~Aleomagnetism of Frecambrian Rhyolites from Southeastern

Home , Geology of Missouri, Ring dike, Russell Mountain, Taum Sauk Mountain

-~aleomagnetism of frecambrian Rhyolites

from Southeastern Missouri

A Thesis

Presented in Partial Fulfillment of the

Requirements for the Degree, Bachelor of Science

Mark David Gilliat

The Department of Geology and Mineralogy

Ohio State University

1984

Approved by

Hallan C. Noltimier, Advisor Department of Geology and Mineralogy ABSTRACT

Twenty four samples of rhyolite from six sites in the Taum Sauk Mountain region were sampled for paleomagnetic investigation. Paleomagnetic techniques were applied to the rocks in an attempt to identify a component of rota tional movement along a mapped fault. All samples indicate multi-component behavior, with remanence beinq carried by magnetite and/or hematite. Within core specimen magnetic dispersion was low, however between sample dispersion was significantly high. This high between sample dispersion made any attempt to identify a rotational component of movement along the fault impossible. Sources of the dis persion of magnetic results include: 1) variation in the primary magnetic combnent due to inhomogeneous aquisition of remanence within a cooling unit.; 2) differential wea thering within a unit. ; 3) lightning strikes. ACKNOWLEDGMENTS

I would like to thank Dr. Hal Noltimier, my advisor, for the "long distance" assistance. His critical reviews were very helpful.

Eric Cherry contributed knowledge and support throgh out the project. Thanks Eric, it couldn't have been com pleted without your help.

Thanks to Nick Schear for help with the word process ing program SCRIBE.

Financial support for the field work was provided by a grant from Sigma Xi.

Financial support for the laboratory work was provided by a grant from the Friends of Orton Hall. 1

Introduction

The St. Francois Mountains of southeast Missouri lie near the crest of the Ozark dome (fig 1). The terrain consists of Precambrian granites and rhyolites, dated at 1.5 b.y. (Bickford and Mose, 1975), cropping out as structural and topographic highs (fig 2). The St Francois Mountains are signifigant in that they are one of the few mid-continent exposures of Precambrian rocks, and these rocks offer an opportunity to study the structural and chemical evolution of the mid-continent, basement complex. The purpose of this investigation is to apply paleomagnetism to the structural relationships between two crustal blocks separated by a fault that outcrops on Taum Sauk Mountain.

Taum Sauk Mountain (elevation 1772 ft.) is located in the western St. Francois Mountains, and has been mapped in detail by Berry (1970, 1976). Berrys' geologic map shows a major northeast trending fault in the saddle between Taum Sauk and Russell Mountains ( elevation 1726 ft.) (fig 3). The repeated section of rock across the fault offers an opportunity to observe the structural relationships between the fault blocks.

In the present investigation these rocks are studied by paleomagnetic techniques in an attempt to identify a component of rotational movement across the fault. The rocks were subjected to standard laboratory alternating field and thermal demagnetization techniques to establish a stable virtual geomagnetic pole (VG P) for each unit. Reflected light studies have been used to better understand magnetic properties of the individual units. In addition, information obtained on the general igneous petrology provided an independent test of the previous petrologic correlation of the units across the fault.

Identification of any rotational component of movement along the fault is made possible by comparing the established VGP's for each unit, with its counterpart across the fault. Assuming the relative movement along the fault is simply translational, paleomagnetic results should show no statistically signifigant difference between the VG P's on either side of the fault. On the other hand, if rotational movement has occured, the rotation between the blocks can be estimated both in terms of local angle and sense of relative rotation. EXPLANATIC" m Rocks of Ordo•,c-c,. -S• ~ Rocks of Co111b,,c,. •;• E~ Pr1ca111llricn rocas

lll1noi1 Basin

Modifiod from Stoto (1932)

Figure 1 Location of the St. Francios Mountains in relation to the Ozark uplift.

