AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Origin of the Mexican Ridges Passive Margin Foldbelt Based on Seismic and Well Integration from the Shelf-Slope-Deep Basin and Structural Restoration
(1) Jackson School of Geosciences, The University of Texas at Austin, Austin, TX.
Passive margin foldbelts (PMFB) form by three different mechanisms: 1) toe-of-slope shortening related to sliding on tectonically-tilted surfaces; 2) gravity-spreading deformation as a result of a regional bathymetric gradient by updip sediment deposition or salt movements; and 3) mixed-mode deformation involving both 1 and 2. The Mexican Ridges province (MRP) is an example of the third, mixed mode type of PMFB that extends for ~130-150 km across strike and ~360-400 km along strike adjacent to onland Laramide (Paleogene), east-verging, convergent structures and the Tamaulipas-Oaxaca fault zone formed during early GOM opening. Vintage University of Texas seismic lines tied to published Pemex wells and SEGY-converted seismic lines illustrate the main structural features of the MRP: 1) an outer shelf and upper slope area characterized by east-dipping, listric normal faults that juxtapose syn-faulting Miocene to Recent sediments with Eocene to Oligocene sedimentary rocks in their footwall blocks; 2) an upper slope area characterized by an undeformed upper section overlying large half-grabens that sole into the thin-skinned MRP fold-thrust belt that is morphologically prominent on the middle slope; and 3) a lower slope and basinal area of large, open folds that may be underlain by the downdip extension of the low-angle detachment surface. Sedimentary growth wedges adjacent to normal faults indicate that sliding and toe of slope compression initiated in the late Miocene and has continued to the present day. Structural restorations illustrate that downslope shortening structures balance total upslope extensional structures. The ~20 Ma time delay between the end of the Laramide orogeny in late Eocene and the initiation of MRP sliding in the late Miocene indicates that Laramide shortening is not the driving mechanism for oversteepening the slope and initiation of the MRP gravity slide and toe of slope shortening structures. A more likely cause for the MRP is accelerated Oligo-Miocene uplift, regional volcanic activity and erosion of the Mexican landmass with loading and destabilization of the narrow, eastern Mexican shelf. Regional isochron maps of the Mexican GOM tying University of Texas deeper basinal lines with published Pemex slope and shelfal lines and wells distinguish an early Laramide pulse of sediment followed by a later Oligo-Miocene pulse and a Miocene and younger, clockwise shift of U.S. GOM clastic sources derived from the Mississippi fan.