Numerical modelling of the Mexican Ridges foldbelt, Gulf of Mexico
Abstract
The Mexican Ridges foldbelt is located on the continental slope off eastern Mexico. It consists of a 3-km-thick section of folded Tertiary clastic turbidites overlying a thick, relatively undeformed shale unit. Seismic profiles show that the folds have average wavelengths of 10 km and amplitudes of up to 250 m. A sequence stratigraphic analysis yields eleven seismic sequences. Below an average depth of 3200 m, the sequences are considered overpressured, consisting mostly of shales, with sandy turbidites near the top. Above this depth, the sequences are interpreted as being mostly turbidites, interbedded sporadically with pelagic and hemipelagic units. The mechanism for folding is considered to be gravity sliding over a decollement in the overpressured shales. A nonlinear one-dimensional compaction flow study computes the fluid pressure generated during sedimentation to be 64% of lithostatic pressure. Higher fluid pressure can be generated if a seal of reduced hydraulic conductivity occurs above the basal shales. The hyperbolic stress-strain relationship, obtained from the soil mechanics literature, successfully describes the mechanical behavior of sediments in young sedimentary basins. The model predicts a high contrast in tangent modulus between sandstones and shales, a contrast that decreases with depth. Implications for folding are that, at shallow depths, fold wavelengths are controlled by the thickness of individual sandstone beds. At great depths, wavelengths are controlled by the thickness of the composite sandstone-shale unit. A finite element analysis of the Mexican Ridges foldbelt, employing the hyperbolic stress-strain relationship, predicts three orders of folding: wavelengths of 9000 to 15000 m for first order folds, 1600 to 3000 m for second order folds, and 400 m for third order folds. These predicted values agree with those actually observed in seismic profiles of the Mexican Ridges: wavelengths of 9900 to 17500 m for first order folds, 1600 to 2800 m for second order folds, and 140 to 420 m for third order folds.
Degree
Ph.D.
Advisors
Tharp, Purdue University.
Subject Area
Geology
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