gravity and magnetic modeling sergipano belt, brazil: tectonic control...

International Journal of Geophysics and Geochemistry

2019; 5(2): 58-71 http://www.aascit.org/journal/ijgg ISSN: 2381-1099 (Print); ISSN: 2381-1102 (Online)

Gravity and Magnetic Modeling Sergipano Belt, Brazil: Tectonic Control and Crustal Thickness of the Basement Adjacent to the Sergipe-Alagoas Basin

Alanna Costa Dutra*, Alexandre Barreto Costa, Roberto Max de Argollo

Department of Earth Physics and Environment, Physics Institute, Federal University of Bahia, Salvador, Brazil

Email address

*Corresponding author

Citation Alanna Costa Dutra, Alexandre Barreto Costa, Roberto Max de Argollo. Gravity and Magnetic Modeling Sergipano Belt, Brazil: Tectonic Control and Crustal Thickness of the Basement Adjacent to the Sergipe-Alagoas Basin. International Journal of Geophysics and

Geochemistry. Vol. 5, No. 2, 2019, pp. 58-71.

Received: April 22, 2019; Accepted: June 13, 2019; Published: July 8, 2019

Abstract: This work used gravimetric and magnetic data to investigate the Sergipano Belt that occupies the Southern Borborema Province, Brazil. The main objective was to interpret tectonic relationships between the geological domains, crustal lateral variation of the physical properties and the behavior of the Moho relief. The gravity and magnetic inversion was performed to determine the physical properties magnetic susceptibility and density contrast to delineate the geometry of the true source. The regional gravity anomaly was used to obtain solutions depth of the interface crust-mantle in which it was necessary to know the initial model of the crustal thickness and density contrast. The geophysical measures was used to delineate the initially crustal thickness and compared to the results based on a compilation of data published in the literature mainly derived from seismic database such as deep seismic refraction experiments. These magnetic sources have signals with different amplitudes that originate from different geometric sources, situated at different depths and with different magnetic properties. As to the crustal thickness results, we found that the southern region of the Sergipano Belt has a crustal (34-35 km) and mantle uplift, mainly in the Girau do Ponciano Dome. The Rio Coruripe domain as well as the PEAL Terrain has a thicker crust (38-40 km), with magnetic and gravimetric sources that reach from 15 to 20 km deep marked in sections.

Keywords: Gravity and Magnetic Modeling, Edge Detection, Tectonic Control, Sedimentary Basin

1. Introduction

The Sergipano Belt is located in northeast Brazil, being the result of the collision between the Pernambuco-Alagoas terrain (PEAL) to the north and the São Francisco Craton (SFC) to the south and occupies the Southern Borborema Province ([6, 16, 30, 31]). During the development of the main collisional event in the Sergipano Belt, several granites were generated that intrude the supracrustal rocks. The establishment of the structural relationships of these granites with the deformational events together with the dating of these bodies allows the establishment of the duration interval of the main collisional event in the Sergipano Belt. The Figure 1 represents a simplified tectonic and geological map of the Southern Borborema Province.

The tectonic evolution of the Sergipano Belt is characterized by a series of contractionary slices, stacked from north to south on the north shore of the São Francisco Craton. In general, it is accepted the subdivision of this band into tectonic domains known as: Canindé, Poço-Redondo-Marancó, Macururé, Vaza Barris and Estância, which are separated by important Neoproterozoic shear zones such as Macururé, Belo Monte-Jeremoabo and Itaporanga ([15, 16, 31]).

The Sergipano Belt is often considered to be the product of a continental collision between the PEAL Terrain and the São Francisco Craton. Given the overall southward vergence of the Sergipano Belt, the São Francisco Craton should represent the lower plate whereas the PEAL Terrain would constitute the upper plate. The Macururé Complex and the

International Journal of Geophysics and Geochemistry 2019; 5(2): 58-71 59

low-grade to unmetamorphosed domains of the southern Sergipano Belt correspond to distal and proximal facies, respectively, of platform sediments deposited onto the passive continental margin of the São Francisco Craton. In this model, the suture zone separating the two plates would be located close to the transition zone [17].

The Poço Redondo-Marancó and Canindé domains represent, respectively, the registration of a Tonian magmatic

arc (979-952 Ma) of the Cariris Velhos orogenic cycle and a subsequent continental extension of Brazil, being composed of plutonic, volcanic and metasedimentary rocks. On the other hand, the Macururé, Vaza-Barris and Estância domains are age Ediacaran, being formed by homonymous supracrustal rocks that present incipient to moderate metamorphism ([16, 17])

Figure 1. Tectonic and geological map of the region studied, with identification of the main regional structures modified of [31]. PEAL - Pernambuco-Alagoas

Lineament; ZCSM = Serra do Mascarenhas Shear Zone; ZCBJ. = Belo Monte-Jeremoabo Shear Zone; ZCI. = Itaporanga Shear Zone.

There are few discussions about the northern boundary of the Sergipan Belt and its contact with the Pernambuco-Alagoas Block. In this work we study the contacts of the main areas and its relationship with the tectonics of the Belt. Gravity and Magnetic images are frequently used to delineate geologic contacts and border of geological formation. These maps have signals with various amplitudes that originate from different geometric sources, situated at different depths and with different properties. The interpretation and modeling of geophysical data (gravimetry and magnetometry) provide these tectonic limits precisely.

