ScienceAsia ScienceAsia ScienceAsia ScienceAsia ScienceAsia 31 (2005):31 (2005):31 (2005):31 (2005):31 (2005): 283-298

New Processing of Airborne Magnetic andElectromagnetic Data and Interpretation for Subsurface

Structures in the Loei Area, Northeastern Thailand

Kachentra Neawsuparpa, *, Punya Charusiria, ** and Jayson Meyersb

a Research Unit for Earthquake and Tectonic Geology (EATGRU), c/o Department of Geology, Faculty ofScience, Chulalongkorn University, Bangkok 10330, Thailand.

b Exploration Geophysics, Curtin University of Technology, PO Box1987, Perth, WA 6845, Australia.

* Present address: Geotechnics Division, Department of Mineral Resources, Bangkok 10400, Thailand, e-mail: kachen@dmr.go.th

** Corresponding author e-mail: cpunya@chula.ac.th

Received 19 Jan 2005Accepted 25 may2005

ABSTRACT: The northern part of the roughly north-south trending Loei suture zone has been investigatedextensively by several geoscientists for more than five decades not only for its complex geology but also formineral exploration. In this study, we have reprocessed existing detailed aeromagnetic data for interpretingthe continuity of geological units and structures where most of the pre-Cenozoic rock units are overlain byregolith cover comprised of recent soils, alluvial deposits, and colluvium surrounding hills and mountains.The aeromagnetic data were run through a series of filter routines to highlight deep and shallow magneticfeatures. These include analytic signal, reduction to the pole, first vertical derivative, directional cosinefiltering, and upward continuation. Interpretation of all the processed geophysical data has been carried outby integration of aeromagnetic data with electromagnetic data, radiometric data, enhanced Landsat imagesand GIS geological information. Three geological domains (eastern, central, and western) were interpretedfrom the geophysical data to correspond to assemblages of contrasting rock types, as well as differentregional structures identified in this study. These three domains are interpreted to be markedly separated bythrusts and sub-vertical shear zones. The magnetic data were also used to model the geometry of mafic unitsand granitic intrusions in 3 dimensions. Magnetic mafic bodies in the Loei suture zone were found to displaytheir dip direction to the east. Magnetic units running along the eastern side of the Loei suture zonecorrespond fairly well to folded and thrust faulted basalt flows of Devonian age. Moreover, some Permo-Triassic granitoid intrusions have a strong magnetic fabric, and a few have surrounding magnetic rings arelikely caused by magnetic minerals in hornfels. The others turn out to be a single smaller magnetic rings atdepths suggesting only one feeding magma chamber. A few Permo-Triassic felsic to intermediate lava flowsare identified by their hummocky magnetic texture. Northeast-southwest trending faults observed in themagnetic data cross-cut Triassic granite intrusions and pre-Jurassic stratigraphy, mostly producing morethan 1 km of sinistral offsets. Our new interpretation agrees with the existing geological bedrock mapping ina broad sense, but shows differences in the continuity of features and extent of granitoid intrusions, andcontains more large-scale structural detail. Our interpretation overlain onto the mineral occurrences mapwith GIS application will help to improve subsurface exploration and will help mineral explorers by highlightingnew mineral target areas under thin Quaternary regolith cover.

List of abbrList of abbrList of abbrList of abbrList of abbreviations:eviations:eviations:eviations:eviations: GIS, Geograpic Information System; DMR, Department of Mineral Resources; MRDP,Mineral Resources Devepoment Project; MORB, Mid-Oceanic Ridge Basalt; AEM, Airborne ElectromagneticSurvey; TM, landsat Thematic Mapper; RTP, Reduction To the Pole; Pb, lead; Cu, copper; Zn, zinc; Au, gold;Fe, iron.

KEYWORDS: aeromagnetic, tectonics, lineament, structural geology, Loei.

INTRODUCTION

Loei is a major province located in the northeasternregion of Thailand, and has been of interest due to its

significant mineral resources. The Loei area has beensurveyed and studied extensively by severalgeoscientists for many decades in the contexts ofgeological mapping, geochemistry, structural geology

doi: 10.2306/scienceasia1513-1874.2005.31.283

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and mineral deposits. Geology of the Loei area wasmapped in various scales, such as 1:250,000 byChareonpawat et al.1, 1:100,000 by Mineral ResourcesDevelopment Project2 and 1:50,000 by Chairangseeet al.3. In addition, Intasopa and Dunn4 and Charusiri5

dated some Triassic and Permo-Triassic igneous suitesin this area by using the stepheating 40Ar/38Ar technique.Panjasawatwong et al.6 studied geochemistry andtectonic setting of basaltic rocks of the Loei-Phetchabun-Ko Chang volcanic belt in Pak Chom area. Recently,Neawsuparp and Charusiri7 studied the lineamentsdetermined from the enhanced Landsat TM5 in theLoei and nearby areas. Seusutthiya and Maopeth8

studied petrography and whole-rock geochemistry ofsome ultramafic rocks in Ban Bun Tan, Suwan Khuhaarea of Nong Bua Lumphu Province, east of theLoei area.

The Loei area has been selected for mineral depositinvestigation by using airborne geophysical surveysperformed by the Department of Mineral Resources(DMR) since 19879. Helicopter airborne geophysicalsurveys flown at 60m terrain clearance and 400m linespacing measuring electromagnetic (AEM) andmagnetic fields were conducted by Kenting EarthScience International Limited (KESIL), Canada, in 1987-1988. Subsequently, these airborne geophysical datawere used in several aspects, such as mineral exploration9-10 and preliminary structural interpretation in theLoei area11.

Galong and Tulayatid9 used the processed airborneelectromagnetic data for assisting in geological mapping

in the eastern part of the Loei area. Rangubpit10 appliedimage processing and interpretation of aeromagneticfor geological mapping in Ban Yuak and Ban Sup, locatedbetween Loei and Udon Thani provinces. However,these two research works delineated simple geologicalboundaries, and suggested the occurrences of possiblemineral deposits.

Tectonically, the Loei area is situated in the “LoeiFold Belt” 12 or the easternmost part of the Nakhon Thaitectonic block13. Very recently, Charusiri et al.13

proposed that the eastern Loei suture, within the Loeiarea, forms the boundary between the Indochina andthe Nakhon Thai tectonic plates (Fig. 1A). However,the Loei suture is poorly studied and difficult to delineatedue to the fact that some parts of the suture (or “tectoniclines”) are underneath thick regolith cover or Jurassicto recent sedimentary covers.

Though the Loei area has been surveyed and studiedby several geoscientists, particularly for the surfacegeological mapping. The subsurface geologic mappinghas never been performed by integrating geologicaland geophysical results, and no attempt has been madeso far to understand the detailed relationship betweenstructural features observed on the ground and thoseextending into the subsurface.

This paper is aimed at re-processing aeromagneticdata to study major surface and subsurface structures,and their relationship with surface structural featuresin the Loei study area. A new interpretation map hasbeen generated showing magnetic units and structures,and the model of subsurface structures are presented

Fig 1.(A) Map of Thailand and adjoining countries showing tectonic blocks and sutures (modified after Charusiri et al.13). Notethat the inset is the study area.