0 IOO 200 MILES ...._~~~~~~~~...... ~~~~~--~~-___J

Figure 2 Generalized cross-section of the St. Francios Mountains. T 33 N

...· -,... t --. ·. ~ ... ,. ::/' J.· ···.', 4 . ,"- _....;. "- ,. ·, .. --

1000 0 1000 2000 3000 4000 5000 FEET

RHYOLITE D CAMBRIAN SEDIMENTS D RUSSELL MOUNTAIN MOUNTAIN RHYOUTE D TAUM SAUK RHYOLITE w LINDSEY D ROYAL GORGE RHYOLITE CJ IRONTON RHYOLITE BUCK MOUNTAIN SHUT -INS D BELL MOUNTAIN RHYOLITE D FORMATION D WILDCAT MOUNTAIN RHYOLITE D POND RIDGE RHYOLITE FAULT; BAR ON DOWNTHROWN · SIDE

Figure 3 Geologic map of the Taum Sauk Mountain region. 2

Regional Geology

of the The St. Francois Mountains of southeast Missouri are located at the crest igneous Ozark dome. The St. Francois terrain is characterized by Precambrian principle rocks exposed through a cover of lower Paleozoic sedimentary rocks. The and rock types exposed in the St. Francois terrain include silicic volcanics, tuffs, associated granitic plutons. The silicic volcanics, chiefly alkali rhyolite ash-flow types of form about two-thirds of the outcrop area {Pratt et. al, 1979). Three granites granite have been identified using bulk chemical methods; 1) Amphibole representing ring fracture and porphyry intrusives 2) Biotite granites representing (E. sub-volcanic massifs 3) Tin granites representing the main batholiths cut Kisvarsanyi, 1980). Hypabyssal basic dikes and sills of the Skrianka diabase youngest across all volcanic and plutonic rocks of the region and represent the period of igneous activity. The St. Francois terrain is interpreted as representing several volcanic-plutonic ring complexes, identified on the basis of petrographic: 1980). geochemical, structural subsurface, and aeromagnetic data (E. Kisvarsanyi, ages of The regional terrain has been dated at approximately 1.5 b.y. using U-Pb to a separated zircons (Bickford and Mose, 1975). This time interval corresponds the mid time when anorogenic volcanic-plutonic complexes were emplaced in continent basement (Muehlberger et. al, 1967; Silver et. al, 1977). as The hills in the St. Francois terrain, composed of igneous rocks, rise as much lying 250 m. above the valleys. Upper Cambrian sedimen_try rocks fill the valleys, unconformably on the Precambrian igneous rocks. Stratigraphic relationships of indicate that there was an extensive period of erosion, prior to the deposition igneous the Cambrian sediments, that produced considerable relief throughout the pre-late terrain (Sides et. al, 1981). The present landscape is primarily the exhumed volcanic Cambrian terrain, where the topographic highs are supported by resistant of low rocks, and the topographic lows are easily weathered granites, or areas paleorelief filled with Cambrian or younger sedimentary rocks. 3

Volcanic Terrian

The Precambrian volcanic terrain of the St.Francois Mountains consists primarily of ash-flow tuffs, lava flows, and minor bedded tuffs, chiefly of rhyolitic composition (R. Anderson, 1962, 1970). These volcanics are preserved only locally on the tops of buried basement knobs and along the flanks of the volcanic terrain (fig 4) (E. Kisvarsanyi, 1981). These sites of preservation correspond to former structural depresions, such as cauldron subsidence stuctures and extra cauldra depresions, where the volcanics were protected while the surrounding highlands were eroded.

