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Structural implications of potential field data on Southeastern North America

Structural implications of potential field data on Southeastern North America The fault system of Eastern Piedmont could be extensive in the East of the USA. Debates remain regarding the sutures zone, contacts and faults between terranes, especially underneath the coastal sediment. However, in this study, a new interpretation of the structures and contacts of the southeastern margin was based on regional land gravity anomaly and magnetic anomaly maps. To delineate and investigate the subsurface geological structures in the southern Appalachian belt and underneath the coastal sediment that covers the southeastern half of the study area, the gravity and magnetic maps were subjected to several filter techniques. The anomalies maps were enhanced by applying the reduction to pole (RTP), analytical signal (AS), tilt derivative (TDR), horizontal gradient (HG), direction filter and power spectrum techniques. The power spectrum filter was applied to separate the regional-residual anomaly. The results of regional anomaly maps display elongate high amplitude anomalies lie in the south that are related to deep-seated igneous mafic intrusive and basaltic lavas emplacement. The directional filter was used to eliminate the sutural trend of the Jurassic dikes that intruded the study area. The result displays the structural contacts clearly after removing the anomalies of the Jurassic. Finally, the edge detection filters (TDR, HG, AS) from the gravity and magnetic maps helped in mapping the anomaly contact of the subsurface bodies. The apparent structural signature of the interpreted contacts confirmed the presence of these structural features in all edge detection methods. Keywords: aeromagnetic, land gravity, Appalachian belt, filter techniques 1. Introduction which abducted toward the Grenville province rocks. The sediments of the coast have now covered much of the ter- The study area contains several major crust lithospheric ranes, which presents a challenge in this region. The central boundaries (gfi ure 1A). The southeastern margin is experi- Piedmont suture is defined as a tectonic suture between two encing twoorogeniccyclesofcrash andbreakup.Duringthe terranes, the Carolina and Piedmont (West 1998). The Car- orogeny of Grenville age (1.2 Ga), a supercontinent Rodinia olina terrane, known as the last outboard magmatic belt of formed, when continental collisions developed along the Neoproterozoic to early Paleozoic age, extends from Alabama easternmargin(Hatcher 2010). Following the Iapetus, the to Virginia (Horton et al. 1989;Steltenpohl et al. 2008). margin edge experienced renewed plate merging, episodic The Alleghanian tectonics made it hard to observe the loca- tectonic and strike–slip motion (Parker et al. 2013). The tec- tion of terrane contacts; in some circumstances, the coastal tonic activity along the margin of Appalachian is made up sediments covered much of the outboard terrane bound- of mostly Neoproterozoic to early Paleozoic meta igneous- aries. Despite the extensive studies; including magnetic, dominated islands of the Laurentian and peri-Gondwanan, 142 © The Author(s) 2022. Published by Oxford University Press on behalf of the Sinopec Geophysical Research Institute. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 1. (A) Lithotectonic terrane map of the southeastern North American margin (modified from Hatcher 1989). (B) Bouguer gravity map. (C) Total magnetic intensity map. (D) Reduced-to-magnetic-pole (RTP) map of Georgia and South Carolina. The white dashed line is the fall line and the black dotted line is Brunswick magnetic anomaly (BMA). gravity and seismic refraction studies on the geology and tec- In this paper, the structure of the southeast province, in- tonic evolution of the southernmost Appalachians (Sheridan cluding the Inner Piedmont terrane and the Carolina terrane et al. 1966;Sheridan 1974;Cook et al. 1979;Hack 1982; is evaluated using potential eld fi data. This study aims also Prodehl et al. 1984;Nelson et al. 1985a,b;Behrendt 1986; to delineate the terrane boundaries beneath the South Car- McBride et al. 1989, 2005;McBride &Nelson, 1988,oh et al. olina and Georgia coastal sediments after applying different 1991;Aleinikoff et al. 1995; Hutchinson et al. 1995;Cook& filtering techniques on the potential field data, where some Vasudevan 2006;Parker 2014), a debate on the tectonic expected contacts and faults are hidden under the coastal framework of this margin remains unanswered. Some of sediments. these debates are related to locating the sutures zone, con- tacts and faults between terranes. In this subject, there 2. Regional geology has been dispute over the mechanisms and tectonics of the Alleghanian orogen, and kinematics closing of the ter- The edge of southeastern North American is composed of rane, including the collision direction between the east- different geological histories and ages of Neoproterozoic ern Laurentia margin with western Gondwana (e.g. Secor to Cambrian (Pollock et al. 2012). Several studies have et al. 1986;Sacks &Secor 1990;Hibbard et al. 2002; attemptedtodelineatethecontactsofterranesthatremained Mueller et al. 2014). linked to the southeastern margin after the Jurassic–Triassic 143 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi separating (Wilson 1966;Higgins &Zeitz 1983;Horton The central Piedmont suture (Towaliga fault), southern et al. 1989;West 1998;Hibbard et al. 2002;Steltenpohl Appalachians, was identified as a large zone of cataclastic et al. 2008;Hatcher 2010; Tull et al. 2010;Mueller et al. rock (Crickmay 1933). The central Piedmont suture was 2013). Three orogenies (Taconic, Acadian and Alleghanian shown as a fault on the geologic map of Georgia (Lawton orogeny) created the Appalachian belt in North American et al. 1976). The fault in Georgia state was increased by during the Paleozoic as a result of the continent island arc reconnaissance surface geologic mapping and the use of collision. Accretionary tectonics of the Carolina to North regional aero-radioactivity and aeromagnetic maps (Bent- America has been debated (e.g., Hibbard 2000;Hatcher ley et al. 1974a,b). Central Piedmont is thought to run NE 2002;Hibbard et al. 2002). Some researchers showed that through Georgia state and likely toward South Carolina state. the Carolina belt docked to the Laurentia crust when one (Howell & Pirkle 1976; Pickering & Murray 1976). During of the three events takes place, the Middle Ordovician the Alleghanian, central Piedmont (mid-Paleozoic) reacti- (∼490–440 Mya) Taconic, the Devonian (∼420–350 Mya) vated locally, separating peri-Gondwanan from Laurentia Acadian or the Carboniferous-Permian (∼340–250 Mya) terranes (Hatcher 2010). Alleghanian (Hibbard 2000). Within the Carolina terrane, a Nelson et al. (1985a,b), McBride & Nelson (1988), fall line exists between the Appalachian and Atlantic. South Austin et al. (1990) indicated that the major geophysical of the fall line, the terranes are little understood due to a lack magnetic feature, the East Coast anomaly and its contin- of seismic surveys and well data, as well as a lack of coastal uation Brunswick magnetic anomaly (BMA), pass over plain sediment coverage. Georgia and Alabama to represent the Alleghanina suture. The Alleghenian compressional structures were super- Others argued about the origin of the magnetic anomaly, in- posed by extension, and basalt injections and flows as a dicated that it resulted from extension of Jurassic rifting (e.g., result of the Atlantic seafloor’s opening in the Triassic– Popenoe & Zietz 1977;Higgins&Zietz 1983; Hutchinson Jurassic period (Seeber & Armbruster 1981;McBride et al. et al. 1983;Klitgord et al. 1983;Bird et al. 