Besides the upper section of the Neoproterozoic cover around the São Francisco Craton has been interpreted as foreland basin deposits recording the erosion of the belts themselves, implying a late-post tectonic radial sedimentary in towards the centre of the craton (e.g. [19]). However, based on detailed studies carried out in the area surrounding the Itabaiana and Simão Dias domes, this model has been ruled out in the southern part of the Sergipano Belt and an alternative explanation has been required for the cratonic sediments [18]. To reach this alternative, the results of these

studies allowing discussing the evidence for mantle uplift of the ancient São Francisco Plate, such events are applicable to other cratons and belts of western Gondwana as well.

The gravity and magnetic measurements are important to produce a geological interpreting, understanding the tectonic environment. The continental boundaries of terrain were commonly recognized by contrast in all contact and knowing the gravimetric and magnetic anomaly, modeling and depths solutions of the crust. This modeling allows us to interpret the crustal domains and their tectonic relationships with the behavior of the crust-mantle interface. The structures in shallow sections often reside on the basement, at least in some depth, and failures in shallow sections are often controlled by the reactivation of basement faults.

2. Geologic Context

The Sergipano Belt is an orogenic belt located in the southern Borborema Province and composed of the Canindé, Pozo Redondo-Marancó, Macururé, Vaza Barris, Estância and Rio Coruripe ([15, 16, 31]). This belt is one of the most

60 Alanna Costa Dutra et al.: Gravity and Magnetic Modeling Sergipano Belt, Brazil: Tectonic Control and Crustal Thickness of the Basement Adjacent to the Sergipe-Alagoas Basin

significant orogenic belt in the Brazilian Northeast because it contains lithologic structures and domains that can be compared to those of Phanerozoic orogens ([31]).

The stratigraphy of the area is expressed as a function of

the main geological ages within each tectonic compartment. The distribution of the main units and structures observed, as well as the layout of the geology throughout the entire transection can be observed in Figure 2.

Figura 2. Simplified geological map with identification of the main regional structures modified of Argollo et al. (2012). ZCSM = Serra do Mascarenhas

Shear Zone; ZCCCN = Congo-Cruzeiro do Nordeste Shear Zone; LPE = Pernambuco Lineament; ZCBJ. = Belo Monte-Jeremoabo Shear Zone; AMT = Alto

Moxotó Terrain; RCT = Rio Capibaribe Terrain; PEAL = Pernambuco-Alagoas Terrain; RCD = Rio Coruripe Domain; SEB = Sergipano Belt ; ITB =

Itabaiana Dome; GPD = Girau do Ponciano Dome; SFC = São Francisco Craton.

The Sergipano Belt with ESE±WNW trending orogenic belt lying between the São Francisco Craton to the south, and the Pernambuco-Alagoas Massif, which is part of the Borborema Province ([1, 31]). The belt occupies the central part of thee 2000 km long megaorogen (Figure 2 and 3). The São Francisco Craton is surrounded by several Pan-African/Brasiliano deformation belts and because the Sergipano Belt contains basement domes mantled by metasediments with well preserved sedimentary features, lying less than 5 km away from the craton margin (the Itaporanga Fault, as will be seen ahead), understanding the tectonic evolution of the craton and the belt may provide answers to questions concerning the evolution of Proterozoic supercontinents [17].

The northern portion of the Sergipano Belt is further subdivided into the Canindé and Rio Coruripe domains, whose limit would be a sinistral transcurrent shear zone. Metasedimentary rocks in these two latter domains were attributed, respectively, to the Araticum and Arapiraca complexes, whereas those in the PEAL Domain were included in the Cabrobró Complex. Most of the central and

northern parts of the Sergipano Belt are constituted by metasedimentary rocks of the Macururé Complex, with basement rocks exposed in the Jirau do Ponciano gneissic dome (Figure 2). The northwestern portion of the Sergipano Belt is separated from the PEAL Domain by the Poço Redondo-Maranco and Canindé e domains.

The models interpreted to the Sergipano Belt assume the existence of a large ocean between the São Francisco Craton and the Borborema Province before the Brasiliano Orogeny ([16, 17, 31]). However, none of the features expected from a collisional event following oceanic closure are supported by in previous works.

3. Materials and Methods

The development of the databases involved merging numerous surveys with gravity and aeromagnetic data with highly variable specifications and quality [13]. The integrated and corrected anomaly maps were processed and interpreted (Figure 4). Knowing the gravity and magnetic anomaly it was possible to estimate the depth top and bottom, magnetic


lineaments, faults, blocks, the lateral extension, the width of the sources. From these results, we can produce a geological interpreting and understanding the tectonic environment. For regional exploration, magnetic measurements were important for example, continental boundaries of terrain were commonly recognized by magnetic contrast in all contact. Such regional interpretations required continental scale for magnetic databases. The method allows solving inversion problems of magnetic and gravimetric surveys, recovery of anomalous magnetic susceptibility, density and geometry of magnetic objects.

The aeromagnetic projects have a spacing of flight lines of 500 m oriented in the N-S direction, with flight height of 100 m, the interval between measurements of the magnetometer of 0.1 s and the spectrometer 1.0 s. The survey was carried out in two different blocks with different flight and tie-line directions, and data acquisition was performed perpendicular to the main structures of the surveyed area [13]. The magnetic data of the study area generated from the pre-processed data, using different combinations of parameters, the cell size of 1/4 of the flight line. Grids of the magnetic anomaly were generated and then a database of each grid generated around the Sergipe-Alagoas Basin defining the study area. The magnetic system used was na optically pumped (cesium vapor) magnetometer that was installed in a stinger extension behind the tail of the aircraft. The output from the magnetometer was sampled at 0.1 s to a resolution of 0.001 nT with a noise envelope less than 0.01 nT.