Several structural complexes in the St. Francois terrain have been suggested to represent cauldron subsidence structures. This interpretation is supported by several lines of evidence. 1) The presence of sub-volcanic massif granites surrounding granite-porphyry ring intrusives 2) The ring intrusives of granite porphyry enclosing remnants of the volcanic terrain, suggesting the structural control involved in preserving the rhyolites. The granite porphyry is assumed to have intruded along the Precambrian ring faults that bound the cauldron subsidence structure 3) Random stuctural attitudes in the volcanics, suggesting large scale brecciation often associated with cauldron subsidence and cauldron collapse stuctures. 4) Downfaulting of the volcanics allowing preservation while the surrounding region was eroded (E. Kisvarsanyi, 1981). It should be noted that observed contacts between rhyolites and biotite granites (sub-volcanic massifs) generally dip at low angles, and lack evidence of forceful intrusion (Tolman and Robertson, 1969). This relationship further suggests that the volcanic pile was supported by its own subvolcanic massif and lacked a solid floor. The sub-volcanic massif granites are essentially the intrusive equivilants of the rhyolites. These rising plutons charged with volatiles exploded through the surface, producing the volcaniclastic deposits and lava flows, and subsequently consolidated below the volcanic pile (E. Kisvarsanyi, 1981). The Eminence region of the St. Francios terrain exemplifies these structural relationships (fig 5).

The Graniteville pluton northwest of Ironton coritains structural evidence suggesting a resurgent cauldron structure. A resurgent cauldron (Smith and Baily, 1962, 1964) is defined as a cauldron within which the cauldron block, after initial subsidence, has been uplifted, usually in the form of a structural dome. Crosscutting relationships show that the tin granites (those forming the central plutons) of the Graniteville pluton are younger than the amphibole and biotite granites of the subvolcanic massif (E. Kisvarsanyi, 1980). This relationship suggests that the pluton intruded the pre-existing cauldron subsidence structure, producing a resurgent cauldron complex. Subsequent erosion of the supracrust resulted in the exposure of the central pluton (fig 6) (table 1).

lntraplate hot spot activity has been presented as a possible mechanism for emplacement of these mid-continent, volcanic-plutonic complexes. The evidence that suggests this evolutional model includes ensialic melting, and incomplete rifting ( G. Kisvarsanyi, 1976; Lowell, 1976). The association of rocks existing in the St. Francios terrain is typical of the suites that develop in regions of continental doming (pre-rift) and tensional tectonic activity (Bowden, 1974). NW+r, ~Sf

Otll 0"4

GOO• 600•

0 5

r--r, SIUCIC ~IC t...:...:1 IIOCI(

Figure 4 Cross-section illustrating sites where the Precambrian volcanic rocks are preserved. a

O Ring intrusion D Subvolconic massif Q Silicic volcanic rock

Figure ? Schematic evolution of the Eminence cauldron-subsidence structure: a. caldera-forming eruptions and development of ring fractures; b. cauldron subsidence, caldera collapse, emplacement of ring dikes; c. erosion of volcanic highlands. Adapted from Smith and Bailey, 1968. a

[23 Central pluton D Subvolcanic massif EJ Ring intrusion G::::J Silicic volcanic rock Figurf' '=,. Schematic evolution of a ring complex with a central pluton: a. cauldron subsidence and emplacment of ring dikes; b. erosion of the volcanic highlands; c. resurgent doming and attendant fracturing; d. exposure of the central pluton within remnants of volcanic rocks and the ring dike. •

TABLE 1. GEOLOGIC E\'ENTS OP THE R.ESUllGENT-CAULDaON CYCLE Duration in the Staae Suuctural Events Volcanic Events Sedimentary Events Plutonic Events Valles Caldera I Regional tumescence ud Eruptions due to leaka&c Erosion of the Compositional zonation in < 4 x J()I yrs propagation of ring and along radial or ring volcanic highland. magma chamben. Increasing ndial fractures with fractures. magma pressure. Minor possible apical graben intrusion. subsidence. D Major ash-flow eruption. Depssing and skimming < JO yra. (est) S0-500 mi•. of zoned top of chamber. DI Caldera collapse. Overlap with staae ll in Avalanches and Disequilibrium. < JO yrs. (est) tome calderas. slides from caldera waJI~.

