Analysis and Interpretation of Regional Geophysical Data in the Kapuskasing Structural Zone of the Superior Province, Canada

Facultad de Ciencias Departamento de Geociencias

Thesis for the degree of Bachelor in geosciences

Author: SHARON VANESSA CUERVO ARCINIEGAS

Research mentor and advisor: PH.D. BOGDAN NITESCU

Bogotá, Colombia December 2020

Acknowledgements

At first, I want to thank Bogdan Nitescu, my research mentor and advisor, for suggesting me the topic of this thesis and for its great knowledge on the subject. Also, I am grateful for his support and for helping me with all my doubts and concerns during the process of this project. To the Geoscience Department and its excellent teachers, thanks to which I have knowledge and love for geosciences, I would like to thank for supplying me with the necessary Software and materials for the data processing and analysis.

My most loving and sincere thanks to my parents, Luz Janneth and Fernando, for loving me and who always supported me and guided me to be the person I am today. To my brother Santiago and my cousin Diany thanks for encouraging me and listen to me when I needed the most. Also, I want to Thank Theo for making me smile and cheer me up. To my uncle Jaime and my beautiful grandma, who believed in me during all my academical process, thank you very much. Finally, I want to thank my friends and colleagues for their support and help in my personal and professional life. Agradecimientos

En primer lugar, quiero agradecer a Bogdan Nitescu, mi mentor y asesor de investigación, por sugerirme el tema de esta tesis y por su gran conocimiento. Además, agradezco su apoyo y ayuda con todas mis dudas e inquietudes durante el proceso de este proyecto. Al Departamento de Geociencias y sus excelentes profesores, gracias a los cuales tengo conocimiento y amor por las geociencias, quiero agradecer por proporcionarme el Software y materiales necesarios para el procesamiento y análisis de datos. Mi más cariñoso y sincero agradecimiento a mis padres, Luz Janneth y Fernando, por amarme, apoyarme y guiarme para ser la persona que soy hoy en día. A mi hermano Santiago y a mi prima Diany gracias por apoyarme y escucharme cuando más lo necesitaba. Además, quiero agradecer a Theo por hacerme sonreír y animarme. A mi tío Jaime y a mi preciosa abuelita, que creyeron en mí durante todo mi proceso académico, muchas gracias. Finalmente, quiero agradecer a mis amigos y colegas por su apoyo y ayuda en mi vida personal y profesional.

Abstract An analysis and modelling of the Kapuskasing Structural Zone regional data were made to investigate the geometry and depth extent of this block of uplifted crust. First the analysis of the Bouguer anomaly and its first vertical derivative showed that the Kapuskasing Structural Zone presents high anomaly values that correspond to denser material in a region of uplifted crust relative to the Wawa and Abitibi subprovinces.

Using the GM-SYS extension tool from Oasis Montaj, forward 2.5D gravity modelling was completed along five profiles that cross the Kapuskasing Structural Zone. One model was done in a West to East direction in the Groundhog River block, three of the models where done in the same direction but in the Chapleau block, and the last model was done in a South to North direction through the Chapleau block. These models showed that in the Groundhog River block the Kapuskasing block does not present a high depth and that the Ivanhoe Lake fault dips to the northwest and is a thrust fault.

On the other hand, the models suggest that the geometry of the Saganash Lake fault is different in the two blocks, being a west-dipping normal fault over the Groundhog River block and southeast dipping reverse fault over the Chapleau block. These geometries follow the proposal of various authors. As a result of this model’s analysis it is suggested that after the Kapuskasing uplift a compressive event occurred in the Chapleau block and erosion removed part of the Groundhog River block.

Resumen

Se realizó un análisis y modelamiento de los datos regionales del Kapuskasing Structural Zone para examinar la geometría y la extensión de profundidad de este bloque de corteza levantada. Primero, el análisis de la anomalía de Bouguer y su primera derivada vertical expusieron que el Kapuskasing Structural Zone presenta altos valores en la anomalía que corresponden a material más denso en una región de corteza levantada en relación con las subprovincias de Wawa y Abitibi. Usando la extensión de la herramienta GM-SYS de Oasis Montaj, se completó el modelamiento directo 2.5D de los datos de gravedad a lo largo de cinco perfiles que cruzan el Kapuskasing Structural Zone. Uno de los modelos se realizó en dirección oeste a este en el bloque de Groundhog River, tres de los modelos se hicieron en la misma dirección, pero en el Chapleau block, y el último modelo se realizó en dirección sur a norte a través del bloque de Chapleau. Estos modelos mostraron que en el bloque de Groundhog River el Kapuskasing Structural Zone no presenta una gran profundidad y que la falla de Ivanhoe Lake buza hacia el noroeste y es una falla de cabalgamiento. Por otro lado, los modelos sugieren que la geometría de la falla de Saganash Lake es diferente en los dos bloques, siendo una falla normal que se inclina hacia el oeste sobre el bloque de Groundhog River y una falla inversa que se inclina hacia el sureste sobre el bloque de Chapleau. Estas geometrías siguen la propuesta de varios autores. Como resultado del análisis de este modelo, se sugiere que después del levantamiento de Kapuskasing ocurrió un evento de compresión en el bloque de Chapleau y la erosión eliminó parte del bloque de Groundhog River.

Table of contents

1. INTRODUCTION ...... 1 2. OBJECTIVES ...... 2 2.1 General objective ...... 2 2.2 Specific objectives ...... 3 3. GEOLOGICAL BACKGROUND ...... 3 3.1 Geology of the Kapuskasing structural zone ...... 3 3.2 Previous studies ...... 6 4. ANALYSIS OF THE BOUGUER ANOMALIES ...... 8 4.1 Gravity database ...... 8 4.2 Qualitative analysis of the Bouguer anomalies ...... 11 5. MODELLING OF GRAVITY DATA ...... 14 5.1 Gravimetric model for profile A ...... 17 5.2 Gravimetric model for profile B ...... 18 5.3 Gravimetric model for profile C ...... 19 5.4 Gravimetric model for profile D ...... 21 5.5 Gravimetric model for profile E ...... 22 6. ANALYSIS OF THE MAGNETIC ANOMALIES ...... 23 6.1 Magnetic database ...... 23 6.2 Qualitative analysis of the magnetic anomalies ...... 23 7. DISCUSSION ...... 27 8. CONCLUSION ...... 32 9. REFERENCES ...... 33 10. APPENDIX ...... 36 10.1 Theoretical gravity ...... 36 10.2 Free air-gravity anomaly ...... 36 10.3 Bouguer gravity anomaly ...... 36

LIST OF FIGURES

1-1. Location of the Kapuskasing Structural Zone (KSZ) in the South-Central Superior province Canada. (Halls, Uplift structure of the southern Kapuskasing zone from 2.45 Ga dike swarm displacement, 1998). The map in the left corner shows the location of the Superior Province on Canada and the position of the KSZ in it.