In the next processing step, the data were interpolated to a regular grid, using algorithms that maintain data fidelity at the original measurement locations. This step was followed by correction of spurious effects caused by the leveling of the original grids. The fourthorder difference technique was used to track anomalous spikes in the magnetic data and to condition sampling along the flight lines based on the spatial Nyquist frequency. This was performed on the selected interpolated grid, which contained square cells 125 x 125 m. The algorithm was based on linear interpolation along the direction of the flight lines, and on the Akima spline perpendicular to the flight lines. Microleveling and decorrugation techniques were further applied to the data. This procedure resulted several geophysical data products, including thematic maps of both individual variables and composite variables, for use in geologic analysis and interpretation.

Usual linear transformations were applied to the gravity and magnetic data to process changes in the amplitude and/or phase related to the set of the data. These transformations are carried out by multiplying the Fourier transform in the data set in the frequency domain. The inverse Fourier transform returns to the space domain and gives the current field to the upper level. This is equivalent to convolving the field in the space domain by an operator (or filter). All transformations of the magnetic field work on this path.. For the magnetic data processing, a regional-residual separation process was required to obtain the subject of interest of this study. To do so, an upward continuation was performed, where the

estimated depth value used in this process was obtained through radially average power spectrum analysis of the magnetic data.

We did the data acquisition with a Scintrex CG5 relative gravimetric and the necessary planialtimetric survey with differential GPS. The data acquired in the field were georeferencing and processed, applying the gravimetric corrections, interpolation and filtering. Gravity varies from point to point over the entire earth's surface. Variations with different causes overlap and are mutually mixed. When the objective is to study gravity variation caused by differences in density and mantle, it is necessary to eliminate all other known variations (variations caused by: rotation and flatness of the Earth, attraction of the Moon and the Sun, variations in altitude between measurement points, effects of the crust structure and upper mantle). The process of eliminating these variations is called "gravimetric correction" and the resulting values are known as "gravimetric anomalies". The equations used in this procedure are found in didactic texts such as Blakely [5]. The gravimetric anomaly derives from the presence of different densities on Earth, and the gravimetric method is sensitive when these differences are found side by side. Most rocks have densities between 1.6 and 3.2 g/cm³. The density depends on the composition and the porosity. The sedimentary rocks, on average, have lower values, depending on the porosity, and the density of the fluid that fills the pores. Therefore, sedimentary rock sequences usually exhibit increased density as a function of depth due to compaction. Constant density values for the initially presumed sediment pack can later be replaced by interpretive models that allow an increase in sediment density with depth. For example, Chai and Hinze [7] and Cordell [12] assume that the density contrast decays exponentially. Bhaskara Rao [4] established a quadratic variation for density contrast and Sari and Salk [35] use a hyperbolically varying density contrast with depth. The appearance of the anomaly depends on the dimensions, contrast of density and depth of the anomalous body. The horizontal extent of the anomaly is called the apparent wavelength. This is a measure of the depth of the body. Deep bodies tend to produce flatter anomalies that occupy a larger area (long wavelength). Shallower bodies tend to produce narrower anomalies of more definite amplitude (small wavelength). Large-wavelength, deep-source anomalies tend to be termed regional, while small-wavelength anomalies are called residual.

In order to estimate the regional gravimetric effect and subsequent separation, the least square polynomial fit method were applied, where the surface polynomial that best represents the regional behavior of the data were applied. The choice of the best regional was done by analyzing the regional and residual maps, verifying which degree best represents the anomaly and how the regional surface fits the data outside the area of concentration of the anomalies. In Figure 3 the residual Bouguer anomaly was obtained by the removal of a third order regional surface adjusted by the method of least squares.


(a)

(b)

Figure 3. Southern Borborema Province: (a) Residual Bouguer Anomaly; (b) Magnetic Anomaly of the Total Field. Profiles AA’ and BB’ to the gravity and

magnetic data inversion.


The gravimetric data collected by authors and integrated

with data requested from the National Petroleum Agency (ANP) and the Geological Survey of Brazil (CPRM) are in the Figure 3.

The regional gravimetric anomaly was used to obtain solutions depth of the interface crust-mantle. This modeling allows us to interpret tectonic relationships between the crustal domains and the behavior of this interface. The method was applied to the gravimetric data estimating a maximum depth and compared to the estimated value by interpretations of seismological data (the mean depth).

We used an inversion algorithm developed by Gomez-Ortiz and Agarwal [23] to compute three-dimensional geometry of a density interface using the Parker–Oldenburg method. The

function represents auseful tool when the density interface does not intercept the surface topography and when its mean depth is larger than its amplitude. The program is very efficient and can handle large data sets. The program requires a previous knowledge of two parameters: the mean depth and the density contrast of the interface. Using these values and the gravity anomaly due to the density interface as the input, an iterative procedure based on the rearrangement of Parker’s formula proposed by Oldenburg [29] computes the geometry of the interface until a user-defined convergence criterion, or a maximum number of iterations are satisfied. An output file with the depth to the interface is obtained, together with the RMS error value and the iteration at which the process has finished.

Figure 4. The aeromagnetic data represented in terms of total horizontal derivative – THD (nT/m), main magnetic lineaments, shear, and shear zones.

4. Results and Discussion

The gravimetric and magnetic data was used to interpret tectonic relationships between the geological domains, crustal lateral variation of the physical properties and the behavior of the Moho relief: Vaza-Barris, Macururé, Rio Coruripe and Pernambuco- Alagoas Terrain. The gravity and magnetic inversion was performed to determine the physical properties magnetic susceptibility and density contrast to delineate the geometry of the true source and the regional gravimetric anomaly was used to obtain solutions depth of the interface crust-mantle. The distribution of the contrast of

density and magnetic susceptibility in depth in the continental crust was correlated with the crustal model and their tectonic relationships in the domains, as well as delineated structures, such as faults and blocks.