IV Minor pyroclastic eruptions Caldera fill; Consolidation of magma and lavas on caldera talus, a\'~lanches, caught in ring fractures floor in some calderas. slides. fans. (residual ring dikes). lake deposits. Progressive recovery of ) equilibrium. Be,inning of minor ring intrusion. < JO' yrs • V Resurgent doming. Possible ring-fracture Caldera fill R.ise of central pluton and 'I volcanism and/ or continues. (Lake perhaps a rina-intrusion eruption or intrusion overflows and StagW' ) in dome fractures. caldera is breached. Possible regional Ring-fracture volcanism. Caldera fill Final emplacement and 8 x 10' yn VI ± JOS yrs tumescence and Possible stage II eruption continues, late differentiation of ring reopening of ring of next cycle lake sediment. intrusions. Possible fractures. Erosion of fill. Fill > erosion. stoping by central pJuton. vn Terminal fumarolic and Erosion. Crystallization of major > lOS yrs hot spring activity Erosion > fill. plutons. Possibly a (hydrothermal alteration) . major ore-forming stage.

• 4

Geology of the Taum Sauk Mountain Region

• The Taum Sauk Mountain region is located in the western portion of the St. Francois Mountains (fig 7). In this region a thick section of Precambrian ash-flow tuffs of alkali rhyolite composition are exposed (E. Kisvarsanyi, 1980). Detailed mapping by Berry et. al (1970) has suggested that the thickness of these units may exceed 1,500 m.

Several hypotheses have been suggested for the structural setting of the Taum Sauk region. R. Anderson (1962, 1970) concluded that the volcanic rocks of the Taum Sauk region occupy a sructural depression in the roof of an underlying batholith. J. Anderson (1969) interpreted this structural sag as a caldera: the Taum Sauk caldera. Based upon the similar distribution of lithologies and fault patterns between the Taum Sauk region and the Valles caldera (Smith and Baily, 1966), Berry (1970) suggested this region represents a resurgent caldera. Recent studies by Sides et. al (1981) support the interpretations of previous workers as far as the caldera structure is concerned, however they go on to sugges~ that two calderas are present in the Taum Sauk region. Based on structural and stratigraphic relationships, it is concluded that the Butler Hill caldera formed first to the west of the Taum Sauk region. Following resurgent doming in the Butler Hill caldera, a new caldera developed to the east in what is now the present day Taum Sauk • region (fig 8) .

• • ...______...... 38°

91°

O"'"1·, c..

._ ,,...... ·. ;; .~

•

__ .-.. t i I}.,-·i '

O 10 20 30 Miles Precambrian Rocks 1------'------,,.... ------.~---- D o 10 20 30 40 Kilometers

Figure 7 Regional map showing the location of • Tatnn Sauk Mountain • e

Explanation ~Lake Killarney Formation ~Taum Sauk Rhyolite t•:;,]High-Silica Granit~ E":\·.:"dCaldera-Filling Units t::::)Low-Silica Granite M~;·/JGrassy Mountain lgnimbrite 0Pre-taldera Crust Fig. s- Sequential· cross sections showing inferred development of caldera~ in tt:e St. Francois Mountains. 5

Previous Paleomagnetic Studies

Several studies investigating the paleomagnetic properties of the Precambrian rocks of Missouri were conducted in the early 60's. The earliest studies were conducted by Allingham (1960) to help interpret aeromagnetic anomalies. Hays and Scharon (1966) conducted a study on intensity and direction of remanent magnetization, and measured the magnetic susceptability of Precambrian rocks exposed in southeast Missouri. Hsu et. al (1966) investigated the magnetic properties of several volcanic units in the Taum Sauk Mountain region.

The study conducted by Hays and Scharon (1966) investigated the magnetic properties of felsites, granites and diabases exposed in southeast Missouri. Standard stepwise alternating field (AF) demagnetization treatments ranging from 10 to 70 mT were conducted to obtain stable magnetization vectors. Paleomagnetic results are listed in Table 2. Conclusions drawn from the study simply demonstrate that first, the directions of remanent magnetization obtained from these Precambrian rocks differs significantly from the direction of the present day geomagnetic field in southeast Missouri, and second, that partial demagnetization in AF fields ranging from 30 to 70 mT significantly reduces the scatter of the remanent magnetization vector by diminishing the anomolous components of viscous and isothermal remanent magnetization (Hays and Scharon, 1966).