3-1. Geological Sketch of the KSZ taken from Nitescu and Halls (2002). The map shows the fault-bounded blocks of the KSZ (CB: Chapleau block; FB: Fraserdale- Moosonee block; GB: Groundhog River block; BV: Val Rita block), the faults (ILF: Ivanhoe Lake fault; LF: Lepage fault; SLF: Saganash Lake fault) the MB (Michipicoten Belt), and the WGD (Wawa Gneiss Domain).

3-2. General geology of the Southern part of the Kapuskasing Structural Zone (KSZ) and the surrounding Wawa and Abitibi subprovinces (Ontario Geological Survey 2016).

4-1. Observed gravity of the KSZ (mGal). Coordinate System NAD83/ UTM Zone 17N. The thin black lines indicate the geological contacts between the different lithologies presented in the area (see also figure 3-2). 4-2. Topographic map of the KSZ (m). Coordinate System NAD83/ UTM Zone 17N. 4-3. Bouguer Anomaly of the KSZ (mGal). Coordinate System NAD83/ UTM Zone 17N. The thin black lines indicate the geological contacts between the different lithologies presented in the area (see also figure 3-2). Thicker lines indicate the location of profile A, B, C, D and E that were done for the 2.5D forward modelling.

4-4. First Vertical Derivative of the Bouguer Anomaly of the KSZ (mGal/m). Coordinate System NAD83/ UTM Zone 17N. The thin black lines indicate the geological contacts between the different lithologies presented in the area (see also figure 3-2).

5-1. Geological features of the KSZ. The geologic map and legend were took from Estrada et al., (2018) and Ontario Geological Survey (2011). Coordinate System NAD83/ UTM Zone 17N. Lines A, B, C, D, and E represent the profiles used to do the 2.5D gravimetric models. 5-2. Gravimetric model for profile A. General geology of the KSZ. Coordinate System NAD83/ UTM Zone 17N. Goes from 5404802.7 E 301589.002 N to 5405614.44 E 417359.93 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Massive granodiorite to granite density: 2.7g/cm3; Foliated tonalite suite density: 2.69 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; KSZ density: 2.88 g/cm3; Mafic metavolcanic rocks density: 2.89 g/cm3; Intermediate metavolcanic rocks density: 2.78 g/cm3. 5-3. Gravimetric model for profile B. Coordinate System NAD83/ UTM Zone 17N. Goes from 5365190.98 E 316717.02 N to 5364677.03 E 400835.62 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Alkalic intrusive and carbonatite density: 2.64 g/cm3; Massive granodiorite to granite density: 2.7 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; KSZ density: 2.88 g/cm3; Mafic metavolcanic rocks density: 2.89 g/cm3. 5-4. Gravimetric model for profile C. General geology of the KSZ. Coordinate System NAD83/ UTM Zone 17N. Goes from 5329486.07 E 308670.09 N to 5328486.07 E 387217.41 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Foliated tonalite suite density: 2.69 g/cm3;Gneissic tonalite suite density: 2.8 g/cm3; Shawmere anorthosite: 2.82 g/cm3; KSZ density: 2.88 g/cm3; Mafic metavolcanic rocks density: 2.89 g/cm3; Metasedimentary rocks density: 2.8 g/cm3. 5-5. Gravimetric model for profile D. Coordinate System NAD83/ UTM Zone 17N. Goes from 5316663.49 E 2999523.99 N to 5317046.92 E 372752.17 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Alkalic intrusive and carbonatite density: 2.64 g/cm3; Massive granodiorite to granite density: 2.7 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; KSZ density: 2.88 g/cm3. 5-6. Gravimetric model for profile E. General geology of the KSZ. Coordinate System NAD83/ UTM Zone 17N. Goes from 5308425.92 E 368638.56 N to 5412367.87 E 371051.79 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Alkalic intrusive and carbonatite density: 2.64 g/cm3; Massive granodiorite to granite density: 2.7 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; Shawmere anorthosite: 2.82 g/cm3; KSZ density: 2.88 g/cm3. 6-1. Total Magnetic intensity map of the KSZ (nT). Coordinate System NAD83/ UTM Zone 17N. 6-2. First Magnetic Vertical Derivative of the KSZ (nT/m). Coordinate System NAD83/ UTM Zone 17N. 7-1. Generalized west-east cross-section from the Wawa domal gneiss terrane, through the Kapuskasing structural zone into the Abitibi subprovince, showing gross crustal structure done by NATO Advanced Study Institute on Exposed Cross-Sections of the Continental Crust (1988, pág. 248). To the East there is the northwest-dipping thrust Ivanhoe Lake fault. 7-2. Map and cross sections comparing structural configuration of individual components of the Kapuskasing Structural Zone (Percival & McGrath, 1986). Profile V-V” going W-E shows the structural configuration of the Groundhog River block bounding to the west with the west-dipping Saganash Lake listric normal Fault. This Map was taken from Percival and McGrath (1986). 7-3. Bouguer gravity profile, the smooth gravity curve, and the local anomalies removed through smoothing done by Nitescu and Halls (2002, p. 474). Data were projected on a northwest-southeast line perpendicular to the strike of the Saganash Lake fault (Nitescu & Halls, 2002).

LIST OF TABLES

3-1. Evolution of the Kapuskasing Line (Watson, 1980).

1. INTRODUCTION

The Kapuskasing Structural Zone (KSZ) is a northeast-trending, fault-bounded, discontinuous belt of Archean high-grade (granulite to upper amphibolite) metamorphic rocks, which cuts diagonally across the generally east–west subprovince structure of the south-central Superior Province and extends 500 km southwestward from James Bay.

FIGURE 1-1. Location of the Kapuskasing Structural Zone (KSZ) in the South-Central Superior province Canada. (Halls, Uplift structure of the southern Kapuskasing zone from 2.45 Ga dike swarm displacement, 1998). The map in the left corner shows the location of the Superior Province on Canada and the position of the KSZ in it.

Various authors (e.g., Percival and McGrath, 1986) have interpreted the KSZ as an east- verging Proterozoic thrust fault system. In this model, a slab of deep crust was uplifted along the northwest-dipping Ivanhoe Lake fault, which represents the southeastern boundary fault of the KSZ. The northwestern boundary fault of the central and southern KSZ is represented by the Saganash Lake fault, which was interpreted by Nitescu and Halls (2002) as a reverse fault.

The high-grade rocks within this discordant structure are characterized by high values of density, magnetic susceptibility, and seismic velocity. The strong contrast in physical properties between the rocks of the KSZ and the adjacent lower metamorphic-grade rocks of the Wawa and Abitibi subprovinces produces prominent gravity and magnetic anomalies (Dunlop et al., 2010).

The purpose of the current project is to analyze, and model public regional gravity dataset alongside with a qualitative analysis of the aeromagnetic dataset acquired by the Geological Survey of Canada over the Kapuskasing Structural Zone (KSZ). This analysis and modelling allow the geological understanding to investigate the geometry and depth extend in this block of uplifted crust.