4.1. Edge Detection and Tectonic Control

Magnetic maps are frequently used to delineate geologic contacts and border of geological formation. These maps have signals with various amplitudes that originate from different geometric sources, situated at different depths and with different magnetic properties. The calculation of the field at higher levels is called continuation upwards and


proposes the removal of high-frequency anomalies relative to low-frequency anomalies. The process can be used to suppress the effect of shallower anomalies when detailing deeper anomalies is desired.

Many filters commonly used to enhance subtle detail in the magnetic field are based on combinations of the horizontal and vertical derivatives of the data set. The vertical derivative has been used for many years to delineate edges in magnetic field data ([24, 37]). In recent years, improved methods have been developed rapidly. For instance, the tilt angle (TDR) by Miller and Singh [26], the Theta map second Wijns et al. [39], the total horizontal derivative of the tilt angle as an edge detector (THDR) by Verduzco et al. [38], the horizontal tilt angle (TDX) according to Cooper and Cowan [9], the balanced analytic signal Cooper [11] and the normalized horizontal derivative Ma and Li [25]. Cooper [9] applied the various local phase-based filters on aeromagnetic data from Australia. Cooper and Cowan [11] introduced the terracing method as an operator that is applied to potential field data to produce regions of constant field amplitude that are separated by sharp boundaries. Ferreira et al. [22] showed the efficiency of the tilt angle of the horizontal gradient for qualitative interpretation of magnetic data. Eshaghzadeh [20] introduce a derivative operator based on the tilt angle of the horizontal and vertical derivatives of the total horizontal gradient of gravity data set, normalized by the analytic signal amplitude (THA). The amplitude of the analytic signal or total gradient is a commonly used filter that produces bell-shaped anomalies over magnetic bodies [27]. The analytic

signal is a linear combination of horizontal and vertical derivatives.

A commonly used edge-detection filter is the total horizontal derivative (THD) which is given by [10]. The total horizontal derivative of the study area is showed in Figure 5. The effect of the total horizontal derivative signal is to highlight the edges of the sources of the anomalies and is very useful in the case of magnetic data that contain the influence of magnetization remnant or taken at low magnetic latitudes. The amplitude is the symmetric sine-shaped function with exactly the top of each contact and width directly related to the depth of the body.

This filter enhances edges of features of all azimuths. Miller and Singh [26] introduce the tilt angle that is ratio of the vertical derivative to the value of the total horizontal derivative. If the magnetic contrast is positive, the tilt angle value is positive when over the source, passes through zero when over, or near, the edge where the vertical derivative is zero and the horizontal derivative is a maximum and is negative outside the source region.

The tilt angle has a range of -90°to + 90°and is much simpler to interpret than the analytic signal phase angle [9]. Ferreira et al. [21] presented an edge detection method that is based on the enhancement of the THD of magnetic anomalies using the TA. It is referred to as the tilt angle of the horizontal gradient (TAHG). In this study, efficiency of the TAHG is considered for magnetic data set. The TAHG transform range is from −π⁄2 to +π⁄2 (Figure 5).

Figure 5. The aeromagnetic data represented in terms of tilt angle of the total horizontal gradient - TAHG (rad), main magnetic lineaments, shear, and shear

zones.


The results show a change in depth of the sources in which in the northern part of the study area are deeper and the southern sources in the region are shallower. As the region of is a collision area of blocks. We can identify different lineaments and magnetic structures that demarcate the tectonic limits of the subdivision of this belt into domains known as Rio Coruripe, Macururé, Vaza Barris and Estância, which are separated by important neoproterozoic shear zones such as Macururé, Belo Monte-Jeremoabo and Itaporanga (Figure 4 e 5).

4.2. Gravity and Magnetic Data Modeling

The method allows solving forward and inversion problems of magnetic and gravimetric surveys (recovery of anomalous magnetic susceptibility, density and geometry of magnetic objects). The magnetic susceptibility is set in SI system (n*10-5), density in g/cm³, measured values in nanotesla or milligals [40].

The volume of the body is obtained by calculating the gravitational attraction caused by each vertex of the modeled

polygon. In this way, the more vertices the polygon has, the better the accuracy of the adjustment. The calculated anomaly will also depend on the density contrast used between the model bodies, the density being constant for each body.

The magnetic fields on the Earth surface can be represented as a vector sum, where and are normal and anomalous magnetic fields, magnetic field variations. The normal field is divided into a dipole and non-dipole components. The dipole field, which, to a first-order approximation, is the magnetic field of the Earth, is a field of a homogeneous magnetized sphere. The difference between the dipole field (calculated) and areal and normal field (measured by satellite) is a non-dipole part of the normal field. It is often named as a residual field or a field of continental anomalies (size these anomalies are commensurate with the sizes of the continents). Their maximum does not exceed 30% of a dipole field value. Sources of the anomalous magnetic field are magnetized objects, located near the surface of the Earth. Maximum depth of magnetic rocks is about 50 kilometers. At greater depths, magnetic properties of rocks disappear due to high temperatures.

Figure 6. Gravity and Magnetic 2D Inversion showing the main structures observed throughout this work (AA’): (a) Observed and Calculated Gravity (red)

and Magnetic data (blue); (b) Density contrast model in g/cm³; (c) Magnetic susceptibility model is set in SI system (n*10-5).

Gravimetric and magnetic modeling is based on the direct calculation of the gravitational or magnetic field produced by a geometric shape, in which an initial model is constructed

for the source body based on available geological and geophysical information, so that the anomaly generated by this model the gravimetric or magnetic anomaly observed.


