclastic strata below occur near Galeana, where 
the picture above was taken, and show that underlying rocks 
were uninvolved in folding.

The dominant deformation mechanism during folding of the 
Cretaceous carbonate section was flexural slip.  The sheet of 
white fibrous calcite and reddish breccia in this photo mark the 
contact between two carbonate layers in Cañón Cortinas.

It would be easy to rationalize the fracture pattern in the Cretaceous 
carbonate flatirons at Cañón Huasteca as being fold-related.  
However, recent observations suggest that most fractures formed 
long before folding, during early burial diagenesis of the strata and 
in some cases near the paleo-sea floor.

Cross-cutting relations show that the dolomite- and calcite-filled veins 
in these breccia blocks at Cañón de las Palmas were already present 
at the time of brecciation.  Overlying growth strata (near the 120 Ma 
sequence boundary) that in-filled karst pits demonstrate that solution 
collapse occurred during deposition, so the fractures also must have 
been syn-depostional and formed long before Paleogene folding.

Many minor faults in the Monterrey salient also developed during 
deposition of the Cretaceous carbonates.  For example, the middle 
of three thrust faults that imbricate mm-scale laminations at this 
outcrop in Cañón Huasteca is cross cut by a burrow!