(B) Geologic map of the Loei study area, northeast Thailand (modified after MRDP2 and Chairangsee et al.3).

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Fig 2. Aeromagnetic data processing maps of the Loei study area based on (A) total magnetic intensity data, (B) residualmagnetic intensity data, (C) reduction to the pole (RTP) data, and (D) analytic signal data.

and discussed.The area under investigation, covering about 5,280

sq km, is located at latitudes 17o 15’ to 18o 15’ N andlongitudes 101o 15’ to 102o 15’ E. The study areaencompasses the northern and eastern parts of Loeiand the western part of Nong Bua Lumphu provincesin northeastern Thailand, close to the Thailand and LoaPDR border (Fig. 1A).

GEOLOGIC SETTING

The regional geologic setting of the Loei area shownin Fig. 1B is summarized from the reports of Bunopas14

at 1:100,000 scale and of Chairangsee et al.3 at 1:50,000scale.

Rock sequences commence with Middle Palaeozoicmetamorphic rocks containing very rare and poorlydefined fossils, which mainly include quartzite andphyllite in the eastern part of the Loei area. Thesemetamorphic rocks are unconformably overlain byalternated strata of shale and siltstone with tuff andintercalated limestone. Intensely folded chert bedswith interbeds of thin volcanic clastic rocks of UpperPaleozoic age are restricted to the central part. Thechert unit is situated adjacent to spilitic basaltic rockswith the age ranging from Devonian to Carboniferous4.

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Fig 3. Enhanced RTP maps of the Loei study area, with upward continuation (A) at 50 m, (B) at 500 m, (C) at 1000 m,and (D) at 3000 m.

Both chert and basalt units form a long, narrow north-south trending belt in the central part of the study area.This basaltic unit extruded onto the sea floor forminghuge masses of volcanic tuff and pillow lava associatedwith sporadic manganese deposits. In the Late middleDevonian to Carboniferous, thick sequences of greywackeintercalated with shale and reef limestone were observedin several places. Late Palaeozoic reef limestones lieunconformably over the older rocks. Permo-Triassicfelsic tuff, such as rhyolitic tuff, covers a large arealocated mostly in the eastern part of the study area, whereasandesitic rocks with Cu-Pb-Zn-Au mineralization coverin the western part of the investigated area. Graniteand granodiorite of Triassic age, with associated Cu-Fe-Au skarn/porphyry-type mineralization5, 13, arelocated in the western part of the area.

In the Loei study area, the major structural featuresare folds and faults. The main faults and fold axes are

oriented in a north-south direction. Additionally,Bunopas14 and Chairangsee et al.3 reported uncon-formities in the mapped area between strata of differentages, for example, between Permian and Late Triassicand between Lower Carboniferous and Devonian.However, they are poorly stratigraphically andstructurally defined due to discontinuous exposuresbetween rice fields and alluvial cover, and thick naturalvegetation with soil cover.

We regard the Loei study area as part of therecognized “ Loei Fold Belt” 12, which corresponds tothe Loei-Phetchabun–Ko Chang Volcanic Belt15andthe Eastern Granite Belt of Thailand16. Large anticlinesand synclines with axes mainly trending in a north-south direction occur in Silurian and Permian rocksequences. Many folds are dislocated by several setsof strike-slip faults oriented in northeast-southwestand northwest-southeast directions. These faults seem

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dominant, and dislocate major folds and pre-existingthrust faults. Sinistral movement, which is morecommon than dextral movement, is present especiallyin the western part of Loei. This is probably due to thecontinuous clockwise rotation of continental SoutheastAsia12,13. The large, north-south trending overthrustin the eastern part of Loei area separates Silurian-Devonian metamorphic rocks from Carboniferoussedimentary rocks9.

MATERIALS AND METHODS

Data SetsData SetsData SetsData SetsData SetsThe main data set used in this study is an airborne

geophysical survey data acquired during 1987-1988by KESIL with the contract from DMR through MRDP.This survey measured high-resolution AEM andmagnetic responses over the northern part of Loei andnearby Nongbua Lamphu provinces.

In this study, we used the aeromagnetic andelectromagnetic data for a total of 13,256 line-

kilometers. The survey was traversed by helicopterflown in an east-west direction, with a line spacing of400 m and at an altitude of 60 m above the ground levelto cover an area of 5,280 square-kilometers.

Beside the aeromagnetic and AEM data, we usedradiometric, digital elevation, Landsat TM5 andavailable GIS data, both published and unpublished,for integration and interpretation.

A revised geological map compiled from thegeological maps on scales of 1:100,000 by MRDP2 and1:50,000 by Chairangsee et al.3 was used to comparewith the interpreted geophysical map. In addition, thedetailed studies of igneous rocks in the area4,5,6,8 werealso evaluated for visualizing and classifying magneticdomains.

Lineament studied in the Loei area from remote-sensing interpretation7 was used to discuss with thegeophysical lineaments generated from this study.Moreover, the interpreted AEM, magnetic andradiometric data in some parts of the Loei area9,10 werealso applied to add subsurface information in this study.

Fig 4. Enhanced aeromagnetic and electromagnetic maps of the Loei study area using (A) first vertical derivative technique,(B) directional cosine filtering in north-south direction, (C) AEM data of frequency 912 Hz, based on in-phase component,and (D) AEM data of frequency 912 Hz, based on quadrature component.

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Fig 5. Geophysical interpretation map of the Loei study area showing eastern, central, and western magnetic domains and theirassociated subsurface structures.

Fig 6. Geophysical lineament map with significant mineral occurrences in the Loei study area.

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Several works on tectonic evolution, such asBunopas12 and Charusiri et al.3, were used to redefinethe tectonic elements from the geophysical information.

Aeromagnetic Processing MethodsAeromagnetic Processing MethodsAeromagnetic Processing MethodsAeromagnetic Processing MethodsAeromagnetic Processing MethodsAeromagnetic data provides the most significant

information for subsurface interpretation. Therefore,the methodology is explained in detail. Totalaeromagnetic data recorded in analogue form weredigitized as profile data. Magnetic intensity generatedfrom the profile to grid data was then displayed as animage in natural color pallets (magenta high intensity,blue low intensity, see Fig. 2A), using histogramequalization to maximize the color ranges. To emphasizethe expression of anomalies near surfaces, the color-shade grid with illumination inclination and declinationat 45o is displayed.

In this study, it is essential to define the magneticanomalies at the place where they sit over the source,because there is a considerable difference in magneticintensities from the inclination and declination at lowlatitudes. By removing the International GeomagneticReference Field (IGRF), residual magnetic data (Fig.2B) can be either positive (higher than the IGRF) ornegative (lower than the IGRF) depending on theorientation of each magnetic body, but distortions inthe normal response still remain.

Mathematical transformation or filtering techniques,such as reduction to the pole (or RTP, Fig. 2C), analyticsignal (Fig. 2D), upward continuity (Fig. 3), verticalderivative (Fig. 4A) and directional cosine filtering(Fig. 4B), were applied to the magnetic data toenhance features for interpretation. The individualtransformation techniques are explained in more detailin Neawsuparp17. In this current study, we adopted andslightly modified the methodology described by Milliganand Gunn18.