This process of parameter adjustment is performed until the calculated and observed anomalies are as similar as possible [5]. The 2D interpretation of profile and areal multi-level data obtained from magnetic and gravity surveys.

The inversion algorithm allows reaching minimum misfit values. It is recommended to use it in the initial interpretation, in most cases. Occam Inversion by least squares method with using a smoothing operator and additional contrast minimization [8]. As a result of using this algorithm, we get the more smooth parameters distribution.

In this work we have a set of geophysical observations d produced by arbitrary sources, adopting the Cartesian coordinate system. It is assumed that the sources are bi-dimensional bodies. The source is included in a region that represents the substrate and is discretized in M cells. Within each cell it is assumed that the physical property is constant. The observations d are approximated by a continuous functional f (x, q, p) expressing the relation between the physical property and the corresponding geophysical observations; Q is a set of parameters associated with the geometry and position of each cell and p is a dimension

vector M of the physical properties. The linear formulation was used in the inverse problem

whose substrate is represented by a homogeneous mesh, of known position and size, but with contrast of unknown physical property. The inversion of data consists of determining the physical property in each cell and, with this, delineates the geometry of the true source. A priori information about the source was incorporated by the method. All geological information can be translated mathematically and automatically incorporated by method.

The relief of the basement will be characterized gravimetrically and magnetically and models that allow to explain the distribution of the contrast of density and magnetic susceptibility in depth in the continental crust, as well as the structures, such as faults and blocks in Figures 6 and 7.

The Rio Coruripe domain has it has a very strong magnetic and gravimetric signals, with high magnetic susceptibility and density contrast that reach from 12 to 15 km deep marked in sections AA’ (Figure 6). In the eastern part of the BB 'profile reaches the depth of 22 km (Figure 7).

Figure 7. Gravity and Magnetic 2D Inversion showing the main structures observed throughout this work (BB’): (a) Observed and Calculated Gravity (red)

and Magnetic data (blue); (b) Density contrast model in g/cm³; (c) Magnetic susceptibility model is set in SI system (n*10-5).

The Macururé magnetic domain reach from 15 to 20 km deep marked in sections AA’ in the Figure 6. It’s formed by


supracrustal rocks that present moderate metamorphism and magnetic relief with low amplitude and wavelength with positive gravity anomalies.

The Pernambuco-Alagoas Terrain consists dominantly of high-grade, often migmatized, orthogneisses and supracrustal units with strongly irregular magnetic relief, high amplitude and positive gravity anomalies. The main lineaments and lateral contacts between the blocks were marked along the

magnetic relief with high amplitude and dip north. The magnetic domains of the sedimentary basin Sergipe-

Alagoas is represented by essentially negative values of mean to high amplitudes. The negative gravity anomalies with mean amplitudes are represented for density contrast model with negative density contrast. The Figure Schematic geological profile showing the main structures observed throughout this work in profile from the SW to NE (Figure 8).

Figure 8. Schematic geological profile showing the main structures observed throughout of the profile CD in Figure 2. Domains: PEAL = Pernambuco

Alagoas Terrain, SEB = Sergipano Belt. Shear Zones: ZCCN = Congo-Cruzeiro do Nordeste, LPE = Pernambuco Lineament, ZCBJ = Belo Monte-Jeremoabo.

4.3. Crust-Mantle Interface Depth Solutions

The present work aims at the acquisition, processing and interpretation of gravimetric data, along the region of the Sergipano Belt, in order to make a gravimetric modeling of the crustal domains and their tectonic relationships. It is possible to use gravimetric data to estimate the depth of the crust - mantle interface.

This procedure is based on a relation between the Fourier transform of the gravity anomaly and the sum of the Fourier transform of the topography of the crust-mantle interface. Considering the mean depth of the density interface and the density contrast between the two media, the three-dimensional geometry of the interface is iteratively calculated [23]. The iterative process is terminated when the error (RMS) between the two successive approximations is less than a preestablished value, used as a convergence criterion, or even a maximum number of iterations. A high cut-off filter in the frequency domain was incorporated to enhance convergence in the iterative process. The inversion of the gravity anomaly is presented to obtain the depth of Moho.

The inversion procedure uses the equation described by [32] to calculate the gravity anomaly by a uniform layer of material by means of a series of Fourier transformations. [29] rearranged this method to calculate the depth of the undulating interface of the gravimetric anomaly by means of an iterative process. The process allows us to determine the topography of an interface by means of an inversion. In this procedure, we assume the average depth of the interface, z0 and the density contrast associated with the two means, ρ. The gravimetric anomaly is first degraded before the calculation of the Fourier transform. This process is continued until a reasonable solution is reached.

Following [29], the process converges if the interface depth is greater than zero and does not intercept the

topography. In addition, the interface relief amplitude must be less than the average interface depth. As the inversion operation is unstable at high frequencies, high cut filter, HCF (k) is included in the inversion process to ensure the convergence of the series. This filter was used to restrict the high frequency content in the Fourier spectrum of the observed gravity anomaly. The frequency, k can be expressed as 1 / χ; Being χ its wavelength in kilometers.

The iterative process is terminated when a given number of iterations is performed or when the difference between two successive approximations of the topography is less than a pre-set value as convergence criteria. Since the topographic relief is calculated by the inversion procedure, it is desirable to calculate the gravity anomaly produced by the calculated topography. In general, this modeled anomaly should be very similar to that used as input in the first step of the inversion process.

This method of inversion was applied to the regional gravimetric data for the Southern Borborema Province. The results were presented in the image of Figure 4 and compared to the depth results of the crust-mantle interface obtained from the interpretation of the seismological stations both obtained in this project (symbols in blue) and available literature (green symbols).

