Virtual Pole from Magnetic Anomaly (VPMA): A procedure to estimate the age of a
rock from its magnetic anomaly only
Renato Cordani
a,
⁎
, Wladimir Shukowsky
b
a
Reconsult Geofísica, Brazil
b
IAG-USP, Brazil
abstractarticle info
Article history:
Received 14 February 2008
Accepted 6 July 2009
Keywords:
Magnetic anomalies
Paleomagnetic virtual poles
South American tectonic plate
Mesozoic intrusions
Virtual Pole from Magnetic Anomaly (VPMA) is a new multi-disciplinary methodology that estimates the age of a
source rock from its magnetic anomaly, taken directly from available aeromagnetic data. The idea is to use those
anomalies in which a strong remanent magnetic component is likely to occur. Once the total magnetization of the
anomaly is computed through any of the currently available methods, the line that connects all virtual
paleogeographic poles is related with the position, on a paleogeographic projection, of the appropriate age fragment
of the APWP curve. We applied this procedure to five (5) well-known magnetic anomalies of the South American
plate in SE Brazil, all of them associated to alkaline complexes of Mesozoic age. The apparent ages obtained from
VPMA on three of the anomalies where the radiometric age of the source rock is known – Tapira, Araxá and Juquiá –
were inside the error interval of the published ages. The VPMA apparent ages of the other two, where the age of the
source rock is not known (Registro and Pariqueraçu magnetic anomalies) were geologically coherent. We expect that
the application of the VPMA methodology as a reconnaissance geochronological tool may contribute to geological
knowledge over continental areas, especially when the source rocks of the magnetic anomalies are unknown or
buried below superficial sediments.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
The determination of magnetic paleopoles from seamount magnetic
anomalies constitutes the major data source for the Pacific Apparent
Polar Wander Path (APWP); Emilia & Massey (1974); Hilderbrand &
Parker (1987); Gee et al. (1989); Sager & Koppers (2000); Lee et al.
(2003); Kubota & Uchiyama (2005). In these cases the bathymetry
provides an immediate constrain for the geometry of the magnetic
sources, i.e., the submarine volcanic cones.
This article was motivated by the great success in using seamount
magnetic anomalies for APWP mapping. However, the main difficulty is
that in this case we are dealing with continental magnetic anomalies
wheresizeandgeometryofthesourcesareusuallyunknownorvery
difficult to characterize.
One of the traditional applications of paleomagnetism consists in
isolating the primary natural remanent magnetization (NRM) of a given
rock, and then determining the correspondent paleogeographic pole at
the time of the rock emplacement. This result is then compared to
the APWP. As it will be described later, the Virtual Pole from Magnetic
Anomaly (VPMA) technique proposed here is based on the assumption
that the remanent component
⇀
Jr determined from the anomaly, would
be approximately correspondent to the primary NRM of the source
rock.
Therefore, this methodology produces an age estimation of the rocks
without need of sampling them. Here we apply this methodology to five
Mesozoic alkaline intrusions in Southeast Brazil, and compare the VPMA
age results to the known radiometric ages obtained from geochronological
studies.
All the magnetic data used in this work came from CPRM (Brazilian
Geological Survey) regional aeromagnetic database. Line spacing
varied from 1000 to 2000 m. Application of the VPMA methodology to
high-resolution magnetic datasets should yield better results.
2. VPMA methodology description
The main concept of VPMA methodology is to isolate the remanent
component
⇀
Jr of the magnetic anomaly and use it in paleomagnetic
studies. Therefore, first we calculate the total magnetization of the
magnetic source using one of the existing methods, and then we
remove the induced component using the following vectorial equa-
tion.
J
→
= J
→
i + J
→
r ð1Þ
Journal of Applied Geophysics 69 (2009) 96–102
⁎ Corresponding author.
E-mail addresses: rcordani@reconsult.com.br (R. Cordani), wladimir@iag.usp.br
(W. Shukowsky).
0926-9851/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jappgeo.2009.07.001
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