As in the study by Hays and Scharon (1966), samples in Hsu's et. al study on the Precambrian volcanics from the Taum Sauk region, were subjected to AF demagnetization treatments ranging from 10 to 80 mT. Conclusions drawn from the ' study indicate that the stable magnetization in some of the units has high coercivity and resides in fine-grained magnetite. This fine-grained magnetite was formed by devitrification and/ or vapor phase crystallization, and thus the resulting aquired magnetization is inferred to be primary thermoremanent magnetization (TRM) aquired at the time of cooling (Hsu et. al, 1966).

Hays and Scharon (1966) sampled one unit of interest to my study, the Royal Gorge Rhyolite. Table 2 shows the results of the study. In both their study and mine the Royal Gorge Rhyolite is characterized by a small cone of confidence. Hsu et. al (1966) investigated the Russell Mountain Rhyolite which is also included in my study. Statistical analysis of their data is listed in Table 3. J>.\I.l::0:\1.\GXI·:TIS~I IX PRF.C~\:\IIlRI.\X mxEors HOCI~S T.\BI.E L .-\ vc-rn~P ':\ foJ,:nrt ir Pro pert i<·:-' ·-· ------·- - -··------.l/A .l(ac .··\her------nating k Jo /), I, a,s, D, I, a.u, Fi!ld, )!,... k x (XI0-3) cxio-3) cl~g deg deg .\° deg deg df'g oe

-;·~ Crc•·k rhyolia• 103 0.43 4.49 244.8 (H.2 10.5 46 24-'1.3 49.3 ~ 9 300-i'OO Rq,ilGi:•rJ!'' rh_Yolitc 16 0.25 0.6:2 256.8 37.6 51.5 ~50 ; _:,!l )lo:uitam :· :,olit~ . 1-l 0.14 0.57 200.7 31.S 35.8 11 243.7 42.5 U.S 450 ·,:-.1:!.lk.:i d1:.ah:1se 12 2.0-l 2.69 U0.7 42.3 11.0 12 235.5 30.6 8.5 300- -.. =.i!es 3-1 0.79 ~o values obtained > :-. .\I0t:ntain .:.::t!~ite 10 0.20 20.00 24-l.5 58.0 17.1 (HOO

~::i.i.:tic:11 techniques after Fisher [1953]. S - number of samples measured. J.: • magnetic susceptibility. Jc - intensity of relil:lnent ID.llgnetization (at O oe). V =- declin:iticn of mean direction of remanent m:ignetization. I - incliD.3tioo. of mean direction of remanent nugnetiza.tion. aH - sem.iangle of cone of 95% confidence for mean direction. JI" = denotes Mtural remancnt magnetiza.tion. JI.e == denote5 m.:ignetizntion after trell.tment in nc field.

'I'AHLE 3 Statistical An:ilysis or )lean Directions

Unit )! }) I k nu

k 10 233.5 51.7 4·.2 21.3 "'i 7 244.4 43.1 10.0 16.7 i 1 247.5 43.7 la 6 237.7 49.l 20.6 12.4 g 2 240.3 47.S 40.9 15.5 I 5 2°51.7 44.9 159.G 4.9 e 2 2~6.6 43.5 206.7 6.0 d 3 2.56.3 40.8 7·1.3 9.5 c 6 221.1 50.0 7.1 21.4 b 5 237.7 63.6 13.0 17.4 a 2 243.1 3S.8 717.l 3.7 All 4!) 242.5 4S.tl 17.2 5.0 All (7') 40 243.3 41.0 11.1 6.2

N, number of samples; D, declination; J, inclination; k, precision p:uamcter;

Location and Description of Field Work

Field work for this study was conducted during mid-June, 1984. The Taum Sauk Mountain region is located in the western part of the St. Francois Mountains of southeast Missouri. The geographical coordinates of the sampling sites are 37 .56 N, 90.72 w.