At first, a geological background is given in which the geology of the KSZ and previous studies about the origin and evolution are mentioned. Next, the qualitative analysis of the Bouguer anomaly data and its first vertical derivative obtained from the Geoscience Data Repository of Natural Resources Canada was implemented. Using Oasis Montaj GM-SYS extension, five gravimetric models were done based on the Bouguer anomaly presented and a posterior qualitative interpretation was done. Finally, a qualitative Total Magnetic Field anomaly analysis was carried out as well as the analysis of the first vertical Total Magnetic Field derivative.

2. OBJECTIVES

2.1 General objective

The main objective of this thesis is the Analysis and Modelling of public Regional Gravity and Aeromagnetic data acquired in the Kapuskasing Structural Zone in order to investigate

the geometry and depth extent of this block of uplifted crust. Data was downloaded from the Geoscience Data Repository of Natural Resources Canada.

2.2 Specific objectives

• Analysis and Modelling of public Regional Gravity data acquired in the Kapuskasing structural zone. • Analysis and Modelling of public Regional Aeromagnetic data acquired in the Kapuskasing structural zone. • Geological interpretation of the geophysical models.

3. GEOLOGICAL BACKGROUND

3.1 Geology of the Kapuskasing structural zone

The Kapuskasing Structural Zone (KSZ) cuts diagonally the south central Superior Province across the east-striking Archean Abitibi Subprovince to the east and the Wawa Subprovince to the west (see Figure 3-1) and extends over 500 km in length and up to a maximum of 50km in width (Percival J. A., 1983). The KSZ is a northeast-trending discontinuous belt of Archean high grade metamorphic rocks (Nitescu & Halls, 2002), fault bounded by the Ivanhoe Lake fault zone to the east and the Saganash Lake fault to the west. The KSZ is divided into three principal blocks the Chapleau block, the Fraserdale-Moosonee block, and the Groundhog River block.

The Chapleau block is the largest and most southerly of these, forming the end of the KSZ according to Halls (2002). It is dominantly orthogneiss, minor paragneiss and mafic gneiss with metamorphic grade ranging from upper amphibolite to granulite facies, with the degree of migmatization increasing to the north-northwest (Estrada et al., 2018). On the other hand, the Groundhog River block has a heterogeneous sequence of tonalitic orthogneiss, minor paragneiss and mafic gneiss (Estrada et al., 2018). Finally, the northern block, the Fraserdale- Moosonee block, consist mainly of metasedimentary granulites, tonalite and rare mafic gneiss (Estrada et al., 2018).

The KSZ represents the basal part of an oblique crustal section about 120 km wide, from low grade supracrustal rocks of the Michipicoten belt metamorphosed at depths of 5 to 10 km,

through amphibolite gneiss of eastern Wawa Subprovidence formed at 15 to 20 km, to granulite of the zone , subjected to pressures of 7 to 9 kb at depths of 30km. Moreover, this section was exposed by major displacement on a west-dipping thrust followed by normal movements on subparallel faults that broke the thrust sheet into several blocks. Thrusting and uplift occurred in the Aphebian in response to collision along the boundary between the Superior and Churchill provinces (Geological Survey of Canada, 1989).

The Chapleau block lithology is mainly composed by Neoarchean rocks with uncertain relative ages. The youngest ones are syenites and monzonites complexes, next to granite, pegmatite, minor quartz syenite monzonite and granodiorite. In the southern part there are also xenolithic and layered granodioritic to dioritic orthogneiss and associated migmatitic rocks, from metatexites to diatexites, and subordinate paragneisses that produced big amount of tonalitic and granitic melt by anatexy (Krogh, 1993) and it is characterized by northeast- trending, northwest-dipping granulite facies gneiss, and a large layered gabbro-anorthosite complex. In the northern part there are granulite facies paragneiss, associated migmatitic rocks and amphibolitic orthogenesis. On the other hand, on the center there are few Proterozoic alkaline intrusions.

FIGURE 3-1. Geological Sketch of the KSZ taken from Nitescu and Halls (2002). The map shows the fault-bounded blocks of the KSZ (CB: Chapleau block; FB: Fraserdale-Moosonee block; GB: Groundhog River block; BV: Val Rita block), the faults (ILF: Ivanhoe Lake fault; LF: Lepage fault; SLF: Saganash Lake fault), the MB (Michipicoten Belt) and the WGD (Wawa Gneiss Domain).

Hosted within the Chapleau block is the Borden Lake area where the KSZ hosts a supracrustal package composed, from top to bottom, of Timiskaming-type metaconglomerates, and a heterogenous felsic-dominated unit overlying a mafic-dominated volcanic rock package that underwent amphibolite-facies metamorphism. These rocks are intruded by a series of early to intermediate intrusive rocks and by younger syn-kynematic felsic plutonic rocks (Duguet & Szumylo, 2016).

FIGURE 3-2. General geology of the Southern part of the Kapuskasing Structural Zone (KSZ) and the surrounding Wawa and Abitibi subprovinces (Ontario Geological Survey 2016).

3.2 Previous studies

Gaucher (1966) and Thurston et al. (1977) recorded mylonites and highly magnetic ultramylonites in the KSZ. Also, there are gneisses showing a strong, regular planar fabric and foliation that are characteristic of ductile shear zones formed at depth. According to Watson (1980) the deflection of layering in granulites and the nature of the dislocation rocks suggest that displacement of the KSZ was achieved partly by movement on ductile shear zones at high temperatures that stimulate recrystallization and prevent the retrogression of granulites. This displacement would seem to have had a sinistral component (Watson, 1980).

According to Watkinson et al. (1972) and Gittins et al. (1967), the principal igneous intrusions which appear to be localized along the KSZ are the Shawmere anorthosite complex dated at 2519 Ma and several alkaline and carbonitic complexes ranging from 1700 to 1050 Ma. The Shawmere anorthosite complex that is in the southernmost granulite strip of the KSZ, has yielded a K-Ar hornblende mineral age of 2519 Ma (Watkinson et al., 1972). This complex is traversed by geophysical anomalies regarded as belonging to Abitibi dikes (Thurston et al., 1974) and is penetrated by an alkaline complex. Based on the observations that the complex has been strongly metamorphosed and that it is mylonitized and invaded by pseudotachylite near the eastern boundary, Thurston et al. (1977) concluded that intrusion took place prior to the upfaulting of the granulite strips.

On the other hand, there is an association between alkaline igneous complexes and carbonatites and the KSZ (Innes et al., 1967). There is geological and geophysical evidence that indicate that emplacement of the alkaline complexes was a late event in the structural history of the KSZ. These complexes intrude both granulites and lower-grade Archean rocks (Bennett et al., 1966).