2007). Keller 1989). During this period of extension, the Central Atlantic et al. (1954), Austin et al. (1990)and Davis et al. (2018) Province (CAMP) intrusion overprint the Alleghenian describedthe east coastmagneticsignalasseaward dipping structure along the eastern side of the USA (Pollack 2014). containing a deep-seated feature near the ocean-continent Transgression and dextral strike–slip setting across the east- transition on the rifting edge. ern Appalachian margin was followed by terrane transport during the Alleghenian (Carboniferous) orogeny, along Blue 3. Processing techniques and results Ridge and Piedmont subduction (Hatcher 2010;Parker 2014). In the present study, gravity and magnetic data are kindly of- The Blue Ridge terrane, Valley and Ridge terrane are fered by the USGS and University of Texas-El Paso (figure separated by a major thrust fault (Harris & Milici 1977): 1Band 1C). Figure 1A shows a tectonic map (after Hatcher the Central Piedmont fault separates the eastern Blue Ridge 1989). province (Hatcher 1987). The Blue Ridge province is in- The igneous and metamorphic terranes are usually domi- terpreted as being of a complex and fragmented nature. The nated by complex magnetic signatures with discrepancies in Blue Ridge rocks consist of continental basement rocks the susceptibilities and/or densities that characterize these and late Precambrian to early Paleozoic, metavolcanic and terranes (Ibraheem 2009). This makes the interpretation of plutons. The Inner Piedmont is made up of rocks with the the magnetic and Bouguer gravity data not an easy process highest metamorphic grade in the Appalachian Piedmont and the data need to be processed and filtered, especially if terrane. Metasandstone, granitic gneiss, schist, amphibolite the faults dissect these terranes. In this respect, it could be and paragneiss are among the rocks found in the Inner added that magnetic anomalies do not depend only on the Piedmont (Goldsmith et al. 1988). The Brevard fault zone susceptibility and shape of the causative body. They also de- is thought to be a mylonite fault that split the Inner Pied- pend on the magnetization directions of the regional mag- mont and the Blue Ridge Mountains (Hatcher 1978, 2001; netic eld. fi Therefore, the RTP filter (magnetic reduction to Higgins et al. 1988). The Brevard fault was identiefi d as a pole) has been used for directing the anomaly vertically over major crustal fault (Jonas 1932;King 1955; Reed & Bryant the causative source and the magnetization vector accord- 1964; Bryant & Reed 1970). Southeastern Piedmont was ingly directed vertically downward as gravity force. To reduce subdivided into several northeast-trending litho-tectonic polarity effects, a magnetic reduction to the pole is applied belts (Crickmay 1952;King 1955;Hatcher 1972). These to the magnetic anomaly (Blakely & Simpson 1995). The belts are distinguished by low- to medium-grade regional RTP place magnetic anomaly gradients and peaks directly metamorphism, e.g. the Carolina slate belt, alternating over their source bodies (Zahra & Oweis 2016). The RTP by medium- to high-grade belts, e.g. the Kiokee and the anomaly map of the study area is shown in gfi ure 1D. The Charlotte belts (Dallmeyer et al. 1986). RTP result map shows linear magnetic anomalies caused by 144 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 2. RTP map with 125° directional filter over Georgia and South Carolina. thedikeswarmoftheJurassicage.TheJurassicdikesobscure (AS) filtering is used to enhance short-frequency features as and interrupt an interesting structural and geological feature well as reveal anomaly texture. The AS exhibits maximum that could otherwise be determined later after data filtering. amplitudes over the fault and/or contact and highlight dis- Generally speaking, a subtle magnetic texture within struc- continuities (Roest et al. 1992). According to (Nabighian tural units may be ambiguous because of the interference 1972;Roest et al. 1992), the AS (equation (1)) is defined as of the dike magnetic signatures. In the following, the direc- the sum of the three orthogonal derivatives of the field: tional filtering technique is applied to eliminate or minimize ( ) ( ) ( ) 2 2 2 the linear effect of northwest dike magnetic anomalies in the 𝜕 f 𝜕 f 𝜕 f AS = + + . (1) magnetic data. 𝜕 X 𝜕 Y 𝜕 z Several filters are used to achieve the primary goal of the study and to highlight the main geological trends. The first Therefore, the AS of the RTP data of the study area filter is the directional filter (e.g., Hinze et al. 2013;Zahra (figure 3) has been used to interpret the magnetic faults &Oweis 2016;Alarifi et al. 2021). This technique is often and contacts of the subsurface. The AS map shows well po- used to eliminate geological noise, for instance dike anoma- sitioned and defined magnetic sources: the AS anomalies lies that cut through a sequence of interest (Cooper 1997; highlighted and emphasized the variation in magnetization. Alarifi et al. 2021). Since the magnetic anomalies of dikes are The result indicated that the structural trends are parallel to generally caused by sources near or at the ground surface, the regional trend along the NE direction. The amplitude of enhancement technique filters of the magnetic data are likely the AS indicated high values that can divide the area into dif- to aggravate the issue. The directional filter method, after ferent zones. Each zone is characterized by a different suscep- being applied to magnetic data, tends to exaggerate and tibility contrast, which shows a significant signature on the enhance features in specicfi directions. Thus, dike swarm study area. signatures were removed for better interpretation by using TheBouguer gravityand RTPdatawereenhancedus- the directional filter. ing a variety of techniques to facilitate the structural inter- The RTP magnetic data were used to calculate the AS in pretation. In this respect, two edge detection techniques the either the frequency or space domains, creating a high value tilt derivative (TDR) and horizontal gradient (HG) are used over discrete bodies and their boundaries. Analytic signal (Salem & Ravat 2003;Hinze et al. 2013;Airo et al. 2014). 145 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 3. Analytical signal filtered map of the RTP data over Georgia and South Carolina. The tilt derivative is used to emphasize and enhance long- and robust in the detection of deep and near-surface struc- frequency and short-wavelength anomaly signals originating tures and sources (Phillips 2002;Ibraheem et al. 2019). from shallow magnetic and gravity sources. Miller & Singh The edge detection filters were applied to the gravity and (1994) described the tilt derivative (TDR), which was later RTP anomaly maps. Figures 4 and 5 show the tilt derivative developed by Salem et al. (2008). The tilt derivative (equa- and HG over the study area. The HG and tilt derivative re- tion (2)) is a function of the ratio of the vertical and horizon- sults indicated a high value over the gravity and magnetic tal derivatives (x and y directions), sources, thus aiming to locate the geological contact. To bet- ter visualize only higher values of the TDR and HG, maps in ( ) VDR −1 TDR = tan , (2) gfi ure 6 were created to detect the trend and the edge con- THDR tactsofthe subsurface geology. Theagreement betweenthe whereVDR andTHDRare thefirstverticaland totalhori- TDR and HG anomalies of both RTP and Bouguer gravity zontal derivatives (Miller & Singh 1994;Verduzco et al. 2004; (figure 6) confirms the location and trend of the subsurface Salem et al. 2007). The total horizontal derivatives of the RTP sources. Compared to the RTP and Bouguer gravity anomaly and gravity tends to produce large values above the contacts maps, the TDR and HG successfully