The volcanic sequence that was sampled consists of, in geochronological order, Russell Mountain Rhyolite, Wild Cat Rhyolite, Bell Mountain Rhyolite {not sampled), and the Royal Gorge Rhyolite. The sampling plan involved stopping at the Taum Sauk Mountain lookout tower, and sampling downsection along the relatively upthrown block, west of the mapped fault. The same units were then sampled on the downthrown eastern fault block. Four block samples, oriented with a Brunton compass, were obtained from each unit on either side of the fault. 7

Paleo magnetic Procedure

The oriented block samples were mounted in plaster such that the oriented surface was horizontal. Cores 2.5 cm. in diameter were drilled normal to the oriented surface. A vertical line corresponding to the horizontal surface was scribed onto the core. Barbs were scribed onto this figure to represent both up direction and dip direction (fig 9). Cores were then sliced into individual paleomagnetic specimen cores that measure 2.3 cm. in length. Noltimier (1971) has calculated that cores of these dimensions minimize effects of shape anisotropy. Samples were then labled according to site and sample number.

Detailed alternating field and thermal demagnetization treatments were conducted in a stepwise manner on a pilot suite of cores representing each sampling site. The direction and intensity of magnetization at each of the demagnetization stages was measured using a Schonstedt SSM-lA spinner magnetometer. A Schonstedt GSD-1 alternating current fixed axis specimen demagnetizer was used to perform alternating field treatments, and a Schonstedt TSD-1 thermal specimen demagnetizer was used for the thermal demagnetization treatments.

All resulting data were reduced using a FORTRAN program written by Kopacz and Smith (1974), with further modifications by Rhodes, Bartman, Noltimier and Cherry. The pilot suite data was evaluated using the FORTRAN program ZIDERVLED (Cherry, 1984a). This program calculates: 1) The coordinates of the modified Zijderveld vector diagrams (Zijderveld, 1967); 2) normalized moment; 3) Briden Stability Index (Briden, 1972); 4) blocking temperature or coercivity spectra; 5) Paleomagnetic Stability Index (Symons and Stupavsky, 1974); and b) the magnitude and direction of the removed vector. The within specimen homogeneity of magnetization was evaluated using the FORTRAN program CORE (Cherry, 1984b), which is based on the method of Harrison (1980).

Rock Magnetic Procedure

Bulk magnetic susceptibility was measured using a soil test MS-3 susceptibility bridge in conjunction with a Tectronix Type 545 cathode ray oscilloscope. Over 50 cores were measured , and all but several showed susceptibilities at or near the limit of resolution for the instrument ( k less than 1.5 x 10 in SI units).

Two cores from each sampled unit were given an isothermal remanent magnetization (IRM) using a variable field direct current electromagnet (Rhodes~ 197 4; Richardson, 1983). The electromagnet was calibrated by Richardson and Cherry and is capable of producing fields up to 0.5 teslas (T). IRM procedure involves first demagnetizing the specimen core in a 100 mT alternating peak field. Subsequent to demagnetization the cores are subjected to direct current fields along the z axis in a stepwise manner up to 0.5 T. !

/ ,,I I i I

Figure 9 Diagram illustrating directional markings on a paleomagnetic specimen core. Srike of the block sample, and dip direction relative to the strike, are indicated on top of the core. Up directiqn is indicated on the extension of the strike line along the side of the core. 8

Sample Preparation for Silicate and Oxide Petrography

Samples for thin section and polished section were cut from core stubs. Thin sections were prepared in the O.S.U. thin section room. These sections were analyzed to determine the silicate phases existing in the sampled units, and determine the crystallization characteristics and weathering phenomena. Polished thin sections were made in the oxide petrology room of the paleomagnetics laboratory. Thin wafers from the specimen cores were cut and finely polished using Buehler polishing cloth and a polish section machine. These sections were analyzed to obtain information on the oxide phases existing within the sampled units. g

Paleomagnetic Results

Russell Mountain Results

The alternating field (AF) and thermal pilot suite results for Russell Mountain 1 samples ( west side of fault) are presented in figures 10 and 11.