3.2.1 Origin and evolution

According to Watson (1980) there are several stages in the evolution of the Kapuskasing Structural Zone that can be fixed in relation to successive phases of igneous activity. There are three main points (see table 3-1):

1. The origin goes back to a time shortly after ending of mobility in the Superior Province. The early stages of movement had a sinistral transcurrent component (Ayres quoted in Thurston et al. 1977). Also, anomalous igneous activity continued from at least 1700 Ma down to Phanerozoic times, and this activity is an indication that the dislocation had not been healed in the lower crust or upper mantle (Watson, 1980). 2. Dike intrusion took place during a late stage of the phase of sinistral displacement and dike fractures were deflected into the tension-fracture orientation as they entered the Kapuskasing zone (Watson, 1980). 3. Around the time of intrusion of the Matachewan dike swarm (2.45 Ga) (Halls & Mound, 1998) the early tectonic history of the Kapuskasing line appears to have been completed.

In general, Watson (1980) suggested that the KSZ originated as a deep transcurrent shear zone in late Archean or earliest Proterozoic times.

Table 3-1. Evolution of the Kapuskasing Structural Zone (Watson, 1980).

Another study of Nitescu and Halls (2002) used a high-resolution gravity profile showing that the Saganash Lake fault defining the northwestern boundary of the southern KSZ is southeast dipping and of reverse type. Also, they suggested that the Chapleau block and other fault-bounded uplifted blocks may have formed as positive flower structures during dextral shear on a system of left-stepping faults (Nitescu & Halls, 2002).

4. ANALYSIS OF THE BOUGUER ANOMALIES

4.1 Gravity database

The gravity database used in this thesis was acquired from the Geoscience Data Repository of the Geological Survey of Canada. The Canadian Gravity Database is managed by the Canadian Geodetic Survey and it includes more than 755.000 observations, including some 232.000 observations on land (Geophysics Data Center, 2020). The gravity maps are gridded to a 2 km interval with a blanking radius of 20 km and includes data between 1944 and 2015 (Geophysics Data Center, 2020). According to the Geophysics Data Center all the relative

gravity measurements are integrated into the IGSN71 datum to create a coherent dataset at the global scale (Geophysics Data Center, 2020)

FIGURE 4-1. Observed gravity of the KSZ (mGal). Coordinate System NAD83/ UTM Zone 17N. The thin black lines indicate the geological contacts between the different lithologies presented in the area (see also figure 3-2).

For this project, we covered the southern area of the KSZ (Chapleau block) using coordinates of 47°to 49°N and -85° to -82°W. When downloading the data, the original geographical

coordinates where converted to NAD83 UTM Zone 17N. The grids that were downloaded and then mapped where the observed gravity (Figure 4-1), the Topography (Figure 4-2), the Bouguer anomaly with an interval of 2 km (Figure 4-3), and the first vertical derivative (Figure 4-4).

FIGURE 4-2. Topographic map of the KSZ (m). Coordinate System NAD83/ UTM Zone 17N.

The observed gravity grid of Canada shows variations in the gravity field largely caused by the shape and rotation of Earth (Geoscience Data Repository for Geophysical Data, 2016). It

is used to define the ideal shape of earth (the geoid). The map of this observed gravity shown in Figure 4-1 shows a higher gravity to the north-east and a lower gravity in the southern part. Looking at the Topographic map (Figure 4-4) we can see that in the south there are positive topographic features, causing a depression of the force of the gravity as Earth’s surface is further away from the centre of mass. The contrary happens in the northern part, where there is a lower topography and therefore, higher gravity. In general, the KSZ has a higher gravity than its surroundings.

4.2 Qualitative analysis of the Bouguer anomalies

The grid of the Bouguer anomalies reflect lateral variations in density in the Earth’s crust and upper mantle that correspond to variations in rock composition and thickness of geological bodies (Geoscience Data Repository for Geophysical Data, 2016). The Bouguer anomaly data were obtained assuming a crustal density value of 2670kg/m3. The correction of observed gravity data was done assuming the geoid as reference. Terrain corrections were also applied.

High-frequency anomalies are caused by near-surface bodies of rocks that have significantly different densities from their host rocks or neighboring bodies of rocks (Geoscience Data Repository for Geophysical Data, 2016). Longer wavelength anomalies are generally associated with variations in crustal thickness or deeper intra-crustal mass variation (Geoscience Data Repository for Geophysical Data, 2016). To study and understand these anomalies quantitative models can be constructed over the study area with bodies underneath the surface with different densities. In these models the purpose is to fit a set of gravity observations by modifying the geometry of the model bodies, considering the depth, rock type and composition, and extension. Also, for this project the models permit a comparation between the KSZ with its surroundings.

The Bouguer anomaly of the KSZ is shown in Figure 4-3. It is characterized by high values in the Bouguer anomaly with values of approximately -46mGal to -20.2mGal. This high anomaly in the KSZ suggests that there must be a denser region in the subsurface or that the rock bodies underlying that area extend deeper than their surroundings. To the east of the KSZ, in the Abitibi subprovince, the anomaly decreases rapidly, having the lowest peaks of the anomaly in areas that correspond to granitic plutons with low densities (~2.67g/cm3). To

the west, in the Wawa subprovince, the anomaly decreases slowly but the lower values also correspond to granitic plutons of lower densities.

FIGURE 4-3. Bouguer Anomaly of the KSZ (mGal). Coordinate System NAD83/ UTM Zone 17N. The thin black lines indicate the geological contacts between the different lithologies presented in the area (see also figure 3-2). Thicker lines indicate the location of profile A, B, C, D and E that were done for the 2.5D forward modelling.

With the anomaly exhibited (Fig 4-3) it is possible to observe the presence of the Ivanhoe Lake fault (ILF) to the East. For the exact location based on gravity data, the horizontal gradient or the analytical signal would be needed. However, the Bouguer anomaly data is in agreement with the presence of a boundary fault on the eastern site of the KSZ. This does not happen with the Saganash Lake fault (SLF) to the west. On the other hand, in the Groundhog River block to the north-east, the lower value of anomalies can be seen, while in the Chapleau block, to the south, the anomaly presents greater values. This difference suggests that there are different depths extends of the two blocks, the Chapleau Block having a larger depth extend.

The first vertical derivative of the Bouguer is presented in Figure 4-4. To obtain it, the Bouguer anomaly grid was converted to the frequency domain by Geomatics Canada using a fast Fourier transform. The vertical derivative transform function was applied to the frequency domain data. This derivative filter improves the short wavelength component at the expense of longer wavelength anomalies and helps resolve closely spaced or superposed anomalies (Geoscience Data Repository for Geophysical Data, 2016).

Similar to the Bouguer gravity anomaly, the first vertical derivative “sharpens” the shallow anomalies indicating the existence of the Ivanhoe Lake fault (ILF) to the East and the Saganash Lake fault (SLF) to the west. Also, in the Wawa and Abitibi subprovinces, the anomalies corresponding to granitic plutons with low densities are better delimitated.