traced the edges and (Cordell & Grauch 1985) and are determined by the follow- contacts of the subsurface structures. Therefore, the TDR ing equation (3) and HG are suitable for mapping the basement structures and have clearly produced more detailed deep structural contacts ( ) ( ) aM aM and faults. THDR = + , (3) The gravity and RTP anomalies data were separated into ax ay regional-residual components (gfi ures 7 and 8)byapply- aM aM where and are the horizontal derivatives of the field ing the fast Fourier transform (FFT). The isolation pro- ax ay cess is used to separate the anomalies caused by shallower (M). The horizontal gradient (HG) is insensitive to the noise 146 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 4. (A) Tilt derivative of the RTP map. (B) Tilt derivative of the gravity map. Figure 5. (A) Horizontal gradient of RTP map. (B) Horizontal gradient gravity map. geological features from deeper geological features. The fil- shallower sources are associated with the high-frequency seg- tered maps were based on the cutoff frequencies. Several ment. The regional (low pass) filtered maps (gfi ure 7)re- authors (e.g., Bhattacharyya 1966; Spector & Grant 1970; veal deeper gravity/magnetic anomaly sources. The residual Reeves 2005) have described the spectral analysis method. (high pass) filtered of the gravity and RTP in the study area This method is based on gravity and magnetic data analysis (figure 8)reflectedshort wavenumber andhighwavelength using FFT. The FFT method is a wavelength function in the anomalies. The result highlights several high anomalies x and y directions. The data are transformed from space into oriented in the northeast, parallel to the structural trends of the frequency or wavenumber domain via the FFT algorithm terranes.ThemapalsohighlightsseveralCarboniferousgran- application. ite plutons (e.g., Liberty Hill pluton, Pageland granite plu- Cutoff wavenumbers of 0.013 and 0.009 radians per meter ton, etc.), which are correlated with gravity minima. Mag- were used to separate the RTP and gravity data, respectively, netic residuals (gfi ure 8A) showed high anomalies associated depending on the interactive filter of Oasis Montaj. Deeper with some know contacts/faults and dikes, indicating near- sources are associated with the low-frequency segment, while surface geological features. 147 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 6. (A) Tilt derivative of RTP. (B) Tilt derivative of Bouguer gravity. (C) Horizontal gradient of RTP. (D) Horizontal gradient of Bouguer gravity. 4. Interpretation The RTP magnetic anomaly map (gfi ure 1D) shows a magnetic anomaly amplitude in a wide range, ranging from Variations in subsurface density and crustal thickness are −841 to 2570 nT. The RTP map is divided into two zones, frequently reflected by the Bouguer gravity anomaly. Con- each of which is characterized by different magnetic signa- sidering a relatively constant density, a thin crust corre- tures, amplitude and pattern. From south to north of mag- sponds to high positive gravity anomalies, while a thick netic maps, a broader wavelength, generally positive ampli- crust corresponds to negative gravity anomalies (Wang et al. tude magnetic (red to pink colored zones), character zones 2010). According to these criteria, the observed low grav- that represent mafic intrusive units and basaltic lavas that ity anomalies that dominate the northwestern side of the were intruded during Jurassic–Triassic. This intruded is cor- studied area (gfi ure 1B) couldbecorrelatedtothe cou- related with the separating of the Atlantic Sea. This positive pled Bouguer Appalachian Mountains belt, and the pos- amplitude magnetic is definite from the lower wavelength itive high gravity anomalies that dominate the rest of magnetic province of the Piedmont and Carolina terranes. the area could represent the full length of the south- The magnetic anomaly map highlights igneous intrusions ern Appalachian belt. Additionally, the strong broad high and structural features. gravity value belt (red color) shown at the central part Structurally, the RTP in gfi ure 1Dshows broadwave- (figure 1B) generally coincides with the slate belt. A circle length magnetic anomalies that could be correlated to ma- positive gravity anomaly at the southern part of the map is jor tectonic structures in the study area. In the central part of related to mafic intrusions. The local circular negative val- the map, the Carolina terrane was intruded by Carboniferous ues are noted over the gravity, which could be interpreted granites as the Carolina terrane docked with Laurentia. The as granite intrusive features, particularly over the Carolina RTP anomaly strongly reflects the northeast structure of the terrane. 148 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 7. (A) Filtered regional RTP anomaly map. (B) Filtered regional Bouguer anomaly map. Figure 8. (A) Filtered residual RTP anomaly map. (B) Filtered residual Bouguer anomaly map. peri-Gondwana terrane and the northwest-trending Jurassic visualization of other structural trends, especially under the mafic dikes. The map is dissected by a deep-seated igneous coastal plain sediments. The low pass gravity map (gfi ure 7B) mafic intrusive southeast of the fall line during the opening shows a signica fi nt change in the Bouguer gravity anomaly of the Atlantic in the Triassic–Jurassic. A southeast trending from the north to the south, which could reflect deep crustal feature reveals the BMA across Georgia and Alabama possi- variations in thickness and/or density. The central part of the bly results from the Mesozoic extension. area in the Bouguer gravity map is characterized by a high The directional filter map (gfi ure 2)was able to improve broad anomaly in the NE–SW direction over the slate belt. strike-filtering features that lay at an angle of 125 °.The di- Southeast of the fall line, a high magnitude (around 35 mGal) rectional filter improves the detection of high wavelength and large size of anomaly response along the coastal plain in- anomalies scattered throughout the study area. It is impor- dicate that the area is underlain by intrusive zones of high tant to note that the result provides a good indication of the density. The axes of anomalies in the NE–SW direction in- regional structure of the study area. A directional filter masks dicate possible structural or faults. The low pass RTP map the NW-SE anomaly signals over the study area and improves (figure 7A) reveals anomalies with deep basement variations. 149 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 9. Possible interpretation for RTP map with 125° directional filter. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. ST is the Suwannee terrane boundary. The map is characterized in the southeastward by large pos- fault zone underlain the coastal in the Carolina terrane. The itive magnetic with high amplitudes and is primarily associ- southwestern area of the map is expected to be isolated from ated with a deep and large basement source. Two prominent the Carolina terrane by the fourth contact (denoted ST in zones show low magnetism values that expand nearly over gfi ure 10) that possibly indicates the contact of the Suwan- the Carolina terrane and the southwest on the Suwannee ter- nee terrane that is hidden under the Paleozoic sedimentary rane (ST) line. The spatial relationship of low and high pass rocks. The obvious structural signature of the interpreted anomalies (figures 7 and 8)revealedthatmostofthe high contacts in the tilt derivative and the HG maps (gfi ures 11 pass is caused by deep-seated structures. Figure 9 shows a and 12) confirms the existence of these structural features possible interpretation for the direction filter. The interpreta- (contacts/faults). In addition, the three edge detection meth- tion map of the directional filter clearly delineates a NE trend- ods (AS, HG and TDR) successfully highlighted the edges ing anomaly that follows the regional trend of the study area. of the granitic plutons in the Carolina terrane as well as the The map result shows a similar structural feature to the AS, mafic intrusions in the southeast of the fall line beneath the TDR, and HG maps (figures 10–12). coastal sediments in Georgia and South Carolina. Figure 13 The interpretation of the edge detection (AS, TDR and shows the regional tectonic trends that were detected from HG) maps indicate clearly four different structural con- the potential field data after applying different technique tacts separating four structural zones that are characterized filters. by their different anomaly signatures (gfi ures 10–12). In the northern part (denoted as the Brevard fault (BF) in 5. Discussions gfi ure 10), the rst fi contact follows the edge of the Inner Piedmont terrane; possibly indicating the BF line. The sec- The potential field was used to investigate the structural ond contact (denoted CP in gfi ure 10) may indicate the cen- framework of Georgia and South Carolina, especially un- tral pediment zone that divides the high-grade metamorphic der the coastal sediments. The seismic reflection studies of of the Inner Piedmont from lower metamorphosed grade Nelson et al. (1985a), Maher et al. (1994)and Cook & rocks of the Carolina terrane. To the south of the CP contact, Vasudevan (2006) found that the structural sutures under the third contact (denoted CT in gfi ure 10) may relate to a the coastal sediments were dicult ffi to identify. In addition, 150 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 10. Possible interpretation for the analytical signal of the RTP map. ST is the Suwannee terrane boundary. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. terrane of the Carolina. The CT extends from the southeast- previous studies, e.g., Cook et al. (1979; 1981); Daniels ern of Inner Piedmont terrane to the southeast underneath (1974); Popenoe & Zietz (1977); Daniels et al. (1983); thecoastal sediments. TheCTisclearly seen beneaththe Higgins & Zietz (1983); Iverson & Smithson (1983); Cook coastal in the directional (gfi ure 9)and theedgedetection & Vasudevan (2006), failed to give a clear picture about these filters (gu fi res 10–12). These techniques are well imaged in fault zones. Therefore, the present data are used for delin- theterranecontactzones.Hatcher (1972) suggested that the eating the structure beneath the coastal plain sediments after Carolina rocks exist to the east of the Piedmont fault system. applying different filter techniques. The present results propose that the Carolina terrane was af- The interpretation of the present Bouguer and aeromag- fected by igneous intrusions mostly during the last tectonic netic maps successfully trace four fault zones that affected event. The igneous intrusions are indicated by the distinc- the study area: namely BF, central Piedmont shear/fault zone tive circular negative gravity anomalies that characterize the (CP), Carolina terrane contact/fault (CT) and ST. The BF gravity and RTP anomaly maps of the Carolina terrane. The and CP tfi well with the surface geology of the southeast- Bouguer gravity anomaly result (gfi ure 7)exhibitsabroad ernmargin(gfi ure 1). The ST shows a linear anomaly trend high gravity anomaly trending in the NE direction, indicat- along the approximately E–SE direction that truncates all the ing the Appalachian trend and the related metasedimentary magnetic and gravity anomaly features that correspond to and metavolcanic rocks in the Carolina slate belt. A positive the Appalachian structure. Interpreted as the Suwannee su- Bouguer gravity belt could be correlated to high densities of ture boundary, this zone was recorded in Florida by Pojeta metamorphic rocks and mafic rocks that dominate the area et al. (1976)andinAlabamabyNeathery&Thomas(1975); (Alarifi et al. 2021). Magnetically, this area is represented as Horton et al. (1984); Steltenpohl et al. (2013). A high mag- a regional low magnetic anomaly (gfi ure 7), which can be ex- netic anomaly zone that dominates the edge detection maps plained as the Carolina terrane undergone mild, deformation (denoted by CT; gfi ures 10–12) may represent the tectonic 151 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 11. Possible interpretation for (A) tilt derivative of RTP map and (B) tilt derivative of gravity map. ST is the Suwannee terrane boundary. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. Figure 12. Possible interpretation for (A) the horizontal gradient of RTP map and (B) horizontal gradient gravity map. ST is the Suwannee terrane boundary. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. and alteration that may lead to distribute the magnetic prop- lar features in the aeromagnetic and gravity maps of Alabama erty. The southern region of the study area is characterized by state suggested that mafic rocks are within the Suwannee. circular and elongate high amplitude anomalies in the gravity and magnetic maps. These circular and elongate anomalies 6. Conclusion are interpreted here as deep-seated igneous mafic intrusive and basaltic lavas emplacement within the Mesozoic rifting The geological features beneath the coastal plain can be margin along the ocean to continent transition. These results mappedbyusingtheregionalmagneticandgravitymaps.For are confirmed by Gohn et al. (1983) who recorded basaltic better interpretation analysis, filtering is carried out on the layers with intercalations of red beds in the drilled boreholes. Bouguer gravity map and reduced to a magnetic pole map. In addition, Steltenpohl et al. 2013 indicated that high circu- The potential field data compared to previously identified 152 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 13. Regional structural trends of the study area detected from the potential field data. ST is the Suwannee terrane boundary. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. dikes, geologic contacts and faults that are published on ge- the northern half and Gondwanan/peri-Gondwanan under- ologic maps. The generation of a lineament system map as a lying the southern half of the area. method of mapping fault and contact systems is a key goal of the potential field interpretation. Acknowledgements The fault system of Eastern Piedmont might be an exten- sive one in the eastern USA. The fault zones could be traced The paper was improved considerably by discussions with Elkhedr for tens of kilometers in length. The faults are detected and re- Ibrahim, James N. Kellogg and Patrick Duff. This research was flected clearly on the tilt derivative, HG, AS and directional supported by Researchers Supporting project number (grant no. filter maps. Using anomaly lineation, we were able to deter- RSP2022R496), King Saud University, Riyadh, Saudi Arabia. mine the location of the Carolina and Suwannee terranes Conflict of interest statement : None declared. thatarehiddenbeneaththecoastalplain.Theedgedetection and direction filter results indicated that the NE–SW linea- ment anomalies lie parallel to the regional geology and tec- References tonic structure. The area is affected by the tectonics related to the Mesozoic rift and Allegenian collision. 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Structural implications of potential field data on Southeastern North America

Alarifi, Saad S
Journal of Geophysics and Engineering , Volume 19 (2): 15 – Apr 1, 2022
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Oxford University Press
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© The Author(s) 2022. Published by Oxford University Press on behalf of the Sinopec Geophysical Research Institute.
