MAGANOM : a computer program for the modeling and interpretation of marine magnetic anomalies with an example from the Cayman Trough, northwest Caribbean Sea

Abstract

MAGANOM is a multi-purpose menu-driven interactive computer program for residual magnetic anomaly data processing and modeling. It is designed to model the magnetic anomalies above 2-dimensional magnetic bodies. The principal use of the program is to model sea floor spreading anomalies. It can also calculate and display the anomalies above magnetic bodies with irregular shapes. Designed primarily as a research tool, MAGANOM can also be used to teach the theory of magnetic anomaly formation at the undergraduate and graduate level. This text contains two sections. The first chapter is a manual for the MAGANOM program. The manual contains: 1) an introduction to the elements of MAGANOM, 2) instructions for the interactive use of MAGANOM, 3) a reference section on the options of the program, 4) sample sessions to guide the user, 5) a discussion of the theoretical basis of MAGANOM, 6) a discussion of the FORTRAN-77 source code of MAGANOM, and 7) a listing of the supplementary files used in the sample sessions. The second chapter is a example of an application of MAGANOM. This example describes a project which was undertaken to understand the evolution of a marginal basin in the northwest Caribbean Sea. The project synthesizes heat flow, depth to basement, and magnetic anomaly data to describe the tectonic development of the Cayman Trough. All of the magnetic anomaly data modeling was done with MAGANOM. It is important to study the opening of the Cayman Trough because it records aspects of relative motion between the North American and Caribbean plates essential to accurate reconstruction of Caribbean evolution. Evaluation of opening rates based upon crustal subsidence analysis and magnetic anomaly interpretation shows the Trough to have opened at less than half previously proposed rates. Depths to basement show that the crust has subsided from the present axis in two stages, a younger stage characterized by substantial and asymmetric (greater to the east) subsidence preceded by an older stage of slight increases of depth with distance. Depth to age relationships calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the Trough has opened at an overall rate of less than 15 mm/yr since 25 to 30 Ma and at 20 to 30 mm/yr prior to 30 Ma. Residual magnetic anomalies show lineation perpendicular to the Trough, including a distinct central anomaly, but the pattern is neither well defined nor symmetrical about the present axis. Partial anomaly sequences can be identified on both sides of the axis, however, and the distribution of these sequences indicate that spreading involved frequent short ridge jumps, most eastward. The overall rate of opening as indicated by the magnetics is between 15 and 30 mm/yr for the entire history of the Trough. Both subsidence and magnetics indicate that the Trough first opened between 45 and 50 Ma.