AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Platform-Scale Fracture Patterns in Atoll-Like Carbonate Platforms: The Latemar Case Study (Dolomites, Italy)
(1) Geotechnology, Delft University of Technology, Delft, Netherlands.
(2) Tetconics-Structural Geology, VU University, Amsterdam, Netherlands.
We present the first reconstruction of distributed fractures and causative stress and strain fields in atoll-like carbonate platforms on the basis of the outcropping Latemar platform (Dolomites, Italy), an analog for buried systems. We consider structural and sedimentological features ranging from platform- to layer scale.
The Middle Triassic Latemar platform is 3-5km across, 800m thick and is composed of a circular, well-bedded platform interior surrounded by a poorly organized slope with the local interposition of margin facies. Rocks in the Latemar are intensively fractured.
Using innovative data acquisition and processing tools we quantify the outcrop-scale geometry of fracture networks in the domains of the platform. We focus particularly on the poorly documented vertical distribution of fractures across the stratigraphy. We measured >5000 fractures.
Two fracture sets are found and have strikes is constant from one domain to the other. Fractures in the platform slope are higher and have larger spacings than those of the platform interior. Here, bed thickness and bed interfaces exert a weak control on spacing and vertical distribution of fractures, and >50% of the fractures initiates or terminates inside the layers. Grainstone-dominated layers have the highest fracture density.
We apply finite element modeling to interpolate between data points and predict platform-scale stress and strain fields. The platform architecture is the primary input of the model. By changing i) boundary conditions (overburden and tectonic stresses) and ii) mechanic bulk properties of the composing domains we predict stress and strain distributions, and type of fractures (opening vs. shear fractures). Our preferred model is one which best fits i) the direction of fractures in the entire platform, ii) the spatial distribution of fracturing and, iii) the kind of fractures.
Fracture patterns in the Latemar are best modeled by applying an overburden of 1-2km and a significant tectonic stress parallel to the joints. The model predicts a platform interior that is mechanically softer than the margin and the slope. Such differences reflect platform properties during burial before full lithification. This is compatible with the parallelism between joints and the Neptunian and Middle Triassic volcanic dykes.
Building on the Latemar case study and using sensitivity analysis we predict fracture patterns affecting similar platforms subjected to different boundary conditions.