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1742-2132
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10.1093/jge/gxac005
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Abstract

The fault system of Eastern Piedmont could be extensive in the East of the USA. Debates remain regarding the sutures zone, contacts and faults between terranes, especially underneath the coastal sediment. However, in this study, a new interpretation of the structures and contacts of the southeastern margin was based on regional land gravity anomaly and magnetic anomaly maps. To delineate and investigate the subsurface geological structures in the southern Appalachian belt and underneath the coastal sediment that covers the southeastern half of the study area, the gravity and magnetic maps were subjected to several filter techniques. The anomalies maps were enhanced by applying the reduction to pole (RTP), analytical signal (AS), tilt derivative (TDR), horizontal gradient (HG), direction filter and power spectrum techniques. The power spectrum filter was applied to separate the regional-residual anomaly. The results of regional anomaly maps display elongate high amplitude anomalies lie in the south that are related to deep-seated igneous mafic intrusive and basaltic lavas emplacement. The directional filter was used to eliminate the sutural trend of the Jurassic dikes that intruded the study area. The result displays the structural contacts clearly after removing the anomalies of the Jurassic. Finally, the edge detection filters (TDR, HG, AS) from the gravity and magnetic maps helped in mapping the anomaly contact of the subsurface bodies. The apparent structural signature of the interpreted contacts confirmed the presence of these structural features in all edge detection methods. Keywords: aeromagnetic, land gravity, Appalachian belt, filter techniques 1. Introduction which abducted toward the Grenville province rocks. The sediments of the coast have now covered much of the ter- The study area contains several major crust lithospheric ranes, which presents a challenge in this region. The central boundaries (gfi ure 1A). The southeastern margin is experi- Piedmont suture is defined as a tectonic suture between two encing twoorogeniccyclesofcrash andbreakup.Duringthe terranes, the Carolina and Piedmont (West 1998). The Car- orogeny of Grenville age (1.2 Ga), a supercontinent Rodinia olina terrane, known as the last outboard magmatic belt of formed, when continental collisions developed along the Neoproterozoic to early Paleozoic age, extends from Alabama easternmargin(Hatcher 2010). Following the Iapetus, the to Virginia (Horton et al. 1989;Steltenpohl et al. 2008). margin edge experienced renewed plate merging, episodic The Alleghanian tectonics made it hard to observe the loca- tectonic and strike–slip motion (Parker et al. 2013). The tec- tion of terrane contacts; in some circumstances, the coastal tonic activity along the margin of Appalachian is made up sediments covered much of the outboard terrane bound- of mostly Neoproterozoic to early Paleozoic meta igneous- aries. Despite the extensive studies; including magnetic, dominated islands of the Laurentian and peri-Gondwanan, 142 © The Author(s) 2022. Published by Oxford University Press on behalf of the Sinopec Geophysical Research Institute. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 1. (A) Lithotectonic terrane map of the southeastern North American margin (modified from Hatcher 1989). (B) Bouguer gravity map. (C) Total magnetic intensity map. (D) Reduced-to-magnetic-pole (RTP) map of Georgia and South Carolina. The white dashed line is the fall line and the black dotted line is Brunswick magnetic anomaly (BMA). gravity and seismic refraction studies on the geology and tec- In this paper, the structure of the southeast province, in- tonic evolution of the southernmost Appalachians (Sheridan cluding the Inner Piedmont terrane and the Carolina terrane et al. 1966;Sheridan 1974;Cook et al. 1979;Hack 1982; is evaluated using potential eld fi data. This study aims also Prodehl et al. 1984;Nelson et al. 1985a,b;Behrendt 1986; to delineate the terrane boundaries beneath the South Car- McBride et al. 1989, 2005;McBride &Nelson, 1988,oh et al. olina and Georgia coastal sediments after applying different 1991;Aleinikoff et al. 1995; Hutchinson et al. 1995;Cook& filtering techniques on the potential field data, where some Vasudevan 2006;Parker 2014), a debate on the tectonic expected contacts and faults are hidden under the coastal framework of this margin remains unanswered. Some of sediments. these debates are related to locating the sutures zone, con- tacts and faults between terranes. In this subject, there 2. Regional geology has been dispute over the mechanisms and tectonics of the Alleghanian orogen, and kinematics closing of the ter- The edge of southeastern North American is composed of rane, including the collision direction between the east- different geological histories and ages of Neoproterozoic ern Laurentia margin with western Gondwana (e.g. Secor to Cambrian (Pollock et al. 2012). Several studies have et al. 1986;Sacks &Secor 1990;Hibbard et al. 2002; attemptedtodelineatethecontactsofterranesthatremained Mueller et al. 2014). linked to the southeastern margin after the Jurassic–Triassic 143 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi separating (Wilson 1966;Higgins &Zeitz 1983;Horton The central Piedmont suture (Towaliga fault), southern et al. 1989;West 1998;Hibbard et al. 2002;Steltenpohl Appalachians, was identified as a large zone of cataclastic et al. 2008;Hatcher 2010; Tull et al. 2010;Mueller et al. rock (Crickmay 1933). The central Piedmont suture was 2013). Three orogenies (Taconic, Acadian and Alleghanian shown as a fault on the geologic map of Georgia (Lawton orogeny) created the Appalachian belt in North American et al. 1976). The fault in Georgia state was increased by during the Paleozoic as a result of the continent island arc reconnaissance surface geologic mapping and the use of collision. Accretionary tectonics of the Carolina to North regional aero-radioactivity and aeromagnetic maps (Bent- America has been debated (e.g., Hibbard 2000;Hatcher ley et al. 1974a,b). Central Piedmont is thought to run NE 2002;Hibbard et al. 2002). Some researchers showed that through Georgia state and likely toward South Carolina state. the Carolina belt docked to the Laurentia crust when one (Howell & Pirkle 1976; Pickering & Murray 1976). During of the three events takes place, the Middle Ordovician the Alleghanian, central Piedmont (mid-Paleozoic) reacti- (∼490–440 Mya) Taconic, the Devonian (∼420–350 Mya) vated locally, separating peri-Gondwanan from Laurentia Acadian or the Carboniferous-Permian (∼340–250 Mya) terranes (Hatcher 2010). Alleghanian (Hibbard 2000). Within the Carolina terrane, a Nelson et al. (1985a,b), McBride & Nelson (1988), fall line exists between the Appalachian and Atlantic. South Austin et al. (1990) indicated that the major geophysical of the fall line, the terranes are little understood due to a lack magnetic feature, the East Coast anomaly and its contin- of seismic surveys and well data, as well as a lack of coastal uation Brunswick magnetic anomaly (BMA), pass over plain sediment coverage. Georgia and Alabama to represent the Alleghanina suture. The Alleghenian compressional structures were super- Others argued about the origin of the magnetic anomaly, in- posed by extension, and basalt injections and flows as a dicated that it resulted from extension of Jurassic rifting (e.g., result of the Atlantic seafloor’s opening in the Triassic– Popenoe & Zietz 1977;Higgins&Zietz 1983; Hutchinson Jurassic period (Seeber & Armbruster 1981;McBride et al. et al. 1983;Klitgord