FIGURE 4-4. First Vertical Derivative of the Bouguer Anomaly of the KSZ (mGal/m). Coordinate System NAD83/ UTM Zone 17N. The thin black lines indicate the geological contacts between the different lithologies presented in the area (see also figure 3-2).

5. MODELLING OF GRAVITY DATA

Even though there are differences between the density values of the different lithologies of the KSZ, the objective is to investigate the geometry and depth extent of this block of uplifted

crust. For this reason, in the modelling of the gravity data, the KSZ was considered as a homogeneous block.

In this project, the gravity anomaly observed across the KSZ was interpreted using five different 2.5D models. To select the profiles location the geological formations at the surface (Figure 5-1) and the location of the main boundary faults (SLF and ILF) were considered. The densities used in these models were taken from Eshaghi et al. (2019) and the NATO Advanced Study Institute on Exposed Cross-Sections of the Continental Crust (1988). For the KSZ a density of 2.88 g/cm3 was used, this being an average of the main lithologies presented in the area. On the other hand, since the Bouguer Anomaly was calculated using an average density of 2.67 g/cm3, the density used for the continental crust in the model is the same.

Forward gravity models were obtained with the Oasis Montaj GM-SYS tool along 5 profiles, the locations of which are shown in Figures 4-3 and 5-1. The parameters of the models are the depths, the density contrast, and the faults dip.

FIGURE 5-1. Geological features of the KSZ. The geologic map and legend were took from Estrada et al., (2018) and Ontario Geological Survey (2011). Coordinate System NAD83/ UTM Zone 17N. Lines A, B, C, D, and E represent the profiles used to do the 2.5D gravimetric models.

5.1 Gravimetric model for profile A

Profile A (Figure 5-2) was made in a West to East direction through the northern part of the Chapleau block (Figure 5-1). It starts in the Wawa subprovince (5404802.7 E 301589.002 N), crosses the Groundhog River block of the KSZ, and ends in the northern part of the Swayze Greenstone belt in the Abitibi subprovince (5405614.44 E 417359.93 N) having a length of 115km approximately.

FIGURE 5-2. Gravimetric model for profile A. Coordinate System NAD83/ UTM Zone 17N. Goes from 5404802.7 E 301589.002 N to 5405614.44 E 417359.93 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Massive granodiorite to granite density: 2.7g/cm3; Foliated tonalite suite density: 2.69 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; KSZ density: 2.88 g/cm3; Mafic metavolcanic rocks density: 2.89 g/cm3; Intermediate metavolcanic rocks density: 2.78 g/cm3.

In figure 5-2 it can be seen that for this profile the Bouguer anomaly in the Wawa subprovince starts to increase towards the east, having higher values in the eastern part of this area. In the

KSZ the anomaly decreases until it reaches the Abitibi subprovince where lower values of the Bouguer anomaly can be found. The greater anomaly is presented in a gneissic tonalite suite lithology with a density of 2.8g/cm3. Since this type of rock is less dense than the rocks that occur in the KSZ, it is possible to suggest that this block is over a denser block that can be seen in the model (Figure 5-2) as a continuation at depth and towards west of the KSZ Groundhog River block, with a density of 2.88g/cm3.

The highest depth that the KSZ block reaches in this model is approximately 8.20km and it is under the gneissic tonalite suite. On the other hand, in the west of the KSZ, the Saganash Lake Fault is dipping to the west as a normal fault agreeing with the interpretations of Percival and Card (1983) mentioned in Nitescu and Halls (2002). On the east of the KSZ, the Ivanhoe Lake Fault dips to the west and is parallel to the Saganash Lake Fault.

The model throws an error of 1.127 due the difficulties in accommodating the anomaly in a simple geometry used for the KSZ block. Since the data obtained from the Geoscience Data Repository was regional, a complex geometry of the KSZ block could not be done.

5.2 Gravimetric model for profile B

Profile B (Figure 5-3) extends in a West to East direction through the center part of the Chapleau block (Figure 5-1). It starts in the Wawa subprovince (5365190.98 E 316717.02 N), passes through the KSZ, ends in the Swayze Greenstone belt of the Abitibi subprovince (5364677.03 E 400835.62 N) having a length of approximately 84km.

This profile passes through a wider exposed part of the KSZ and in these regions the maximum value of the Bouguer anomaly is located. The lower peak of the anomaly can be found in the Abitibi subprovince where massive granodiorite to granite lithologies are exposed. The profile intersects a carbonatite intrusion that is characterized by a low-density value of 2.64g/cm3. The current model shows that it is not deep and does not have a significant expression in the Bouguer the anomaly.

The KSZ reaches a depth of 6.94 km which is a larger depth compared to the one in profile A.

Regarding the main faults, the Ivanhoe Lake fault is similar in attitude to what is observed in the model along profile A. Compared to the model along profile A, the Saganash Lake fault

seems to have a different attitude, the current model indicating an east dipping fault of reverse nature.

FIGURE 5-3. Gravimetric model for profile B. Coordinate System NAD83/ UTM Zone 17N. Goes from 5365190.98 E 316717.02 N to 5364677.03 E 400835.62 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Alkalic intrusive and carbonatite density: 2.64 g/cm3; Massive granodiorite to granite density: 2.7 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; KSZ density: 2.88 g/cm3; Mafic metavolcanic rocks density: 2.89 g/cm3.

This profile has an error of 2.11 due to the same difficulties as in profile A and impediments in accommodating the eastern part of the anomaly where the massive granodiorite to granite lithology has a low density of 2.7g/cm3 and is not known what lies under it.

5.3 Gravimetric model for profile C

Profile C (Figure 5-4) was made in a West to East direction through the southern part of the Chapleau block (Figure 5-1). It starts in the Wawa subprovince zone (5329486.07 E

308670.09 N), passes through the KSZ, and ends in the Swayze Greenstone belt (5328486.07 E 387217.41 N) with a length of 80km approximately.

FIGURE 5-4. Gravimetric model for profile C. Coordinate System NAD83/ UTM Zone 17N. Goes from 5329486.07 E 308670.09 N to 5328486.07 E 387217.41 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Foliated tonalite suite density: 2.69 g/cm3;Gneissic tonalite suite density: 2.8 g/cm3; Shawmere anorthosite: 2.82 g/cm3; KSZ density: 2.88 g/cm3; Mafic metavolcanic rocks density: 2.89 g/cm3; Metasedimentary rocks density: 2.8 g/cm3.

In this profile is possible to see that the Bouguer anomaly is bigger in the KSZ compared to its surroundings. Here, the KSZ block reaches a depth of 7.47km which is similar to the maximum depth observed along profile B. Moreover, there is the presence of the Shawmere anorthosite complex which have a high density of 2.82 g/cm3 and has a big Bouguer anomaly.

As in profile B, the Saganash lake fault shows a dipping to the east reverse fault geometry and the Ivanhoe Lake fault shows the same geometry as in profiles A and B. In this model the Saganash Lake fault shows a bigger inclination to the east than it does in profile B.

