3‐D Modeling of Vertical Gravity Gradients and the Delimitation of Tectonic Boundaries: The Caribbean Oceanic Domain as a Case Study

Geophysical data acquisition in oceanic domains is challenging, implying measurements with low and/or nonhomogeneous spatial resolution. The evolution of satellite gravimetry and altimetry techniques allows testing 3‐D density models of the lithosphere, taking advantage of the high spatial resolution and homogeneous coverage of satellites. However, it is not trivial to discretise the source of the gravity field at different depths. Here, we propose a new method for inferring tectonic boundaries at the crustal level. As a novelty, instead of modeling the gravity anomalies and assuming a flat Earth approximation, we model the vertical gravity gradients (VGG) in spherical coordinates, which are especially sensitive to density contrasts in the upper layers of the Earth. To validate the methodology, the complex oceanic domain of the Caribbean region is studied, which includes different crustal domains with a tectonic history since Late Jurassic time. After defining a lithospheric starting model constrained by up‐to‐date geophysical data sets, we tested several a‐priory density distributions and selected the model with the minimum misfits with respect to the VGG calculated from the EIGEN‐6C4 data set. Additionally, the density of the crystalline crust was inferred by inverting the VGG field. Our methodology enabled us not only to refine, confirm, and/or propose tectonic boundaries in the study area but also to identify a new anomalous buoyant body, located in the South Lesser Antilles subduction zone, and high‐density bodies along the Greater, Lesser, and Leeward Antilles forearcs.

Plain Language Summary: The knowledge of the density structure of the different layers that compose the solid Earth is important, for example: in the study of earthquakes, in plate tectonics reconstructions, or for the modeling of petroleum systems. These density variations affect (in small scale) the intensity of the gravity field on each point of the Earth's surface. The gravity field can be globally measured with satellites, reaching areas where the direct measurements are expensive and time consuming, such as in the ocean. In this work, we propose a new methodology with the purpose of recognizing tectonic and/or terrain limits, located in the outer most layer of the solid Earth, named crystalline crust. We calculate the gravity field of different density distributions, using four layers: seawater, sediments, crystalline crust, and mantle (a layer located below the crust), and compare the results with satellite global measurements. With our methodology it is possible to refine, confirm, and/or propose terrain limits, but additionally, we are able to estimate the average density configuration of the crystalline crust. This methodology is validated in the oceanic domain of the Caribbean, where a complex geologic history exists, due to its evolution since approximately 144 million years ago.

Key Points:

Vertical gravity gradients are especially sensitive to density contrasts in the upper layers of the lithosphere. We propose a new method for identifying tectonic boundaries based on the gradient's residuals and tested it in the Caribbean oceanic region. Using 3‐D lithospheric models, we forward modeled these gradients to infer the average density structure of the crystalline crust.