et al. 1983;Bird et al. 2007). Keller 1989). During this period of extension, the Central Atlantic et al. (1954), Austin et al. (1990)and Davis et al. (2018) Province (CAMP) intrusion overprint the Alleghenian describedthe east coastmagneticsignalasseaward dipping structure along the eastern side of the USA (Pollack 2014). containing a deep-seated feature near the ocean-continent Transgression and dextral strike–slip setting across the east- transition on the rifting edge. ern Appalachian margin was followed by terrane transport during the Alleghenian (Carboniferous) orogeny, along Blue 3. Processing techniques and results Ridge and Piedmont subduction (Hatcher 2010;Parker 2014). In the present study, gravity and magnetic data are kindly of- The Blue Ridge terrane, Valley and Ridge terrane are fered by the USGS and University of Texas-El Paso (figure separated by a major thrust fault (Harris & Milici 1977): 1Band 1C). Figure 1A shows a tectonic map (after Hatcher the Central Piedmont fault separates the eastern Blue Ridge 1989). province (Hatcher 1987). The Blue Ridge province is in- The igneous and metamorphic terranes are usually domi- terpreted as being of a complex and fragmented nature. The nated by complex magnetic signatures with discrepancies in Blue Ridge rocks consist of continental basement rocks the susceptibilities and/or densities that characterize these and late Precambrian to early Paleozoic, metavolcanic and terranes (Ibraheem 2009). This makes the interpretation of plutons. The Inner Piedmont is made up of rocks with the the magnetic and Bouguer gravity data not an easy process highest metamorphic grade in the Appalachian Piedmont and the data need to be processed and filtered, especially if terrane. Metasandstone, granitic gneiss, schist, amphibolite the faults dissect these terranes. In this respect, it could be and paragneiss are among the rocks found in the Inner added that magnetic anomalies do not depend only on the Piedmont (Goldsmith et al. 1988). The Brevard fault zone susceptibility and shape of the causative body. They also de- is thought to be a mylonite fault that split the Inner Pied- pend on the magnetization directions of the regional mag- mont and the Blue Ridge Mountains (Hatcher 1978, 2001; netic eld. fi Therefore, the RTP filter (magnetic reduction to Higgins et al. 1988). The Brevard fault was identiefi d as a pole) has been used for directing the anomaly vertically over major crustal fault (Jonas 1932;King 1955; Reed & Bryant the causative source and the magnetization vector accord- 1964; Bryant & Reed 1970). Southeastern Piedmont was ingly directed vertically downward as gravity force. To reduce subdivided into several northeast-trending litho-tectonic polarity effects, a magnetic reduction to the pole is applied belts (Crickmay 1952;King 1955;Hatcher 1972). These to the magnetic anomaly (Blakely & Simpson 1995). The belts are distinguished by low- to medium-grade regional RTP place magnetic anomaly gradients and peaks directly metamorphism, e.g. the Carolina slate belt, alternating over their source bodies (Zahra & Oweis 2016). The RTP by medium- to high-grade belts, e.g. the Kiokee and the anomaly map of the study area is shown in gfi ure 1D. The Charlotte belts (Dallmeyer et al. 1986). RTP result map shows linear magnetic anomalies caused by 144 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 2. RTP map with 125° directional filter over Georgia and South Carolina. thedikeswarmoftheJurassicage.TheJurassicdikesobscure (AS) filtering is used to enhance short-frequency features as and interrupt an interesting structural and geological feature well as reveal anomaly texture. The AS exhibits maximum that could otherwise be determined later after data filtering. amplitudes over the fault and/or contact and highlight dis- Generally speaking, a subtle magnetic texture within struc- continuities (Roest et al. 1992). According to (Nabighian tural units may be ambiguous because of the interference 1972;Roest et al. 1992), the AS (equation (1)) is defined as of the dike magnetic signatures. In the following, the direc- the sum of the three orthogonal derivatives of the field: tional filtering technique is applied to eliminate or minimize ( ) ( ) ( ) 2 2 2 the linear effect of northwest dike magnetic anomalies in the 𝜕 f 𝜕 f 𝜕 f AS = + + . (1) magnetic data. 𝜕 X 𝜕 Y 𝜕 z Several filters are used to achieve the primary goal of the study and to highlight the main geological trends. The first Therefore, the AS of the RTP data of the study area filter is the directional filter (e.g., Hinze et al. 2013;Zahra (figure 3) has been used to interpret the magnetic faults &Oweis 2016;Alarifi et al. 2021). This technique is often and contacts of the subsurface. The AS map shows well po- used to eliminate geological noise, for instance dike anoma- sitioned and defined magnetic sources: the AS anomalies lies that cut through a sequence of interest (Cooper 1997; highlighted and emphasized the variation in magnetization. Alarifi et al. 2021). Since the magnetic anomalies of dikes are The result indicated that the structural trends are parallel to generally caused by sources near or at the ground surface, the regional trend along the NE direction. The amplitude of enhancement technique filters of the magnetic data are likely the AS indicated high values that can divide the area into dif- to aggravate the issue. The directional filter method, after ferent zones. Each zone is characterized by a different suscep- being applied to magnetic data, tends to exaggerate and tibility contrast, which shows a significant signature on the enhance features in specicfi directions. Thus, dike swarm study area. signatures were removed for better interpretation by using TheBouguer gravityand RTPdatawereenhancedus- the directional filter. ing a variety of techniques to facilitate the structural inter- The RTP magnetic data were used to calculate the AS in pretation. In this respect, two edge detection techniques the either the frequency or space domains, creating a high value tilt derivative (TDR) and horizontal gradient (HG) are used over discrete bodies and their boundaries. Analytic signal (Salem & Ravat 2003;Hinze et al. 2013;Airo et al. 2014). 145 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 3. Analytical signal filtered map of the RTP data over Georgia and South Carolina. The tilt derivative is used to emphasize and enhance long- and robust in the detection of deep and near-surface struc- frequency and short-wavelength anomaly signals originating tures and sources (Phillips 2002;Ibraheem et al. 2019). from shallow magnetic and gravity sources. Miller & Singh The edge detection filters were applied to the gravity and (1994) described the tilt derivative (TDR), which was later RTP anomaly maps. Figures 4 and 5 show the tilt derivative developed by Salem et al. (2008). The tilt derivative (equa- and HG over the study area. The HG and tilt derivative re- tion (2)) is a function of the ratio of the vertical and horizon- sults indicated a high value over the gravity and magnetic tal derivatives (x and y directions), sources, thus aiming to locate the geological contact. To bet- ter visualize only higher values of the TDR and HG, maps in ( ) VDR −1 TDR = tan , (2) gfi ure 6 were created to detect the trend and the edge con- THDR tactsofthe subsurface geology. Theagreement betweenthe whereVDR andTHDRare thefirstverticaland totalhori- TDR and HG anomalies of both RTP and Bouguer gravity zontal derivatives (Miller & Singh 1994;Verduzco et al. 2004; (figure 6) confirms the location and trend of the subsurface Salem et al. 2007). The total horizontal derivatives of the RTP sources. Compared to the RTP and Bouguer gravity anomaly and gravity tends to produce large values