For this model, the error is 1.129, lower than the one in profile B and similar to the error in profile A.

5.4 Gravimetric model for profile D

FIGURE 5-5. Gravimetric model for profile D. Coordinate System NAD83/ UTM Zone 17N. Goes from 5316663.49 E 2999523.99 N to 5317046.92 E 372752.17 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Alkalic intrusive and carbonatite density: 2.64 g/cm3; Massive granodiorite to granite density: 2.7 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; KSZ density: 2.88 g/cm3.

Profile D (Figure 5-5) has a West to East direction and lies in the southern part of the Chapleau block (Figure 5-1). It starts in the Wawa subprovince (5316663.49 E 2999523.99 N), passes through the KSZ, and ends in the Abitibi subprovince (5317046.92 E 372752.17 N) with a length of 72 km approximately.

In this model as in the others the KSZ is characterized by the largest values of the Bouguer anomaly. A little local gravity low is also observed in the KSZ block, superimposed over the

large gravity high associated with the KSZ. This decrease of the anomaly is exactly where the carbonatites are. This can also be seen in Profile B where there is a small local low in the presence of the carbonatites that have a lower density than the KSZ.

The geometry of the main faults stays the same as in profile B with small changes in the inclination. On the other hand, the depth of the KSZ is lower compared to the other profiles with a maximum depth of 6.7 km.

The error is 1.688 and it is mainly caused by the basic geometry used for the KSZ, the presence of the carbonatites with low density, and the density of the gneissic tonalite suite in the Wawa subprovince.

5.5 Gravimetric model for profile E

Profile E (Figure 5-6) was made in a South to North direction and it starts in the Swayze Greenstone belt (5308425.92 E 368638.56 N), goes through the Chapleau block intersecting the four previews profiles, and ends in the Wawa subprovince (5412367.87 E 371051.79 N) and has a length of 104km approximately (Figure 5-1).

FIGURE 5-6. Gravimetric model for profile E. Coordinate System NAD83/ UTM Zone 17N. Goes from 5308425.92 E 368638.56 N to 5412367.87 E 371051.79 N. The profile shows the main faults (ILF: Ivanhoe Lake fault; SLF: Saganash Lake fault). Alkalic intrusive and carbonatite density: 2.64 g/cm3; Massive granodiorite to granite density: 2.7 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; Shawmere anorthosite: 2.82 g/cm3; KSZ density: 2.88 g/cm3.

For this model, the lowest peak of the anomaly is in the Swayze Greenstone belt while the biggest anomaly value is in the KSZ block. The depths of the model at the intersections with the West-East profiles are consistent with the depths in the models along those profiles. Also, the deeper zone of the KSZ block corresponds to the biggest value of the Bouguer Anomaly. In this model the geometry of the faults is the same as the ones in profile B, C, and D.

Profile E is the one with the biggest error of 4.83. The reason of this is the difficulties in accommodating the anomaly in a simple geometry used for the KSZ block while having consistent depths with the models A-D at the points of intersection.

6. ANALYSIS OF THE MAGNETIC ANOMALIES

6.1 Magnetic database

The magnetic dataset used in this thesis was downloaded from the Geoscience Data Repository of the Geological Survey of Canada. The magnetic dataset is gridded to a 50 m interval and is based on data acquired by Scintrex Ltd and FUGRO SIAL Airborne Surveys INC. (FSAS).

For this project, we covered the southern area of the KSZ (Chapleau block) using coordinates of 47°to 49°N and -85° to -82°W. When downloading the data, the original geographical coordinates where converted to NAD83 UTM Zone 17N. The grids that were downloaded and then mapped where the total magnetic intensity (Figure 6-1), and the first vertical derivative (Figure 6-3).

6.2 Qualitative analysis of the magnetic anomalies

The total magnetic intensity data measures variations in the density of the Earth magnetic field that is caused by the variation in the magnetism of the rocks due to different content of rock-forming minerals in the Earth crust.

In the map shown by Figure 6-1 the total magnetic intensity of the KSZ range from 57494.8 nT to a maximum value of 58398.5 nT. The biggest values can be seen in the Groundhog River block and the fault that divides this block with the Chapleau block can be seen. There is a connection between areas where tonalites and mafic gneisses are exposed with high values of the total magnetic field. The Shawmere anorthosite shows the lowest values specially in the eastern area. On the other hand, the carbonatites can be recognized due to their high values.

FIGURE 6-1. Total Magnetic intensity map of the KSZ (nT). Coordinate System NAD83/ UTM Zone 17N.

FIGURE 6-2. First Magnetic Vertical Derivative of the KSZ (nT/m). Coordinate System NAD83/ UTM Zone 17N.

The total magnetic intensity also permits to see some of the dykes that are in the south of the KSZ and the Wawa subprovince. These dykes are shown by high values. Moreover, The Saganash Lake fault and the Ivanhoe Lake fault can be located with the total magnetic field.

The first vertical derivative (Figure 6-2) represents the rate of change of the total magnetic field with respect to the vertical. This derivative enhances the short wavelength components at the expense of longer wavelengths. With this derivative, other geological features as minor dykes can be seen all over the map. There is also a better delimitation of the Saganash Lake fault and Ivanhoe Lake fault in the northern area.

7. DISCUSSION

The general objective of this project was to carry out the analysis and modelling of public Regional Gravity data and the qualitative assessment of public Aeromagnetic data acquired in the Kapuskasing Structural Zone (KSZ) in order to investigate the geometry and depth extent of this block of uplifted crust. 2.5D forward gravity modelling was conducted along five different profiles to infer the geometry and depth of the KSZ and compare the Bouguer anomaly with its surroundings.

The Bouguer anomaly grid shows that the KSZ is characterized by high values of the anomaly compared to the Swayze greenstone belt to the east and the Wawa subprovince to the west. The Chapleau block is characterized by a strong gravity high, especially in the north-center area of the block (33.4 mGal to -20.2 mGal approximately). On the other hand, the Groundhog River block presents lower values in the Bouguer anomaly (-57.9 mGal to -36.5 mGal approximately). This suggests that the Chapleau Block contains denser rocks than the Groundhog River block, but looking at the geological map it can be seen that the Groundhog River block have rocks with higher densities, with presence of metasedimentary rocks (2.8g/cm3) and Migmatized supracrustal rocks (2.97 cm3), compared to the ones in the Chapleau block, which is primarily composed by a gneissic tonalite suite (2.75 cm3) and a foliated tonalite suite (2.69 cm3). So, the lack of a higher anomaly in this block with denser rocks suggests that the high-density material forms only a thin layer in the surface. Despite the difference of the values in gravity anomalies between the two blocks, the values are high, and it is possible to say that there is a continuity of the anomalies along the two blocks. The eastern limit of the gravity Bouguer anomaly correlates with the Ivanhoe lake fault, while the Saganash lake fault is not visible with the anomaly.