Many of the linear anomalies for the Loei area werenot completely revealed by shaded-relief total magneticimagery. Interpretations using vertical derivative plusautomatic gain control methods remove the influenceof the large amplitude, long wavelength anomalies. Thedirectional cosine filter method, in particular, workedwell to reveal a comprehensive pattern of faulting withinthe basin fill19. Upward continuation filtering helpeddetermine the depth range of deeper sources. All themethods together provided a view of the pattern andgeneral depth ranges of intra-basin faults within thearea that aid research on the geology, intrusions, andfaults with respect to the mineral resources.

Electromagnetic Processing MethodsElectromagnetic Processing MethodsElectromagnetic Processing MethodsElectromagnetic Processing MethodsElectromagnetic Processing MethodsElectromagnetic data recorded in the frequency

domain are divided into 4 channels. including x and yin-phase and quadrature for the frequencies: 736, 912

and 4200 Hz. In-phase and quadrature data wereconverted to apparent resistivity for 4200 Hz. The dataprocessing of AEM data were displayed in grid imagesthat can be overlaid with the other data by using GIS(Arcview version 3.2 program).

All data were moved to the zero level. This techniquewas applied to decrease the leveling problems. In orderto create grid, we used the spline grid method becausethis grid method has been useful for the wide linespacing data and for reducing the trend enforcementgridding problem.

Apparent resistivity grids were imaged asconductivity grids. Additionally, since anomalies inAEM showed negative amplitudes, the in-phase andquadrature data were multiplied by -1 to convert thedata to high amplitude anomalies. The electromagneticdata of 912 Hz were displayed as in-phase andquadrature components (Figs. 4 C and D, respectively).

Interpretation MethodInterpretation MethodInterpretation MethodInterpretation MethodInterpretation MethodVisual interpretation was made on hard copies for

all geophysical data processed at 1:100,000 scale, todiminish parallax problems caused by computerdisplaying. The boundaries were then mapped moreaccurately using the source edge maps by placingmargins at the position of the source edges coincidingwith major amplitude change. The interpretedaeromagnetic and AEM data were integrated with othersGIS data for mapping geological structures andcontinuity of lineaments, which represented faults andfolds in the study area. Domain boundaries andstructural features, such as folds, faults and thrusts,and their relations to intrusions, were also digitized(Fig. 5).

From the interpretation map, the subsurfacemodeling of the area was created by using theModelVison software in both forward and inversionmodeling methods. Magnetic response profiles of thereduction to the pole were selected for modeling. Tosimplify models, line profile and cross sections wereconstructed based on latitudes of the area and thelocations of latitudes were used for the name of lineprofile. We applied three profile lines for magneticmodeling including L1920000N, L1950000N andL1970000N in the east-west direction (Fig. 7).

Magnetic modeling in this study was used for bothof linear inversion and non-linear inversion methods.The linear inversion technique20 involves subdividingthe space below an observed magnetic field into seriesof geometric bodies and then finding values ofmagnetization for the shapes in such a way that thesummed magnetic effect of all the bodies matched theobserved magnetic field. The non-linear inversiontechnique was applied in an attempt to obtain a matchbetween observed and calculated magnetic fields by

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Fig 7. Aeromagnetic modeling of the Loei study area showing (A) selected aeromagnetic line at different latitudes displayingon the RTP map, (B) cross-section along the line L 1,920,000N, (C) cross-section along the line L 1,950,000N, (D)cross-section along the line L 1,970,000N, and (E) 3D perspective view in azimuth 335 °and inclination 10° showinginterpreted subsurface magnetic structures.

Fig 8. Lineament maps of the Loei study area showing structural patterns in comparison with (A) major faults from thegeological map (Fig. 2), and (B) patterns from the enhanced Landsat map7, with new structure and domains from thegeophysical interpretation.

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iteratively varying the unknown parameters. Usinga trial and error method, variations that improved thefit between the observed field and calculated resultsbased on the model were stored and used as a basis fornew parameter estimating19. In this study, the forwardmodel is initially obtained to best fit between theobserved and calculated magnetic field. Then theinversion to produce the final perfect fit by interactivemodeling was applied.

RESULT AND INTERPRETATION

ResultsResultsResultsResultsResultsThe total magnetic field (Fig. 2A) shows regional

high magnetic intensities in the northern part and lowintensities in the southern part of the Loei study area.After we removed the IGRF from the total magneticfield, the result is a residual magnetic map (Fig. 2B)showing the difference in locations of high and lowmagnetic intensities. The relative residual magneticmap (IGRF-corrected) shows a rather complex crustalmagnetization pattern. Magnetic intensity level rangesfrom -820 to 980 nT, with a base level of -4.25 nT. Localvariations of field intensity always exceed 500 nT in theeastern part and diminish rather abruptly to less than100 nT in the central and northwestern parts.

The residual magnetic intensity map (Fig. 2B)exhibits three regional magnetic zones that roughlytrend in a north-south direction. From the east, thereexists a high magnetic intensity zone clearly visible onthe enhanced map. The most prominent low magneticintensity zone is in the central part, between Na Dungand Pak Chom districts, and continues northward tothe Pak Chom district and Lao PDR. Within this zone,there are series of elongated and relatively highermagnetic intensities trending in a northwest-southeastdirection. Additionally, the boundary between thecentral zone and the eastern zone is displayed by afairly sharp magnetic contrast. To the west, the zone ischaracterized by groups of strong, positive, and roughlycircular anomalies (approximately 2 km in diameter).These are located in the southern part, whereas in thenorth, the large circular features have an averagediameter of about 10 km.

Both RTP (Fig. 2C) and analytic signal (Fig. 2D) mapsdisplay similar magnetic features and suchenhancements lead to more outstanding magneticboundaries. The RTP map shows the average peak topeak at about 700 nT. These high anomalies are orientedin a north-south trend, with a total length of about 40km. The western edge of this anomaly is marked by asharp magnetic gradient in a northwest-southeasttrend, extending from Suwan Khuha district to NamSom district (in Nong Bua Lumphu province). Theanalytical signal image shows the marked narrow zone

between the eastern and central parts better than thoseof the previously described images. It should beemphasized that the boundary contrast withinindividual zones is quite clear.

The filtered images following RTP and upwardcontinuation (Figs. 3) show the high magnetic anomaliesvarying in geometry and locations at different depth.For example, groups of circular features in the centralpart (A in Fig.3) are changed to the small circularmagnetic body in the deeper part whereas the largecircular magnetic body (B in Fig. 3) in the western partremains similar in size and geometry.

