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Gravity inversion helps solve pluton emplacement debate

by Virginia Heffernan on December 1, 2016 applied

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Figure 1: Geological map of Southwest England. The surface outcrops of plutons associated with the Cornubian batholith have been labelled. Map axis are labelled with British National Grid coordinates. This map was produced using Edina’s geology Digimap service.

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Figure 2: Gravity model which was produced using an intermediate density contrast. The model is colour shaded with the same key as the depth map on Figure 3. British National Grid coordinates are plotted along the model axis.

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Figure 3: (a.) Observed and (b.) calculated Bouguer gravity anomaly grids, and (c.) a map of the depth to the top of the granite for the gravity model shown by Figure 2. The granite depth map is contoured with the absolute misfit between the observed and calculated anomaly grids at a contour interval of 1 mGal. British National Grid coordinates are plotted along the map axis. Dashed black lines have been included on the depth map to outline shallow and deep sections of the gravity models which were interpreted to represent separately intruded sheets of granite.

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Figure 4: (a.) Plot of modelled granite thickness and (b.) a schematic of pluton emplacement based on the geometry of the gravity models. A dashed black line has been included on the plot to mark the maximum granite sheet thickness which is supported by power-law expressions that describe the relationship between the width and thickness of granitic intrusions.

A longstanding debate about whether granitic magmas in England ascended through the crust as diapirs or along networks of dykes to form tabular structures has taken a step closer to resolution.

A gravity inversion performed to model the 3D structure of the St Austell pluton, one of six major plutons associated with the Cornubian batholith of Southwest England (Figure 1), suggests the pluton is comprised of at least two separately intruded sheets of granite.  

“This finding is consistent with the hypothesis that the pluton was emplaced by dykes and contributes to the debate about whether dyking, or the long-held view of diapirism, is the dominant granitic magma ascent mechanism,” says Adam Goddard, who recently graduated with a BSc in geophysics from the University of Durham.

Drawn to gravity work as an undergraduate, Goddard chose to study the St. Austell pluton for his final year thesis because the pluton is associated with a large Bouguer gravity anomaly (approximately 50 mGal) and he was able to tap into a rich body of research that he could integrate with his own findings.

Goddard performed the inversion using data from 2,702 gravity stations from the British Geological Survey (BGS)’s gravity databank and a separate gravity survey designed to supplement the BGS data. First, he reduced the data from both sources to Bouguer gravity anomaly data by applying Drift, Latitude, Free-Air, Bouguer and Terrain corrections using Geosoft Oasis montaj. Then he gridded the data using a minimum curvature algorithm.

Goddard used density contrasts of 110-180 kgm-3 to model a range of possible pluton thicknesses. He produced a model for each density contrast by performing a structural inversion on a low density sheet representing the roof of the pluton (Figure 2; Figure 3). By varying the depth of the base of the model at each density contrast, he found the thickness of granite that produced the most realistic model (i.e. that had small mean and residual misfit errors and an upper surface geometry that fit the St Austell pluton’s outcrop and depth constraints from boreholes.)

“I used Oasis montaj to grid my gravity data and GM-SYS 3D to make my 3D models,” says Goddard, who is currently pursuing a master’s degree at the University of Leeds. “Thanks to Geosoft’s excellent help guides and the clarity of the user interface in Oasis montaj, I mastered the software over a short period of several weeks.”

Using an Oasis montaj research subscription, Goddard modelled pluton thicknesses ranging from 11 to 25 km (Figure 4a), but concluded that intermediate density contrasts (140-150 kgm-3) produced the most accurate results. The resulting 14.5-16.5 km thickness estimate suggests the pluton cannot be underlain by a basal thrust at depths of 11-15 km as earlier theories proposed. The estimate also backs up recent geothermal studies that have shown that the Cornubian plutons must be 16 km thick to satisfy heatflow data.

All the models included a shallow east-west trending section of granite that outcropped at the surface and a broader, northeast-southwest trending sheet at depth. Goddard concluded that these shallow and deep sections of the models could represent two separately intruded sheets of granite, although gravity data would also support a more complex structure including at least three sheets of granite.

Goddard aspires to a career in geophysical exploration when he completes his MSc but is keeping an open mind about working in a different sector of geoscience or pursuing a PhD.

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