FIGURE 7-1. Generalized west-east cross-section from the Wawa domal gneiss terrane, through the Kapuskasing structural zone into the Abitibi subprovince, showing gross crustal structure done by NATO Advanced Study Institute on Exposed Cross-Sections of the Continental Crust (1988, pág. 248). To the East there is the northwest-dipping thrust Ivanhoe Lake fault.

For the forward 2.5D modelling a simple geometry of the KSZ block was used. The locations on the surface of the geological contacts of the surrounding of the KSZ were constrained based on the mapped geology, and appropriate density values were used for each lithology. The density values used for modelling were: Massive granodiorite to granite density: 2.7g/cm3; Foliated tonalite suite density: 2.69 g/cm3; Gneissic tonalite suite density: 2.8 g/cm3; Mafic metavolcanic rocks density: 2.89 g/cm3; Intermediate metavolcanic rocks density: 2.78 g/cm3; Metasedimentary rocks density: 2.8 g/cm3. Since this is a regional project and is based on the analysis of a regional gravity dataset, the KSZ was considered in the forward models as a homogeneous block with a density of 2.88 g/cm3.

The main faults can be seen in the five profiles models. The Ivanhoe Lake fault dips to the west in the 5 models suggesting that this fault zone is the surface expression of a northwest-

dipping thrust fault. This suggestion follows various publications including Exposed Cross- Sections of the Continental Crust (1988) and Seismic image of the Ivanhoe Lake Fault Zone in the Kapuskasing Uplift of the Canadian Shield (Wu et al., 1992). In Figure 7-1 a generalized west-east cross-section from the Wawa domal gneiss terrane, through the Kapuskasing structural zone into the Abitibi subprovince was done by the NATO Advanced Study Institute on Exposed Cross-Sections of the Continental Crust (1988) confirms this geometry of the Ivanhoe Lake fault.

On the other hand, in profile A (Figure 5-2) the Saganash Lake Fault dips to the west and appears to be a normal fault. This is in agreement with the proposal of Percival and McGrath (1986), who interpret the Groundhog River block as a perched thrust tip, that is bounded to the west by a west-dipping Saganash Lake normal fault. This can be seen in profile V-V’’ (Figure 7-2) done by Percival and McGrath (1986).

On the contrary, profiles B-E done in the Chapleau block shows that the Saganash Lake Fault is a southeast dipping reverse fault. This geometry is consistent with the results of Nitescu and Halls (2002), who show that the Saganash lake fault dips to the southeast and is of a reverse nature. The gravity profile (Figure 7-3) done by Nitescu and Halls (2002) is similar to the gravity models obtained for this project, suggesting that the data obtained and the models are correct.

This difference in the attitude of the Saganash Lake fault between the Groundhog River block and the Chapleau block suggests that subsequent to the of the KSZ along the Ivanhoe Lake fault, the southern part of the KSZ (Chapleau block) suffered a compression that converted the Saganash Lake fault to a southeast dipping reverse fault. This could have happened during an intraplate compressive episode in the early Proterozoic (NATO Advanced Study Institute on Exposed Cross-Sections of the Continental Crust, 1988). On the other hand, the Groundhog River block may have been eroded.

FIGURE 7-2. Map and cross sections comparing structural configuration of individual components of the Kapuskasing Structural Zone (Percival & McGrath, 1986). Profile V-V” going W-E shows the structural configuration of the Groundhog River block bounding to the west with the west-dipping Saganash Lake listric normal Fault. This Map was taken from Percival and McGrath (1986).

Observing model A (Figure 5-2) it is possible to see that in the Groundhog River block the depth extent of the denser rocks of the KSZ is about 2km. This demonstrates that even though this block presents rocks with high densities, the depth of the block is low and that is why the Bouguer Anomaly is not the highest. On the other hand, in the Chapleau block all the profiles indicate a depth higher than 6km, showing that the rocks in this zone, although of lower densities than those in the Groundhog River block, extend deeper in the crust.

All the models presented an error between 1.127 – 5.152, this is due to the difficulties in doing an adjustment of the KSZ block with a basic geometry. This geometry needed to be used since the Data from Geoscience Data Repository of Natural Resources Canada is regional and only the upper crust lithologies are known.

FIGURE 7-3. Bouguer gravity profile, the smooth gravity curve, and the local anomalies removed through smoothing done by Nitescu and Halls (2002, p. 474). Data were projected

on a northwest-southeast line perpendicular to the strike of the Saganash Lake fault (Nitescu & Halls, 2002).

The Total Magnetic Field data indicate a very strong aeromagnetic response in the Groundhog River block and it correlates with the Saganash Lake fault to the west, the Ivanhoe Lake fault to the east and with a minor fault to the south that separates this block with the Chapleau block. On the other hand, the Chapleau block presents a small broadscale aeromagnetic response, with higher values of the Total Magnetic Field Intensity over the carbonatite complexes. For the anorthosite bodies in the Chapleau block, the Total Magnetic Field is low.

The first vertical derivative of the Total Magnetic Field Intensity shows an enhanced expression of the Saganash Lake fault and the Ivanhoe lake fault in the Groundhog River block.

8. CONCLUSION

The Kapuskasing Structural Zone (KSZ) in the Superior Province is represented by a gravity high that corresponds to a region of uplifted denser, deeper crustal material (Geoscience Data Repository for Geophysical Data, 2016). The Groundhog River block is made up by denser rocks that forms only a thin layer in the surface, while the Chapleau block is thicker. Both blocks present a higher Bouguer anomaly compared to their surroundings. The magnetic data indicate high magnetic values in the north and center of the Chapleau block, and lower magnetic values in the southern part.

The Ivanhoe Lake fault is a northwest-dipping thrust fault. On the other hand, the Saganash lake fault is a west-dipping normal fault over the Groundhog River block, while in the region of the Chapleau block it is a southeast dipping reverse fault. This may be due to a compressive event in the southern part of the KSZ.

More studies can be done in this area. It is important to make future projects with more local and specific data that permits a better modelling and interpretation of the geometry and depths in the KSZ. Also, a more detailed project focused on the Saganash Lake fault can be done to learn about the processes that account for the different geometries between blocks.

9. REFERENCES

Bennet, G., Brown, D. D., & George, P. T. (1966). Operation Kapuskasing-District of Cochrane. Summary of field work, 1966. Geological Branch, Ontario Department of Mines.

Duguet, M., & Szumylo, N. (2016). Project NE-16-002. Archean and Porterozoic Geology of the Borden Lake Area, Kapuskasing Structural Zone, Abitibi-Wawa Terrane. In O. G. Survey, Summary of Field Work and Other Activities 2016 (pp. 56-75).

Eshaghi, E., Smith, R., & Ayer, J. (2019). Petrophysical characterisation (i.e. density and magnetic susceptibility) of major rock units within the Abitibi Greenstone Belt. Laurentian University Mineral Exploration Research Centre.