The first vertical derivative (Fig. 4A) and thedirectional cosine filter (Fig. 4B) maps reveal asignificant magnetic pattern, such as linear structures,better than those of the other maps. As shown in Fig.4A, linear patterns are more prominent in the east thanin the central and western parts. Most of these aresteeply dipping vergence volcanic horizons. However,the linear patterns in the west can be better definedusing the directional cosine filter (see Fig. 4B).Application of the directional cosine filters of totalmagnetic maps in the north-south direction showswell-defined lineaments, particularly those trending inthe northeast-southwest direction. The long andcontinuous lineaments of this direction seem to cross-cut three regional magnetic zones earlier mentioned.Additionally, some lineaments, such as those in thesouth of Pak Chom district, show minor displacements(less than 0.5 km). The results shown in these mapsclearly indicate that most of the northwest lineamentsare cross-cut by the northeast-southwest trendinglineaments.

InterpretationInterpretationInterpretationInterpretationInterpretationFrom the results of enhanced aeromagnetic maps

we were able to distinguish the magnetic responses inthe bedrock geology due to the difference in magneticsusceptibilities, structures and deformation styles ofthe magnetic units in the area. Variable source depthswithin a domain may also contribute significantly tochanges in anomaly sizes and shapes sizes.

Magnetic DomainsMagnetic DomainsMagnetic DomainsMagnetic DomainsMagnetic DomainsFrom the enhanced image maps, we created a new

interpretation map shown in Fig. 5. With dataintegration and interpretation, we divided the Loeistudy area into three magnetic domains based uponthe magnetic intensities, structural styles, and geologicalfeatures, namely Eastern, Central, and Westerndomains. Boundaries of the individual domains clearlycoincide with the abrupt changes in average magneticintensities, anomaly variabilities and orientations. Eachdomain was further divided into sub-domains on thebasis of local detailed magnetization, interpreted

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subsurface geology and structure, lineament patterns,and circular features described in the previous section.The characteristics of individual sub-domains aredescribed by comparing the interpretation withproposed geological features (see Fig. 1B).

Eastern DomainThe eastern domain is the geophysical domain

clearly trending in an almost north-south direction.The sharp boundary between the eastern and centraldomains is regarded as a shear zone. It is chieflycomposed of moderate to high magnetic intensities ofvolcanic rocks. Our field investigation revealed thevery high magnetic intensity units in the southwesternpart corresponding to the newly exposed mafic andultramafic rocks. These rock units are not shown inany previous geological maps (Fig. 1B). We furtherdivided the eastern domain into four main sub-domainsnamely E1, E2, E3 and E4 sub-domains, based on theaverage magnetization and structural styles.

The E1 sub-domain shows the highest magneticintensities and is located in the southwestern part ofthe domain. Our field investigation and recentpetrographical studies8 indicate the occurrences ofserpentinized ultramafic rocks. These serpentinizedrocks account for nearly 70 percent of the E1sub-domain. Shear zones are the main deformationstructures, commonly exceeding 40 km in length andmostly trending in a north-south direction. The shearzones are characterized by overlapping lineaments,and form the ‘S’ shaped curvi-lineaments from foldingand thrust. Structural geophysical trends within theseultramafic rocks are largely aligned to the north andnorthwest trends. In addition, intense folds of variablymagnetized strata, shown as curved and bandedmagnetic patterns, are observed in the southern part.Such banded patterns are quite similar to thoseinterpreted to represent folded structures ofsedimentary rock strata in Finland21. A series ofinterpreted synclines and anticlines show the averagenorth-south trending axial plane. Two strike-slip faults,of similar trend, are also recognized in the northernpart of the E1 sub-domain, and they show minor leftlateral movement of about 0.5 km.

The E2 sub-domain is located in the northernhalf of the domain. This sub-domain is characterizedby low to moderate magnetization corresponding tothe felsic volcanic rocks (rhyolite and rhyolitic tuffs),as well as granite plutons mapped by Chairangsee et al. 3.This sub-domain may be related to the E1 sub-domainwith a shear contact at depth and ultramafic rocks areapparent contact of the boundaries between E1 and E2sub-domain.

The E3 sub-domain is located in the northeasternpart of the eastern domain. This sub-domain is

dominated by a large circular high magnetic unit witha north-south structural trend. In the geological map,this sub-domain is mapped as Mesozoic continentalsedimentary rocks of the Khorat Group, which has lowmagnetization. The contrast in the magnetic responsesleads us to believe that there are some felsic intrusionsunderneath these non-marine strata.

The E4 sub-domain is located in the eastern part ofthe domain, and displays moderate magnetic intensities.They are interpreted to represent sedimentary rocksunderlying the E2 felsic volcanic rocks. A syncline, asindicated by a high magnetic unit, has a length of about10 km, and its axial trace trends in a north-southdirection. The E4 sub-domain is separated from the E2sub-domain by a strike-slip fault.

Central DomainThe central domain is largely composed of low

magnetic intensity features. It can be divided into foursub-domains namely C1, C2, C3 and C4 sub-domainsusing magnetic shape, pattern, and intensities, as wellas distribution of small magnetic intensive stocks.

The C1 sub-domain forms a north to northwest-southeast trending low magnetic zone, and correspondsto thick meta-sedimentary rock sequences with mostlythe northwest strike2, 3. These metasediments wereobviously affected by tectonic deformation andmetamorphism much more intensely than any otherrocks in the Loei area. The second oldest sedimentaryrocks of Ban Nong Formation have a definite age ofSilurian-Devonian by using the evidence of several coralssuch as Heliolites sp, H. barrandei and Favosite sp22.Thus, the metamorphic rocks were at least of middleSilurian in age3. This sub-domain shows several long,north to northwest-southeast trending magnetic linearpatterns interpreted to be associated with major faultsrunning parallel to a regional structure. These faultsare up to 30 km long and are identified clearly by theAEM image data17, because the extension of poorlymagnetized faults with similarly magnetizedsedimentary rocks is difficult to determine from theprocessed aeromagnetic data. To the south of the sub-domain, a series of AEM conductive zone occursfollowing the major fault trend.

The C2 sub-domain is dominated by a low magneticpattern with lineaments similar in style to those of theC1 sub-domain. In the western part of this sub-domain,to the north of Na Duang, circular magnetic highsindicate intrusive stocks. This sub-domain was mappedas Carboniferous clastic sequences of the WangSaphung Formation3. The circular magnetic anomaliesobserved in the central part were mapped as the felsicintrusive bodies3. As shown in Fig. 5, the boundarybetween C1 and C2 sub-domains is marked by a majornorth-south trending thrust fault. To the west, the

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north-south trending anticline is dominant.Embedded into the C2 sub-domain, in a roughly

north-south direction, is the C3 sub-domain, whichconsists of an elongate high magnetic intensity trend.This sub-domain is equivalent to the mafic volcanic beltof basalt to basaltic andesite in composition3. This highmagnetic unit shows many short northeast- southwesttrending lineaments with a series of intricate folds inthe same trend, and two small thrust faults in the north-south direction.

The C4 sub-domain is mainly composed of lowmagnetic intensities. This sub-domain corresponds tothick Permian carbonates and clastic sequences of theLoei Group. The-long wavelength regional foldingprobably resulted in a symmetrical variation of magneticfield intensity23. The clearly defined lineament trend isillustrated by the curved lineament patterns (see Fig. 6)interpreted to represent a large folded structure withthe regional axial plane in the northwest trend.The boundary between the C2 and C4 sub-domains isinterpreted to follow thrust faults trending in anorthwest direction.