above the contacts maps, the TDR and HG successfully traced the edges and (Cordell & Grauch 1985) and are determined by the follow- contacts of the subsurface structures. Therefore, the TDR ing equation (3) and HG are suitable for mapping the basement structures and have clearly produced more detailed deep structural contacts ( ) ( ) aM aM and faults. THDR = + , (3) The gravity and RTP anomalies data were separated into ax ay regional-residual components (gfi ures 7 and 8)byapply- aM aM where and are the horizontal derivatives of the field ing the fast Fourier transform (FFT). The isolation pro- ax ay cess is used to separate the anomalies caused by shallower (M). The horizontal gradient (HG) is insensitive to the noise 146 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 4. (A) Tilt derivative of the RTP map. (B) Tilt derivative of the gravity map. Figure 5. (A) Horizontal gradient of RTP map. (B) Horizontal gradient gravity map. geological features from deeper geological features. The fil- shallower sources are associated with the high-frequency seg- tered maps were based on the cutoff frequencies. Several ment. The regional (low pass) filtered maps (gfi ure 7)re- authors (e.g., Bhattacharyya 1966; Spector & Grant 1970; veal deeper gravity/magnetic anomaly sources. The residual Reeves 2005) have described the spectral analysis method. (high pass) filtered of the gravity and RTP in the study area This method is based on gravity and magnetic data analysis (figure 8)reflectedshort wavenumber andhighwavelength using FFT. The FFT method is a wavelength function in the anomalies. The result highlights several high anomalies x and y directions. The data are transformed from space into oriented in the northeast, parallel to the structural trends of the frequency or wavenumber domain via the FFT algorithm terranes.ThemapalsohighlightsseveralCarboniferousgran- application. ite plutons (e.g., Liberty Hill pluton, Pageland granite plu- Cutoff wavenumbers of 0.013 and 0.009 radians per meter ton, etc.), which are correlated with gravity minima. Mag- were used to separate the RTP and gravity data, respectively, netic residuals (gfi ure 8A) showed high anomalies associated depending on the interactive filter of Oasis Montaj. Deeper with some know contacts/faults and dikes, indicating near- sources are associated with the low-frequency segment, while surface geological features. 147 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 6. (A) Tilt derivative of RTP. (B) Tilt derivative of Bouguer gravity. (C) Horizontal gradient of RTP. (D) Horizontal gradient of Bouguer gravity. 4. Interpretation The RTP magnetic anomaly map (gfi ure 1D) shows a magnetic anomaly amplitude in a wide range, ranging from Variations in subsurface density and crustal thickness are −841 to 2570 nT. The RTP map is divided into two zones, frequently reflected by the Bouguer gravity anomaly. Con- each of which is characterized by different magnetic signa- sidering a relatively constant density, a thin crust corre- tures, amplitude and pattern. From south to north of mag- sponds to high positive gravity anomalies, while a thick netic maps, a broader wavelength, generally positive ampli- crust corresponds to negative gravity anomalies (Wang et al. tude magnetic (red to pink colored zones), character zones 2010). According to these criteria, the observed low grav- that represent mafic intrusive units and basaltic lavas that ity anomalies that dominate the northwestern side of the were intruded during Jurassic–Triassic. This intruded is cor- studied area (gfi ure 1B) couldbecorrelatedtothe cou- related with the separating of the Atlantic Sea. This positive pled Bouguer Appalachian Mountains belt, and the pos- amplitude magnetic is definite from the lower wavelength itive high gravity anomalies that dominate the rest of magnetic province of the Piedmont and Carolina terranes. the area could represent the full length of the south- The magnetic anomaly map highlights igneous intrusions ern Appalachian belt. Additionally, the strong broad high and structural features. gravity value belt (red color) shown at the central part Structurally, the RTP in gfi ure 1Dshows broadwave- (figure 1B) generally coincides with the slate belt. A circle length magnetic anomalies that could be correlated to ma- positive gravity anomaly at the southern part of the map is jor tectonic structures in the study area. In the central part of related to mafic intrusions. The local circular negative val- the map, the Carolina terrane was intruded by Carboniferous ues are noted over the gravity, which could be interpreted granites as the Carolina terrane docked with Laurentia. The as granite intrusive features, particularly over the Carolina RTP anomaly strongly reflects the northeast structure of the terrane. 148 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 7. (A) Filtered regional RTP anomaly map. (B) Filtered regional Bouguer anomaly map. Figure 8. (A) Filtered residual RTP anomaly map. (B) Filtered residual Bouguer anomaly map. peri-Gondwana terrane and the northwest-trending Jurassic visualization of other structural trends, especially under the mafic dikes. The map is dissected by a deep-seated igneous coastal plain sediments. The low pass gravity map (gfi ure 7B) mafic intrusive southeast of the fall line during the opening shows a signica fi nt change in the Bouguer gravity anomaly of the Atlantic in the Triassic–Jurassic. A southeast trending from the north to the south, which could reflect deep crustal feature reveals the BMA across Georgia and Alabama possi- variations in thickness and/or density. The central part of the bly results from the Mesozoic extension. area in the Bouguer gravity map is characterized by a high The directional filter map (gfi ure 2)was able to improve broad anomaly in the NE–SW direction over the slate belt. strike-filtering features that lay at an angle of 125 °.The di- Southeast of the fall line, a high magnitude (around 35 mGal) rectional filter improves the detection of high wavelength and large size of anomaly response along the coastal plain in- anomalies scattered throughout the study area. It is impor- dicate that the area is underlain by intrusive zones of high tant to note that the result provides a good indication of the density. The axes of anomalies in the NE–SW direction in- regional structure of the study area. A directional filter masks dicate possible structural or faults. The low pass RTP map the NW-SE anomaly signals over the study area and improves (figure 7A) reveals anomalies with deep basement variations. 149 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 9. Possible interpretation for RTP map with 125° directional filter. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. ST is the Suwannee terrane boundary. The map is characterized in the southeastward by large pos- fault zone underlain the coastal in the Carolina terrane. The itive magnetic with high amplitudes and is primarily associ- southwestern area of the map is expected to be isolated from ated with a deep and large basement source. Two prominent the Carolina terrane by the fourth contact (denoted ST in zones show low magnetism values that expand nearly over gfi ure 10) that possibly indicates the contact of the Suwan- the Carolina terrane and the southwest on the Suwannee ter- nee terrane that is hidden under the Paleozoic sedimentary rane (ST) line. The spatial relationship of low and high pass rocks. The obvious structural signature of the interpreted anomalies (figures 7 and 8)revealedthatmostofthe high contacts in the tilt derivative and the HG maps (gfi ures 11 pass is caused by deep-seated structures. Figure 9 shows a and 12) confirms the existence of these structural features possible interpretation for the direction filter. The interpreta- (contacts/faults). In addition, the three edge detection meth- tion map of the directional filter clearly delineates a NE trend- ods (AS, HG and TDR) successfully highlighted the edges ing anomaly that follows the regional trend of the study area. of the granitic plutons in the Carolina terrane as well as the The map result shows a similar structural feature to the AS, mafic intrusions in the southeast of the fall line beneath the TDR, and HG maps (figures 10–12). coastal sediments in Georgia and South Carolina. Figure 13 The interpretation of the edge detection (AS, TDR and shows the regional tectonic trends that were detected from HG) maps indicate clearly four different structural con- the potential field data after applying different technique tacts separating four structural zones that are characterized filters. by their different anomaly signatures (gfi ures 10–12). In the northern part (denoted as the Brevard fault (BF) in 5. Discussions gfi ure 10), the rst fi contact follows the edge of the Inner Piedmont terrane; possibly indicating the BF line. The sec- The potential field was used to investigate the structural ond contact (denoted CP in gfi ure 10) may indicate the cen- framework of Georgia and South Carolina, especially un- tral pediment zone that divides the high-grade metamorphic der the coastal sediments. The seismic reflection studies of of the Inner Piedmont from lower metamorphosed grade Nelson et al. (1985a), Maher et al. (1994)and Cook & rocks of the Carolina terrane. To the south of the CP contact, Vasudevan (2006) found that the structural sutures under the third contact (denoted CT in gfi ure 10) may relate to a the coastal sediments were dicult ffi to identify. In addition, 150 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 10. Possible interpretation for the analytical signal of the RTP map. ST is the Suwannee terrane boundary. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. terrane of the Carolina. The CT extends from the southeast- previous studies, e.g., Cook et al. (1979; 1981); Daniels ern of Inner Piedmont terrane to the southeast underneath (1974); Popenoe & Zietz (1977); Daniels et al. (1983); thecoastal sediments. TheCTisclearly seen beneaththe Higgins & Zietz (1983); Iverson & Smithson (1983); Cook coastal in the directional (gfi ure 9)and theedgedetection & Vasudevan (2006), failed to give a clear picture about these filters (gu fi res 10–12). These techniques are well imaged in fault zones. Therefore, the present data are used for delin- theterranecontactzones.Hatcher (1972) suggested that the eating the structure beneath the coastal plain sediments after Carolina rocks exist to the east of the Piedmont fault system. applying different filter techniques. The present results propose that the Carolina terrane was af- The interpretation of the present Bouguer and aeromag- fected by igneous intrusions mostly during the last tectonic netic maps successfully trace four fault zones that affected event. The igneous intrusions are indicated by the distinc- the study area: namely BF, central Piedmont shear/fault zone tive circular negative gravity anomalies that characterize the (CP), Carolina terrane contact/fault (CT) and ST. The BF gravity and RTP anomaly maps of the Carolina terrane. The and CP tfi well with the surface geology of the southeast- Bouguer gravity anomaly result (gfi ure 7)exhibitsabroad ernmargin(gfi ure 1). The ST shows a linear anomaly trend high gravity anomaly trending in the NE direction, indicat- along the approximately E–SE direction that truncates all the ing the Appalachian trend and the related metasedimentary magnetic and gravity anomaly features that correspond to and metavolcanic rocks in the Carolina slate belt. A positive the Appalachian structure. Interpreted as the Suwannee su- Bouguer gravity belt could be correlated to high densities of ture boundary, this zone was recorded in Florida by Pojeta metamorphic rocks and mafic rocks that dominate the area et al. (1976)andinAlabamabyNeathery&Thomas(1975); (Alarifi et al. 2021). Magnetically, this area is represented as Horton et al. (1984); Steltenpohl et al. (2013). A high mag- a regional low magnetic anomaly (gfi ure 7), which can be ex- netic anomaly zone that dominates the edge detection maps plained as the Carolina terrane undergone mild, deformation (denoted by CT; gfi ures 10–12) may represent the tectonic 151 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 11. Possible interpretation for (A) tilt derivative of RTP map and (B) tilt derivative of gravity map. ST is the Suwannee terrane boundary. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. Figure 12. Possible interpretation for (A) the horizontal gradient of RTP map and (B) horizontal gradient gravity map. ST is the Suwannee terrane boundary. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. and alteration that may lead to distribute the magnetic prop- lar features in the aeromagnetic and gravity maps of Alabama erty. The southern region of the study area is characterized by state suggested that mafic rocks are within the Suwannee. circular and elongate high amplitude anomalies in the gravity and magnetic maps. These circular and elongate anomalies 6. Conclusion are interpreted here as deep-seated igneous mafic intrusive and basaltic lavas emplacement within the Mesozoic rifting The geological features beneath the coastal plain can be margin along the ocean to continent transition. These results mappedbyusingtheregionalmagneticandgravitymaps.For are confirmed by Gohn et al. (1983) who recorded basaltic better interpretation analysis, filtering is carried out on the layers with intercalations of red beds in the drilled boreholes. Bouguer gravity map and reduced to a magnetic pole map. In addition, Steltenpohl et al. 2013 indicated that high circu- The potential field data compared to previously identified 152 JournalofGeophysicsand Engineering (2022) 19, 142–156 Alarifi Figure 13. Regional structural trends of the study area detected from the potential field data. ST is the Suwannee terrane boundary. BF is the Brevard fault. CP is the central Piedmont shear/fault zone. CT is the Carolina terrane boundary. dikes, geologic contacts and faults that are published on ge- the northern half and Gondwanan/peri-Gondwanan under- ologic maps. The generation of a lineament system map as a lying the southern half of the area. method of mapping fault and contact systems is a key goal of the potential field interpretation. Acknowledgements The fault system of Eastern Piedmont might be an exten- sive one in the eastern USA. The fault zones could be traced The paper was improved considerably by discussions with Elkhedr for tens of kilometers in length. The faults are detected and re- Ibrahim, James N. Kellogg and Patrick Duff. This research was flected clearly on the tilt derivative, HG, AS and directional supported by Researchers Supporting project number (grant no. filter maps. Using anomaly lineation, we were able to deter- RSP2022R496), King Saud University, Riyadh, Saudi Arabia. mine the location of the Carolina and Suwannee terranes Conflict of interest statement : None declared. thatarehiddenbeneaththecoastalplain.Theedgedetection and direction filter results indicated that the NE–SW linea- ment anomalies lie parallel to the regional geology and tec- References tonic structure. The area is affected by the tectonics related to the Mesozoic rift and Allegenian collision. 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Journal

Journal of Geophysics and Engineering – Oxford University Press

Published: Apr 1, 2022

Keywords: aeromagnetic; land gravity; Appalachian belt; filter techniques

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