Estrada, N., Tinkham, D. K., & Jorgensen, T. C. (2019). Assessing the Potential for Metal Mobility During Lower Crustal Evolution, Kapuskasing Structural Zone, Ontario. Summary of Field Work and Other Activities, 2019, Ontario Geological Survey, Open File Report 6360, 461-46-8.

Estrada, N., Tinkham, D. K., Jorgensen, T. C., & Marsh, J. H. (2018). Identification of Partial Melting Relationships in the Southern Kapuskasing Structural Zone, Ontario. Ontario Geological Survey, 31-1 - 31-7.

Gaucher, E. H. (1966). Elsas-Kapuskasing-Moosonee magnetic and gravity highs. Geological Survey of Canada, 66(1), 189-191.

Geological Survey of Canada. (1989). Geology, Timmins, Ontario - Québec. Geological Atlas(1 of 5). (K. D. Card, & B. V. Sanford, Compilers) Canada.

Geophysics Data Center. (2020, May 5). Government of Canada; Natural Resources Canada; Geological Survey of Canada. Retrieved from https://open.canada.ca/data/en/dataset/5a4e46fe-3e52-57ce-9335-832b5e79fecc

Geoscience Data Repository for Geophysical Data. (2016). Gravity_Bouguer_Anomaly_description.

Geoscience Data Repository for Geophysical Data. (2016). Gravity_Observed_Description.

Geoscience Data Repository for Geophysical Data. (2016). Gravity_VD1_Bouguer_Anomaly_description.

Gittins, J., MacIntyre, R. M., & York, D. (1967). The ages of carbonate complexes in eastern Canada. Canadian Journal, 4, 651-654.

Halls, H. (1998). Uplift structure of the southern Kapuskasing zone from 2.45 Ga dike swarm displacement. Geology, 26(1), 67-70.

Halls, H., & Mound, J. (1998). The McEwan Lake fault: gravity evidence for a new structural element of the Kapuskasing zone. Canadian Journal of Earth Sciences(35), 696-701.

Innes, M. J., Goodacre, A. K., Weber, J. R., & McConnell, R. K. (1967). Structural implications of the gravity field in Hudson Bay and vicinity. Canadian Journal of Earth Sciences, 4, 977-993.

Krogh, T. (1993). High precision U-Pb ages for granulite metamorphism and deformation in the Archean Kapuskasing structural zone, Ontario: Implications for structure and development of the lower crust. Earth and Planetary Science Letters, 119, 1-18.

NATO Advanced Study Institute on Exposed Cross-Sections of the Continental Crust. (1988, September 17-27). Exposed Cross-Sections of the Continental Crust. (M. Salisbury , & D. Fountain , Eds.) 317, 657.

Nitescu, B., & Halls, H. (2002). A gravity profile across southern Saganash Lake fault: implications for the origin of the Kapuskasing Structural Zone. Canadian Journal of Earth Sciences, 39, 469-480.

Ontario Geological Survey. (2011). 1:250 000 Scale Bedrock Geology of Ontario; Ontario Geological Survey, Miscellaneous Release–Data 126 - Revision 1. .

Percival, J. A. (1983). High-grade metamorphism in the Chapleau–Foleyet area. American Mineralogist, 68, 667-686.

Percival, J., & McGrath, P. (1986, August). Deep crustal structure and tectonic history of the Northern Kapuskasing Uplift of the Northern Kapuskasing Uplift of Ontario: An integrated petrological‐geophysical study. Tectonics, 5(4), 553-572.

Thurston, P. C., Sage, R. P., & Siragusa, G. M. (1974). Ottawa Department of Mines, Map Compilation Series, Sheet 2221 (Chapleau-Foleyet).

Thurston, P. C., Siragusa, G. M., & Sage, R. P. (1977). Geology of the Chapleau area, districts of Algoma, Sudbury. Ontario Division of Mines, Geoscience report(157), 1-293.

Watkinson, D. H., Thurston, P., & Shafiqullar, M. (1972). The Shawmere anorthosite of Archean age in the Kapuskasing belt, Ontario. Journal of Geology(80), 736-739.

Watson, J. (1980). The origin and history of the Kapuskasing structural zone, Ontario, Canada. Can. J. Earth SCI., 17, 866-875.

Wu, J., Mereu, R., & Percival, J. (1992, February 21). Seismic image of the Ivanhoe Lake Fault Zone in the Kapuskasing Uplift of the Canadian Shield. GEOPHYSICAL RESEARCH LETTERS, 19(4), 353-356.

10. APPENDIX

The follow information was given by the Geological Survey of Canada.

10.1 Theoretical gravity

Measurements of gravity yield little direct geological information unless corrections are made to account of variations in Earth’s shape and topography. To isolate the effect of lateral variations in density within Earth, the bulk gravity effects of Earth due to latitude must be removed. The theoretical gravity is given in mGal by the International Gravity Formula that is based on the Geodetic Reference System of 1980 (GRS80): 1 + 푘푠푖푛2휙 훾휙 = 훾푒 √1 − 푒2푠푖푛2휙

Where 휙 is the latitude in degrees of any point on Earth, 훾푒 is normal gravity at the equator (978032.67715 mGal), 푒 is the eccentricity (푒2 = 0.00669438002290) and k is an ellipsoidal parameter (0.001931851353).

10.2 Free air-gravity anomaly

To correct variations in elevation, the vertical gradient of gravity is multiplied by the elevation (퐻) of the station: 2 휕훾 훾휙(1 + 푓 + 푚 − 2푓푠푖푛 휙) 훾휙 = −2 + 3 퐻 휕퐻 푎 푎2 where 푎 is the semi-major axis (6378137 m), 푓 is the flattening (0.00335281068118) and 푚 = 0.00344978600308. This additional correction produces the free-air gravity anomaly: 휕훾 ∆푔 = 푔 − 훾 − 퐻 푓푎 푡 휙 휕퐻 where 푔푡 is the observed gravity on Earth’s surface.

10.3 Bouguer gravity anomaly

To isolate the effects of lateral variations in density on gravity, it is also necessary to correct for the gravitational attraction of the slab of material between the observation

point and the mean sea 48 level. This is the Bouguer gravity anomaly, which is given in mGal for static land measurements by:

∆푔퐵 = ∆푔푓푎 + 2휋퐺휌퐻 - where ∆푔푓푎 is the free-air anomaly, G = gravitational constant (6.672 x 10-11 m³•kg 1 s -2 or 6.672 x 10-6 m²•kg-1•mGal),  = density of crustal rock (kg•m-3), and H = elevation above mean sea level (m). In areas of rough terrain, a terrain correction (TC) for the effect of nearby masses above (mountains) or mass deficiencies below (valleys) the gravity measurement point can be calculated and applied to the Bouguer anomaly. The final Bouguer gravity anomaly reflects lateral variations in rock density.