Western DomainThe western domain is essentially composed of

moderate magnetization. This domain is divided intothree sub-domains namely W1, W2 and W3 sub-domains) based on high magnetic patterns andintensities. The boundary between the central andwestern domains is characterized by a north-southtrending thrust fault.

The W1 sub-domain is indicated by moderatemagnetization corresponding to the Permiansandstones with interbedded limestone3. Geologically,this sub-domain shows a structural trend in a north-south direction similar to that of the regional mappedrock units.

Enclosed by the W1 sub-domain in the northernpart, the W2 sub-domain shows a large circular highmagnetic pattern (10 x 15 km in size). This sub-domainis cross-cut by northeast-southwest trending majorfaults. In the field, this sub-domain is comprised offelsic plutonic rocks.

The W3 sub-domain consists of moderate to highmagnetization with a straight linear pattern in a north-south direction. This sub-domain is interpreted torepresent intermediate to mafic volcanic rocks, mainlyandesite2. There are a few small circular features,possibly suggesting volcanic vents occurring in theeastern and western parts.

Magnetic StructuresMagnetic StructuresMagnetic StructuresMagnetic StructuresMagnetic StructuresFig. 6 displays major lineaments of various patterns

and styles drawn using interpretation following theimage manipulation. The lineaments were interpreted

on the enhanced airborne geophysical data from visualhard copy images on a 1:100,000 scale. Geologiclineaments (fractures and faults) were superimposedafterward as a separate layer. Comparison with thegeologic lineaments shows that many of the magneticanomaly trends coincide with geological contacts orfaults.

The eastern domain is characterized by a ratherirregular pattern of magnetic anomalies containing longsegments having predominant north-south lineartrends. This belt was separated by the sharp, long,northwest-southeast lineaments conformable withregional structures. These lineaments show the strike-slip fault motion indicated by the displaced lineamentsand rock units, and some lineaments were interpretedas shear zone by high magnetic units with the ‘S’ shapedfeatures. The magnetic data also show small foldsconsisting of several discontinuous, dextrally side-stepping lineaments with displacement of about 1 km.The structural pattern of this belt shows that the majorstructures were formed in response to compressionaltectonic activity in the region. Moreover, the magneticlineaments can be inferred as axial planes of some ofthe folds. Good examples are those in the E4sub-domain, with north-south trending and northeast-southwest trending.

The central domain comprises sets of northeast-southwest trending structural patterns. The prominentfeatures are characterized by the large-scale, open andupright folds in the northern and southern parts, whichare obvious in both magnetic and geologic maps. In thenorthern part, steeply-dipping and tightly-foldedsedimentary strata of Devonian age are mapped withinthe area with low magnetic intensities. This zone formsa rather strong lineament pattern along most of itslength and cross-cuts the set of folds. In the southernpart, dominant northeast-southwest trendinglineaments are clearly visible and form large foldstructures.

The boundary between the central and easterndomains is mapped as the thrust fault by MRDP2. In thegeological map, the thrust marks the boundary betweenthe metamorphic rocks and sedimentary strata ofdifferent ages. But our aeromagnetic result shows nocontrasting signature at the western and eastern sidesof the fault boundary. However, from the processedaeromagnetic interpretation, a consistent sharp andstrong magnetic gradient was observed at the boundarynear the mapped thrust fault, which is about 5 km tothe east. However, some faults in the Loei basin areahave either no magnetic expression or such a small onethat it was not detected by aeromagnetic data. Inaddition, the extent of poorly magnetized shear zonesand faults yield similar values of magnetization tosedimentary rocks, so they are difficult to be determined

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from the aeromagnetic data. In these cases the AEMmaps are used to delineate the shear zones and faultsin the study area. Our AEM interpretative result (seeFigs. 4C and D) shows the long lineament pattern ofhigh conductors at the same location of the mappedthrust fault as shown in the Fig 5 and 6.

To identify a thrust fault by using geophysical data,seismic and gravity data are commonly used forobserving faults and their dips.

Pluijm and Marshake23 reported that fault systemswere common along the tectonic margins of convergentplates, and faults tend to be composed of relays orparallel arrays. In addition, Airo and Ruotoistenmark21

discussed that thrust faults were typical geologicalboundaries that separated regions or geological blocksshowing different deformation styles and structures.Thrusting, which follows the main deformation stage,was recognized as abrupt termination of onedeformation style with regard to the adjacent one.Moreover, regions of folding were often separated bylarge-scale fault zones or thrust faults running parallelto the axial-plane strike or groups of smaller faults withthe same trend. Therefore, in this study, five thrustfaults with a length of 20 to 60 km were interpretedbased on the magnetic patterns and geologicalcharacteristics.

Fig. 5 and 6 show the interpreted thrust faults (TF)from the interpretation of aeromagnetic andelectromagnetic maps. The TF1 and TF2 represent theboundaries between eastern and central domains, andbetween central and western domains, respectively.The TF2 is interpreted as the thrust fault conformingto the geological map2, 3. The TF3 separates a series ofsmall tight anticline folds in the eastern side (C2 and C3sub-domains) and a large open syncline fold in thewest (C4 sub-domain). The TF4 represents a series ofthrust faults separated the large open fold in the C4sub-domain, which parallels the axial-plane strike, anddeveloped during folding.

Small northeast-southwest trending lineaments aredominant in the western domain, with geophysicaltrends in a north-south direction. In Fig. 5 the boundarybetween central and western domains are interpretedfrom contrasting magnetic patterns. The prominentstructure of this domain is a large circular feature,similar to the mapped Triassic granite intrusion.Additionally, this circular structure was cross-cut bythe northeast-southwest trending lineaments,suggesting that the northeast lineaments are of youngerage than the granite intrusion.

Magnetic ModelingMagnetic ModelingMagnetic ModelingMagnetic ModelingMagnetic ModelingFrom the results of magnetic data interpretation,

we were able to construct a geophysical model for bothforward and inversion methods. In this study, magnetic

response profiles from the RTP data were selected formodeling. To simplify the line profiles, cross sectionswere draw based on latitudes of study area (Fig. 7A).Three profile lines in the east-west direction wereselected for modeling, including line numbersL1920000N, L1950000N and 1970000N.

Fig. 7A shows the selected RTP aeromagnetic dataalong the east-west surveyed line with datainterpretation. Three cross-sections along the linesL1970000N, L1950000N and 1920000N areillustrated in Figs. 7B, C and D, respectively. In thewestern side of the study area (at grid coordinate790000E-800000E), interpreted bodies have magneticintensity higher than those of the surrounding areas,suggesting magnetic intrusions/ or dikes of possiblygranitic composition. The circular feature (W2) in thewestern domain is the largest body, which is east-dippingat depth. On the surveyed line L1970000N (Fig. 7B)at coordinate 815000E, small dike-like bodies areindicated by high magnetic intensities, correspondingto the volcanic (basalt) unit (C3) of the central domain.At grid coordinate 830000E-835000E along the lines1950000N and 1970000N (Figs. 7B and C), a series ofnarrow magnetic dikes are encountered, and they areinterpreted to represent the east-dipping faults whereasthe west-dipping dikes correspond to the mappedultramafic rocks. These anomalies are interpretedto be related to the fault zone with northwest-southeasttrend, and contain mainly ultramafic and meta-sedimentary rocks in the eastern domain (E1).This assemblage probably represents a melange ofophiolite crust.