The southern region of the Sergipano Belt has a crustal depth of 34-35 km and mantle uplift, mainly in the Girau do Ponciano Dome. The Rio Coruripe domain as well as the PEAL Terrain has a thicker crustal depth of the 38-40 km (Figures 9 and 10).

We delineated the relief of the interface crust-mantle in which it was necessary to know the gravimetric anomalies and the density contrast. The geophysical measures was used to delineate the crustal thickness and compared to the results based on a compilation of data published in the literature mainly derived from seismic datasets such as deep seismic refraction experiments ([3, 33]).


Figure 9. Crust-Mantle Interface Depth Solutions from the gravity anomalies and the density contrast. The solutions were compared to the results from

seismological stations.

Figure 10. Gravtity anomalies Crust-Mantle Interface Depth Solutions of the profiles AA’ and BB’.

The Sergipano Belt occupies the Southern Domain, being the result of the collision between the PEAL terrain to the north and the São Francisco Craton to the south. A series of

contractionary slices, stacked from north to south on the north shore of the San Francisco Craton are also well marked along the region's magnetic relief. The magnetic domains of


the sedimentary basin Sergipe-Alagoas is represented by essentially negative magnetic anomalies of mean to high amplitudes (-50 to -200 nT) and negative gravity anomalies of mean to amplitudes (-10 to -60 mGals)

The Macururé, Vaza-Barris and Estância domains are formed by supracrustal rocks that present moderate metamorphism and magnetic relief with low amplitude and wavelength with positive gravity anomalies. The Rio Coruripe domain has it has a very strong magnetic (30-150 nT) and gravimetric (20-50 mGals) signals, with high magnetic susceptibility and density contrast that reaches depths over 12 km. Its lateral extension is marked in the images and the total horizontal gradient analytical signal and the total horizontal gradient.

The PEAL terrain has a different composition from the rest of the region. As can be seen by the important magnetic lineaments mapped in Figure 4 and 5, where the main lineaments and lateral contacts between the blocks were marked along the magnetic relief with high amplitude and wavelength. Ancient subduction zones are characterized by such developments in such works as the [28]. The predominance of a transpressive deformational tectonic represents the most important structural feature of this terrain, as demonstrated by [6, 34]. The main units of PEAL were orthogneisses and migmatites of granite metatonalitic composition and supracrustal rocks rich in biotite and granada, quartzites, amphibolite calcificsilic rocks and locally migmatized orthognaisses. The data of [28] in metasedimentary rocks in the transition zone between the Pernambuco-Alagoas Domain and the Sergipano Belt were studied in order to place constraints on the geological evolution of this portion of West Gondwana. The authors interpreted that during the course of an extensional event at c. 673-647 Ma, Tonian granitic intrusions and synextensional metamorphic rocks were unroofed and eroded to provide zircons for sediments deposited in the north, whereas Paleoproterozoic and synextensional magmatic rocks were the main sources for sediments in the south. Peak metamorphic conditions and contractional deformation are constrained to c. 630-600 Ma.

As to the crustal thickness results, we found that the southern region of the Sergipano Belt has a crustal (34-35 km) and mantle uplift, mainly in the Girau do Ponciano Dome. The Rio Coruripe domain as well as the PEAL Terrain has a thicker crust (38-40 km), with magnetic and gravimetric sources that reach from 12 to 15 km deep marked in sections AA’ and BB'. To determine the location of these profiles, the total horizontal derivative (THD) was applied to the residual magnetic field data, which allowed a better visualization of the source’s edges, delimitating the contacts between the source bodies. It was also possible to identify folds in the Rio Coruripe domain’s region, due to its location at the northeast folding region of the Borborema Province. Contractional shear zones, such as the Belo Monte Jeremoabo shear zone, indicated by the triangles in figure 2, and a sinistral shear zone, indicated by the arrows, located above the Pernambuco-Alagoas source, were also found. It’s

also possible to notice the presence of the Jirau do Ponciano gneissic dome, located below the Rio Coruripe source. The THD map also allowed the interpretation of the source’s strike-dip directions, where the Rio Coruripe source is at the NW-SE direction, with a SW-NE dip direction and the Pernambuco-Alagoas source is at the NE-SW direction, with a SE-NW dip direction.

[16, 17] interpreted the Sergipano Belt in terms of an oblique collision of the Pernambuco-Alagoas Massif and the ancient São Francisco-Congo Plate, after subduction of an ocean to the north. [36] shows that the subduction zone existed since around 1045 Ma, with formation of intraoceanic arcs up to 1940 Ma, and accretion around 715 Ma to an Andean-type margin developed along the southern border of the Pernambuco-Alagoas Massif. Such evolution implies a suture zone that includes the Macururé fault and the Belo Monte Jeremoabo Fault (BMJF), close to which metamorphism attained the highest pressures in the belt [36].

We can identify accommodation of magnetic material in important lineaments that demarcate the tectonic limits and subdivisions of the Sergipano Belt. Such lineaments corroborate the suture between this domain and the São Francisco Craton much more to the south than the location proposed by other authors.

5. Conclusion

We can produce a geological interpreting and understanding the tectonic environment in the Sergipano Belt that occupies the Southern Borborema Province, Brazil. The gravimetric and magnetic measurements were important for continental boundaries of terrain were commonly recognized by magnetic contrast in all contact. The structures in shallow sections often reside on the basement, at least in some depth, and failures in shallow sections are often controlled by the reactivation of basement faults, it is often possible to identify structures from the interpretation of the basement. The tilt angle of the horizontal gradient could be used with efficiency for qualitative interpretation of magnetic data and to help correlate complex systems for fault finding and block contacts. We can identify different lineaments and magnetic structures that demarcate the tectonic limits of the subdivision of this belt into domains.