A perspective view in 3 D is displayed in Fig. 7Ein the azimuth and inclination at 335o and 10o,respectively. The model is a result of the overlayingmaps of the RTP grid at the top surface, and upward1,000 meters at the bottom. The model shows thatmagnetic bodies continue to the deeper zone.

DISCUSSION

Magnetic Responses Related to Rock UnitsMagnetic Responses Related to Rock UnitsMagnetic Responses Related to Rock UnitsMagnetic Responses Related to Rock UnitsMagnetic Responses Related to Rock UnitsTo correlate magnetic anomalies with rock units, it

is noteworthy that sedimentary rocks are generally notmagnetic, whereas igneous rocks rich in iron andmagnesium (mafic to ultramafic) tend to be verymagnetic. Granite intrusions and hornfels contactaureoles can also be magnetic.

Magnetic quiet areas are widely distributed in thecentral and northwestern parts of the study area andexhibit magnetic relief of 50 nT or less. These areascorrespond to a basin filled with Carboniferous clasticand Devonian chert sediments3. These areas are faultedand folded as recognized both in the field and in thegeophysical and Landsat imagery interpretation map.

′

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Meta-clastic rocks in the westernmost part (the C1sub-domain) also have low magnetic intensities similarto those of the Carboniferous sediments. Moreover,results on magnetic susceptibility indicate that severalrock samples yield rather low values17. However, it isgenerally accepted that metamorphic rocks have moremagnetic susceptibility than sedimentary rocks(average sedimentary and metamorphic rocks about75 emu and 350 emu, respectively, Telford et al.24). We,therefore, consider that the meta-clastic unit (in the C1sub-domain) is likely to be the same geologic unit as theCarboniferous clastic unit. Otherwise, magneticmineral contents decrease accompanying collisionalprocess25. Either burial pressure-dominated or weakdynamo-thermal metamorphism of Late Paleozoicclastic rocks associated with folding and thrusting mayhave formed in response to compressional tectonics,similar to that occurring along the Nan Suture.

The magnetically moderate areas have accentuatedmagnetic relief with lineaments and anomalies havingamplitudes of 100 to 400 nT. Most of the magneticanomalies in these areas are observed over plutonicrock exposures. Magnetic field intensity increases bymore than 200 nT over a poorly exposed granodioritestock. The small circular magnetic bodies in the southernpart of the study area suggest that these features arecaused by intrusive rocks, corresponding well withseveral of the known granodiorite stocks. Moreover,the results of this study show more extent and newgranitoid intrusions than the previous geologicalmapping (see Figs. 1B and 5).

Narrow and higher amplitude anomalies in the northof the central part (the C3 sub-domain) indicate theexistence of mafic volcanic rocks corresponding toCarboniferous basalt and basaltic andesite2, 3. Thevolcanic rocks (the C3-sub-domain) are made up mainlyof pillow lava, hyaloclastites and pillow breccias. Thesevolcanic rocks have been assigned to those eruptedin a mid-ocean ridge to back arc basin environment,and have a wholerock Rb-Sr Isochron age of 341 ±11Ma4. Geochemical results from the volcanic rocks inthe Loei area by Panjasawatworng et al.6, suggest thatthe Loei volcanic rocks are comprised of MORB andoceanic island-arc lava.

Additionally, volcanic rocks in the north of the E2sub-domain show lower magnetic intensities thanthe volcanic rocks in the W3 sub-domain. Based onour field visits and geological maps by MRDP2 andChairangsee et al.3, the volcanic rocks of the easternpart are dominated by more felsic variations and thoseof the western part are characterized by more maficvariations.

The magnetically high areas of the E1 sub-domainshow magnetic anomalies with amplitudes of morethan 500 nT, and are characterized by high wavelength

anomalies. Large anomalies are situated close to theborder of metamorphic and volcanic rocks in thesoutheastern part of the studied area. The magneticintensities in this zone are higher than those of themetamorphic and volcanic rocks. It is very interestingthat this zone is not shown in the geological map. Thesestrong magnetic anomalies are oriented in a northwest-southeast direction. The surface geology mapped byMRDP2 and Chairangsee et al.3 does not show an obviouscause of the high positive magnetic anomalies. Theresults of our and previous field data indicate thatthese intense anomalies correspond to the mappedmafic and ultramafic intrusions comprised ofserpentinite, peridotite and gabbro8. The studies ofpetrological and geological characteristics ofserpentinized rocks8 suggest that these rocks wereretrograded or hydrothermally metamorphosed fromdunite, pyroxinite and peridotite.

There are some correspondences between the high,elongated magnetic zone and outcropping serpentinitesalong a fault in the vicinity of Ban Bun Tan, SuwanKhuha District, Nong Bua Lumphu Province. However,the serpentinite is only exposed near the center of ananomaly, so the large magnetic anomaly suggests thatserpentinite is present below the meta-sedimentaryrock types that are generally weakly magnetic.

Magnetic Interpretation Related to StructuralMagnetic Interpretation Related to StructuralMagnetic Interpretation Related to StructuralMagnetic Interpretation Related to StructuralMagnetic Interpretation Related to StructuralFeaturesFeaturesFeaturesFeaturesFeatures

The anomalies display several trends defined byalignment of gradients and shapes of anomalies, andare best illustrated in Figs. 5 and 6. The most dominanttrend or lineament is in the northeast-southwestdirection, followed by the northwest-southeastlineament, and the least dominant one isin a north-south direction.

The northwest-southeast trending lineaments arecross-cut by the northeast-southeast trending faultswith sinistral movement and a horizontal slip of about500 m17. As seen in the central domain, the northeast-southwest trending lineaments are younger than thoseof the northwest trends. The north-south trendinglineaments mostly indicate major strike-slip faults inthe eastern domain and the east-dipping thrust faultsin the central domain (as the C1 sub-domain). Thewestern domain is mainly represented by minor north-south trending lineaments and geophysical trends.

The newly processed of AEM data (see Figs. 4C andD) show the long continuity of lineaments of themoderate to high amplitude anomalies with the roughlynorth-south direction in the C1 sub-domain,corresponding to the thrust fault in the geologic maps2,

3. This result is in good agreement with the structurestudied by Galong and Tulayatid9, which showed thecorrelation of a conductor with the thrust fault from

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the geological map. Additionally, the aeromagneticstudy by Rangubpit10 in Ban Yuak and Ban Sup, locatedin the southern part of the central domain, show themultiple thrust faults in this location. In contrast withthis study, there is only one major thrust fault (TF2) thatcan be observed at this location. This is due to thesmaller scale applied in this study. However, our studiesshow that the other thrust faults (TF1, 3, 4, and 5) inFigs. 5 and 6 are parallel to this thrust fault (TF2).Interestingly, the region faults that have been mappedusing AEM data particularly nos. TF2, 3, 4 and 5,represented a very shallow source down-faultedbeneath the only sedimentary rocks. Additionally,because of a parallel magnetic contour pattern andmagnetic modeling (Fig. 7), the thrust faults in this areaare indicated to have east dipping geometries.