The inversion of data determined the physical properties magnetic susceptibility and density contrast in each cell and, with this, delineates the geometry of the true source. The priori information about the source was incorporated by the method. The relief of the basement will be characterized gravimetrically and magnetically and models that allow to explain the distribution of the contrast of density and magnetic susceptibility in depth in the continental crust, as well as accommodation of magnetic material in important lineaments that demarcate the tectonic limits, such as faults and blocks.

The gravimetric anomaly and a model, solutions to depths of the crust were found along an area. This modeling allows us to interpret the crustal domains and their tectonic


relationships with the behavior of the crust-mantle interface. The combination of the gravity and magnetic The combination of gravimetric and magnetic Field, as well as seismic datasets permitted the joint interpretation and obtaining important models for study in the subsurface region as well as to accurately estimate the variation in depth of the crust mantle interface on terrains studied here coupled to lateral variations of physical properties

Acknowledgements

We are grateful to Promob - Cenpes - Petrobras for its support through the Petrobras project 0050.0082543.13-9, to the CPRM (Brazilian Geology Program) for the Aerogeophysical Projects granted and to the National Observatory (ON) for the concession of the measurement equipment.

References

[1] Almeida de, F. F. M., Hasui, Y., Brito de, Neves B. B., Fuck, R. A., 1981. Brazilian Structural Provinces: an introduction. Earth Sciences Review 17, 1±29 Special Issue.

[2] Argollo, R. M. ; Marinho, M. M. ; COSTA, A. B. ; Sampaio Filho, H. A. ; Santos, E. J. ; Coutinho, L. F. C., 2012. Modelo crustal e fluxo de calor nos domínios Estância, Canudos, Vaza-Barris e Macururé adjacente às bordas emersas da Bacia Sergipe-Alagoas.. BOLETIM DE GEOCIÊNCIAS DA PETROBRAS (IMPRESSO), v. 20, p. 283-304.

[3] Assumpção, M., Bianchi, M., Julià, J., Dias, F. L. França, G. S., Nascimento, R., Drouet, S., Pavão, C. G., Albuquerque, D. F., Afonso, E. V. L., 2013. Crustal thickness map of Brazil: Data compilation and main features, Journal of South American Earth Sciences, Volume 43, Pages 74-85. https://doi.org/10.1016/j.jsames.2012.12.009.

[4] Bhaskara, Rao, D. 1986. Modelling of sedimentary basins from gravity anomalies with variable density contrast. Geophys. J. Roy. Astr. Soc., 84: 207-121.

[5] Blakely, R. J., and Simpson, R. W., 1986, Approximating edges of bodies from magnetic or gravity anomalies, Geophysics, v. 51, p. 1494-1498.

[6] Brito Neves, B. B., Van Schmus, W. R., Santos, E. J., Campos Neto, M. C., Kozuch, M., 1995. O evento Cariris Velhos na Província Borborema: integração de dados, implicações e perspectivas. Rev. Bras. Geociênc. 25, 279e296.

[7] Chai, Y. & Hinze, W. J. 1988. Gravity inversion of an interface above which the density contrast varies exponentially with depth. Geophysics, 53: 837-845.

[8] Constable, C. S., Parker, R. L., and Constable, C. G., 1987, Occam’s inversion: A practical algorithm for generating smooth models from electromagnetic sounding data: Geophysics, 52, 289–300.

[9] Cooper, G. R. J. and Cowan, D. R., 2006. Enhancing potential field data using filters based on the local phase. Computers and Geosciences, 32, 1585–1591.

[10] Cooper, G. R. J. and Cowan, D. R., 2008. Edge enhancement of potential-field data using normalized statistics. Geophysics,

73 (3), H1–H4.

[11] Cooper, G. R. J. and Cowan, D. R., 2009. Terracing potential field data. Geophysical Prospecting, 57, 1067– 1071.

[12] Cordell, L. 1973. Gity analysis using an exponential density–depth function - San Jacinto Graben California. Geophysics, 38: 684-690.

[13] CPRM, 2008. Projeto Aerogeofísico Borda Leste do Planalto da Borborema, CPRM (Programa Geologia do Brasil).

[14] Delgado I. M., Souza J. D., Silva L. C, Silveira Filho N. C., Santos R. G., Pedreira A. J., Guimarães J. T., Angelim L. A. A., Vasconcelos A. M., Gomes I. P,, Lacerda Filho J. V., Valente C. R., Perrotta M. M. & Heineck C. A., 2003. Geotectônica do Escudo Atlântico. In: BIZZI LA, SCHOBBENHAUS C, VIDOTTI RM & GONÇALVES JH (Eds.). Geologia, tectônica e recursos minerais do Brasil: texto, mapas & SIG. CPRM, p. 227-334.

[15] Davison, I.; Santos, R. A., 1989. Tectonic evolution of the Sergipano Fold Belt, NE Brazil, during the brasiliano orogeny. Precambrian Research, v. 45, p. 319-342

[16] D’el Rey Silva, L. J. H., 1995. The evolution of basement gneiss domes of the Sergipano fold belt (NE Brazil) and its importance for the analysis of Proterozoic basins. Journal of South American Earth Sciences, v. 8, n. 3/4, p. 325-340.

[17] D’el-Rey Silva, L. J. H., 1999. Basin infilling in the southern-central part of the Sergi- pano Belt (NEBrazil) and implications for the evolution of Pan-African/Brasiliano cratons and Neoproterozoic cover. Journal of South American Earth Sciences 12, 453–470.