In the northern parts of the central domain (the C3-sub-domain), narrow magnetic highs are related toCarboniferous mafic volcanic rocks (Carboniferousbasalt and andesite) surrounded by Mid-Paleozoic chertand limestone, and crosscut by the sharp and distinctnortheast faults. The regional magnetic anomalyamplitude of this zone is distinctively higher than thoseof the other sub-domains in the central domain. Thishigh anomaly corresponds to MORB and oceanic island–arc lavas reported by Panjasawatworng et al.6. Thiszone is, therefore, interpreted to have formed in avolcanic island arc in the Devonian to Carboniferous17.

Fault and fracture zones at or near surfaces aregenerally expressed as linear magnetic lows because ofthe alteration of magnetite to more weakly magneticminerals at low temperature, high oxygen pressure inthe presence of water. In contrast, the faults occurringat depth within the crust at higher temperature andhigh water vapour pressure, Fe-bearing hydrous silicates(biotite) form in the fault zone. Therefore, the associatedfault is expressed by increased Fe-oxides (magnetite),which show strongly linear magnetic trends21. In thisstudy, the TF1 thrust fault is associated with the positiveanomaly, so we believe that the fault is a deeply seatedstructure that originated in a Permian-Triassicsubduction system that accreted the crust in the studyarea17. The TF1 boundary has been adopted as thesuture zone or tectonic line (called the ‘Loei suture’)described by Charusiri et al.13. The interpretation inthis study suggests that this zone is a tectonically collisionzone between the island arc and the Indochinacontinent. The collision zone is visible as regional andlocal shear zones shown as magnetic minima boundaries(the C1 sub-domain), and marks the accretionaryboundaries of arc terrains. The collision zone is coveredby Middle Paleozoic (?) metamorphic rocks that werebroken by numerous collision-related wedge-shapedthrusts and faults. On the magnetic map, the easterndomain is the profound magnetic maximum where a

significant component is due to magnetite enriched inserpentinites. It is possible that the serpentinites havebeen squeezed into wedge-shaped fractures within thesediment accretionary prism. In addition, the TF1 maybe formed as a fault zone evidenced by magneticmodeling and may be a controlling structurethroughout the geologic history of the area. Moreover,the northwest-southeast trending fault systems seemto change their orientation to north-south trending,especially in the northern part of the study area. Thisis probably due to a major change in tectonic stress andorientation, perhaps leading to the clockwise rotationof Southeast Asia by Indo-Australia collision12, 13.

Within the eastern domain, the magnetic trends arecharacterized by a rather irregular pattern of magneticanomalies containing the long segments withpredominant northwestern and northern trends. Thisdomain shows the north-south shear zone in the E1sub-domain as indicated by en-echolon features ofhigh magnetic intensity. These imbrications may becaused by the compressive tectonic stress roughly inthe north-south direction due to the subduction of theoceanic slab beneath the amalgamated, mainlandSoutheast Asia terrain7. Moreover, this domain containsa set of the northwest-southeast lineaments that cutthe shear zones and the TF1 and TF2 thrust faults in thenorth part of the domain, conforming a younger sinistralstrike-slip motion. The magnetic data also show thatthe domain consists of several discontinuous, sinistrallyside-stepping lineaments. This domain is representedby a zone of high strain, which may be influenced bythe tectonic collision in this area.

In general, there is a good correlation betweenexposed geologic units and the aeromagneticanomalies, with many of the mapped faults displayinga strong magnetic signature. Moreover, a number ofmagnetic anomalies strongly suggest that a number ofsignificant faults are not shown on the existing geologicmap. In some places, faults mapped on the surfacehave no magnetic expression and cut across magneticfeatures.

Comparison of the magnetic lineaments with thegeologic map2, 3 (Fig. 8A) show that many of the magneticanomaly trends (red color) coincide with geologicalcontacts or faults (black color) or closely parallel them.Some poor correlations in the northeast-southwesttrending faults contrast strongly in the central domainand the major north-south trending faults in the easterndomain, where they were not found on the geologicalmap. Good correlation with the northeast-southwesttrending fault segments in the eastern and centraldomains are observed in both magnetic lineamentsand boundaries on the existing geological maps.Comparison of the major lineaments from LandsatTM57 (cyan color) and aeromagnetic lineament (red

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color) maps (Fig. 8B) shows that the mapped lineamentshave similar trends, but the locations are mostly inparallel, suggesting that the sources of the two types ofanomalies are different in depths. Additionally, thecoincidence of the northeast-southwest trending faultsin the central domain of the aeromagnetic image andthe Landsat image7 likely indicate that these faults areassociated with the shallow fault sets that continueacross the Loei basin, but these lineaments are notshown on the geological maps.

Large-scale, open and upright folds dominatedthe central domain, are obvious in magnetic,electromagnetic and Landsat images. The southernpart of the eastern domain is a zone of steeply dippingand tightly folded sub-domains consisting ofsedimentary and serpentinite rocks, and defining anarea with high magnetic intensities. The dominantnorthwest to north trends are clearly visible as themagnetic lineaments surrounding axial surfaces (greencolor in Fig. 8) of the fold structures. However, thedifference of axial plane orientations in the E4 sub-domain, with north and northeast trends, suggests twostages of folding. The first stage probably occurred ascompressive stress developed in a roughly east-westdirection, and the second stage took place as thenortheast-southwest trending compressive stress. Thisresult corresponds with that of Neawsuparp andCharusiri7 for a major change in tectonic regime duringthe India-Asia collision.

CONCLUSIONS

Based on our aeromagnetic interpretation, the Loeiarea is divided into three magnetic domains; namelyeastern, central and western domains. The easterndomain is represented by mainly high magneticintensities with major strike-slip faults and small folds.The central domain consists of low magnetic intensities,thrust faults, large open synclinal fold and small tightanticlinal folds. The western domain containsthe large and prominent circular features withmoderate magnetic intensities and north-southtrending lineaments.

Three main suites of the magnetic lineaments areidentified including northeast-southwest, northwest-southeast and north-south trending features. In thecentral domain, the northwest-southeast trendinglineaments are cross-cut by the northeast-southwesttrending lineaments, indicating that the northeast-southwest trending lineaments are younger than thoseof the northwest-southeast trends. Additionally, thenorthwest-southeast trending lineaments in the centraldomain represent major thrust faults. The north-southtrending lineaments in the eastern part of the area areregarded as major strike slip faults. Some magnetic

lineaments correspond to previous mapped faults, andseveral interpreted lineaments are previously unknownfaults.

Our geophysical interpretation was developed togive more details on the covered bedrock in the Loeiarea. The recently complied geophysical interpretationof the Loei area is highly useful for studying theanomalous composition and Paleotectonic crust in theLoei area that has been formed by arc-continentcollision in the Paleozoic time.

ACKNOWLEDGEMENTS

We thank V. Daorerk, head of Department ofGeology, Chulalongkorn University for allowing us touse all facilities at the department and J. Tulyatid, asenior geophysicist, Geotechnics Division, Departmentof Mineral Resources (DMR), for their fruitful discussionon previous field and remote-sensing information. Weacknowledge S. Pothisat, Director General of DMR, forhis encouragement and permission to publish theresults. This project was sponsored by the ThailandResearch Fund (TRF grant no. PHD/0017/2545).

REFERENCES

1. Chareonpawat A, Wongwanit T, Tantiwanit V and KitiprariwatA (1975) Geological map of sheet Changwat Loei (NE 47-12)scale 1:250,000. Geological Survey Division, Departmentof Mineral Resources, Thailand.

2. Mineral Resources Development Project (MRDP) (1988)Geology of Loei area. Geological Complication and EditingSection, Geological Research and Technology GeologicalSurvey Division, Bangkok, Department of Mineral Resources,Thailand, 61 p.

3. Chairangsee C, Hinze C, Machareonsap S, Nakornsri N,Silpalit M and Sinpool-Anunt S (1990) Geological map ofThailand 1:50,000: Explanation for the sheets Amphoe PakChom 5345 II, Ban Na Kha 5344 I, Ban Huai Khop 5445,King Amphoe Nam Som 5444 IV. Hannover: GeologisuhesJahrbuch, Reihe B, 55p.

4. Intasopa S and Dunn T (1993) Petrology and Syn-Ndisotropic systems of the basalts and rhyolites, Loei, Thailand.Journal of Southeast Earth Sciences, 9 9 9 9 9,167-80.

5. Charusiri P (1989) Lithophile metallogenetic epochs ofThailand: A Geological and Geochronological Investigation.Ph.D Thesis, Queen’s University, Canada, 819 p.

6. Panjaswatwong Y, Chantaramee S, Limtrakun P and PiraraiK (1997) Geochemistry and tectonic setting of eruption ofcentral Loei volcanic in the Pak Chom area, Loei, northeastThailand. The International Conference on Stratigraphy andTectonic Evolution of Southeast Asia and the South Pacific,pp. 287-302. Aug.19-24, Bangkok, Thailand.

7. Neawsuparp K and Charusiri P (2000) Lineament analysisfrom Landsat data for structural geology and mineraloccurrences in Loei area, Northeastern Thailand. InProceedings of the International Conference on AppliedGeophysics, November 9-10, 2000, Chiang Mai, Thailand.

8. Seusutthia K and Maopeth N (2001) Petrography andgeochemistry of ultramafic rocks at Ban Bun Tan, SuwanKhuha Sub-district, Nong Bua Lumphu Province. Department

298 ScienceAsia ScienceAsia ScienceAsia ScienceAsia ScienceAsia 30 (2004)30 (2004)30 (2004)30 (2004)30 (2004)

of Geology, Chulalongkorn University, Bangkok, Thailand,84 p (unpublished).

9. Galong W and Tulyatid J (1992) Airborne geophysical datainterpretation of eastern Loei area: An aid for Geologicalmapping and mineral exploration. In C. Piancharoen(ed.),Proceedings of National Conference on GeologicalResources of Thailand: Potential for Future Development,pp. 70-85. November 17-24, Department of MineralResources, Bangkok, Thailand.

10. Rangubpit W (2003) Image Processing and Interpretation ofAeromagnetic and Resistivity Data over Two 1:50,000 MapSheet Area of Ban Yuak and Ban Sup, Loei area, Thailand.M.Sc. Thesis, University of New South Wales, Australia(unpublished).

11. Neawsuparp K, Charusiri P and Meyers J (2004) Applicationof aeromagnetic data and GIS integration for mappingcontinuity of geotectonic feature in the Loei area , NortheastThailand. In Proceedings of RPJ-Ph.D. Congress V, TheThailand Research Fund , April 23-25, Chonburi, Thailand,p 85.

12. Bunopas S (1992) Regional stratigraphic correlation inThailand. Proceeding of the National Conference onGeologic and Mineral Resources of Thailand: Potential forFuture Development, November 17-24, Department ofMineral Resources, Bamgkok, Thailand, pp.189-208.

13. Charusiri P, Daorerk V, Archibald D, Hisada K andAmpaiwan T (2002) Geotectonic evolution of Thailand: ANew Synthesis. Journal of the Geological Society of ThailandNo.1,1-20,2002: pp.1-20.

14. Bunopas S (1988) Summary of geology of Loei area andaccompanying 1:100,000 Geological map. MineralResources Development Project, Department of MineralResources, Thailand, 61 p.

15. Putthapiban P (2002) Geology and Geochronology ofigneous rocks of Thailand. In the Proceedings of theSymposium on Geology of Thailand, 26-31 August 2002,Department of Mineral Resources, Bangkok, Thailand, pp.261-83.

16. Nakapadungrat S and Putthapiban P (1992) Granites andassociated mineralization in Thailand. In the proceedings ofthe National Conference on Geologic Resources of Thailand:Potential for Future Development, Department of MineralResources, November 17-24. Bangkok, Thailand.

17. Neawsuparp K (2005) Application of Airborne Geophysicaland Remote Sensing Data to Structural Geology and TectonicSetting in Loei Area, Northeastern Thailand. Ph.D. Thesis,Department of Geology, Chulalongkorn University, Bangkok,Thailand. (unpublished).

18. Milligan P R and Gunn P J (1997) Enhancement andpresentation of airborne geophysical data. Journal ofAustralian Geology & Geophysics, 1717171717(2), 63-76.

19. Gunn P J (1997) Quantitative methods for interpretingaeromagnetic data. JGSO Journal of Australian Geology andGeophysics, 1717171717(2), 105-14.

20. Safon C, Vasseur G and Creu M (1977) Some application oflinear programming to the inverse gravity problem.Geophysics, 4242424242, 1215-99.

21. Airo M-L and Ruotoistenmaki T (2000) Magnetic signaturesof finish bedrock and their relationship with geologicalboundaries. In: Pesonen, L., Korja, A. and Hjelt, S-E.(editors): Lithosphere 2000. Report S-41, Institute ofSeismology, University of Helsinki, Finland,(abstract).

22. Fontaine H and Ingavat R (1982) The Lower Carboniferousin Thailand. In Proceedings of the10th InternationalStratigraphy Geology Carboniferous Congress, Madrid 1983,1:129-132, Madrid 21.

23. Plijm B A and Marshak S (1997) Earth Structure, anIntroduction to Structural Geology and Tectonics. WCB/McGraw-Hill, New York, 495 p.

24. Telfold W M, Geldart L P, Sheriff R E and Keys D A (1986)Applied Geophysics. Cambridge University Press, New York,860 p.

25. Whitaker A (1994) Integrated geological and geophysicalmapping of southwestern Western Australia. AustralianGeological Survey Organisation, Journal of Geology andGeophysics 1515151515, 313-28.