[18] D’el-Rey Silva, L. J. H., McClay, K. R., 1995. Stratigraphy of the southern part of the Sergipano Belt, NEBrazil: tectonic implications. Revista Brasileira de Geociências 25, 185–202.

[19] Dominguez, J. M. L., 1993. As coberturas do Cráton São Francisco: uma abordagem do ponto de vista da análise de bacias. In.: Dominguez J. M. L., Misi A. (Eds.), O Cráton São Francisco, SBG. BA-SE, pp. 137–159.

[20] Eshaghzadeh, A., 2014. Anomaly Edge Enhancement of Microgravity Data Using Normalized Standard Deviation. Geodynamics Research International Bulletin, 2, XLVIIILII.

[21] Ferreira, F. J. F., L. G. de Castro, A. B. S. Bongiolo, J. de Souza, and M. A. T. Romeiro, 2011. Enhancement of the total horizontal gradient of magnetic anomalies using tilt derivatives: Part II—Application to real data: 81st Annual International Meeting, SEG, Expanded Abstracts, 887–891.

[22] Ferreira, F. J. F., de Souza, J., Bongiolo, A. B. S., Castro, L. G., 2013. Enhancement of the total horizontal gradient of magnetic anomalies using the tilt angle. Geophysics, 78, J33–J41.

[23] Gomez-Ortiz, D., Agarwal, B. N. P., 2005. 3DINVER. M: a MATLAB program to invert the gravity anomaly over a 3D horizontal density interface by Parker–Oldenburg’s algorithm. Computers & Geosciences vol. 31, p. 13–520.

[24] Hood, P. J. and Teskey, D. J., 1989. Aeromagnetic gradiometer program of the Geological Survey of Canada. Geophysics, 54 (8), 1012–1022.

[25] Ma, G. and Li, L., 2012. Edge detection in potential fields with the normalized total horizontal derivative. Computers and Geosciences, 41 (2012) 83–87.


[26] Miller, H. G. and Singh, V., 1994. Potential field tilt - a new concept for location of potential field sources. Journal of Applied Geophysics, 32, 213–217. Miller, H. G. and Singh, V. (1994) Potential field tilt - a new concept for location of potential field sources. Journal of Applied Geophysics, 32, 213–217.

[27] Nabighian, M. N., 1972. The analytical signal of 2D magnetic bodies with polygonal cross-section: Its properties and use for automated anomaly interpretation. Geophysics, 37, 507–517.

[28] Neves, S. P., Bruguier, O., Silva, J. M. R., Mariano, G., 2015. From extension to shortening: dating the onset of the Brasiliano Orogeny in eastern Borborema Province (NE Brazil). J. S. Am. Earth Sci. 58, 238 e 256.

[29] Oldenburg, D. W., 1974. The inversion and interpretation of gravity anomalies. Geophysics 39 (4), 526–536.

[30] Oliveira, E. P., Toteu, S. F., Araújo, M. N. C., Carvalho, M. J., Nascimento, R. S., Bueno, J. F., McNaughton, N., Basilici, G., 2006. Geologic correlation between the Neoprotero- zoic Sergipano belt (NE Brazil) and the Yaoundé schist belt (Cameroon, Africa). Journal of African Earth Sciences 44, 470–478.

[31] Oliveira, E. P., Windley, B. F., Araújo, D. B., 2010. The Neoproterozoic Sergipano orogenic belt, NE Brazil: a complete plate tectonic cycle in western Gondwana. Precambrian Res. 181, 64 - 84.

[32] Parker, R. L., 1973. The rapid calculation of potential anomalies. Geophysical Journal of the Royal Astronomical Society 31, 447–455.

[33] Pavão, C. G., França, G. S. Bianchi, M., Almeida, T., Huelsen, M. G. V.., 2013. Upper-lower crust thickness of the Borborema

Province, NE Brazil, using Receiver Function. Journal of South American Earth Sciences. Volume 42, March 2013, Pages 242-249. https://doi.org/10.1016/j.jsames.2012.07.003.

[34] Santos dos, E. J., Brito de, Neves B. B., 1984. Província Borborema. In: Almeida, F. F. M., Hasui, Y. (Eds.), O pre- cambriano do Brasil. Editora Edgard Blucher, São Paulo, pp. 123-186.

[35] Sari, Cokun & Salk, Müjgan. 2002. Analysis of gravity anomalies with hyperbolic density contrast: An application to the gravity data of Western Anatolia. Journal of the Balk an Geophysical Society, 5 (3): 87-96.

[36] Silva Filho, A. F.; Guimaraes, I. P.; Brito, M. F. L.; Pimentel, M. M., 1997. Geochemical signatures of main Neoproterozoic Late-Tectonic granitoids from the Sergipano Fold Belt, Brazil: Significance For The Brasiliano Orogeny. International Geology Review, Estados Unidos, v. 39, n. 07, p. 639-659.

[37] Thurston, J. B., and Smith, R. S., 1997, Automatic conversion of magnetic data to depth, dip and susceptibility contrast using the SPITM method, Geophysics, v. 62, p. 807-813.

[38] Verduzco, B., Fairhead, J. D., Green, C. M. (2004) New insights into magnetic derivatives for structural mapping. The Leading Edge, 23 (2), 116–119.

[39] Wijns, C., Perez, C., Kowalczyk and P. (2005) Theta map: edge detection in magnetic data. Geophysics, 70 (4), 39– 43.

[40] ZONDGM2D, 2013. Zond Geophysical Software: Program for 2D interpretation of magnetic and gravity data, Saint-Petersburg: http://zond-geo.com/english/zond-software/gravity-magnetic-self-potential/zondgm2d, 2001-2018.

gravity and magnetic modeling sergipano belt, brazil: tectonic control...

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