Thermal and AF demagnetization show that remanent magnetization is dominated by hematite (Ht). A small viscous moment is removed by 12 mT during AF demagnetization. The Zijderveld diagrams show good general declination correlation between AF and thermal demagnetization data. General inclination directions also correspond well, however the AF Zijderveld diagram shows a much tighter grouping than the thermal diagram.

Stereo plot data do not follow great circle paths, but instead are tightly clustered. Mean directions correspond well between thermal and AF stereo plot data, yielding declinations of 245 °and inclinations of +45~

The coercivity spectra (H,)are characteristicof Ht behavior with a small viscous moment (possibly residing in a few coarse ~agnetite (Mt) grains) removed in low fields. The blocking temperature (T \.:.) .lf;c. distinctly Ht, where over 85% of the remanence is carried by grains with blocking temperatures above 670.

Wild Cat Results

The AF and thermal pilot suite results for Wild Cat 1 samples (west side of the fault) are presented in figures 12 and 13.

AF and thermal demagnetization treatments of W c 1-1 show that the remanent magnetization is composed in two components. Approximately 70% of the remanent magnetization resides in Mt, with Ht being prevalent in the high coercivity and temperature range.

Remanence directions group closely in the thermal stereo plot, with more scatter shown on the AF stereo plot. Zijderveld diagrams also show good correlation in general magnetic directions. Note the steep initial inclination component shown on both diagrams, along with the good correlation between declinations. The two remanence components do agree well between AF and thermal data. Mean inclinations for the Ht and Mt components are about the same at +45 ° to +50° showing normal polarity. The declinations however are significantly different with the Mt component being 180° to 190° and the Ht component is at 230• to 250~

He indicates overlap between Ht and Mt at moderate field strengths. Ht dominates the magnetization above 60 mT. The Tb spectrum is broad with Ht dominance after O C . 0 O 580 ..as evidenced by the steep drop from 580 to 600 C.

AF and thermal pilot suite results for Wild Cat 2 samples (east side of fault) are presented in figures 14 and 15. 10

Both thermal and AF treatments of W c 2-2 show that Ht completely dominates the remanent magnetization. A small viscous moment is recognized, but is removed by 200 C. during thermal demagnetization, and 0.2 mT by AF demagnetization.

Remanence directions follow great circle patterns away from the natural remanent magnetization (NRM). Primary directions are not well defined on Zijderveld diagrams, and no good directional correlation between AF and thermal diagrams can be made. Stereo plot data show close correlation between AF and thermal directional data, yielding declinations of 245 ° with inclinations of +25~

The H c shows Ht dominating the magnetization upwards from 10 mT. the T\, spectrum is very indicative of Ht.

Royal Gorge Results

The AF and thermal pilot suite results for Royal Gorge 1 samples (west side of fault) are presented in figures 16 and 17.

Both AF and thermal demagnetization treatments of Rg 1-5 indicate that the stable remanent magnetization is dominated by magnetite, with two mmor secondary components. A small viscous component is removed by 5mT. A secondary component in Ht is recognized at the highest coercivities, and above the currie temperature of Mt.

Remanence directions follow great circle patterns away from the NRM. The primary magnetization in Mt is well defined on the Zijdervld diagram, and shows a declination of 255: with inclinations of +58~ The Ht direction is not as well defined, having reversed inclinations in a southwesterly direction.

The H,.shows Mt moderately overlapping at high fields with Ht. The Tl.spectrum is broad for Mt with Ht being distinct at temperatures in excess of 620° C.

The AF and thermal pilot suite results for Royal Gorge 2 samples (east side of the fault) are presented in figures 18 and 19.

AF and thermal demagnetization treatments of Rg 2-2 reveal that the remanent magnetization is comprised of two distinct components, one in Mt, and one in Ht. A small viscous component, probably carried by coarse grained Mt, is removed by 10 mT during AF demagnetization. The remanent magnetization is comprised of 60% Ht, and 40% Mt.

Remanence directions follow great circle paths away from the NRM. General directions of magnetization correspond well between thermal and AF Zij derveld diagrams. Stereo plot data also show close correlation, yielding declinations of 206~ and inclinations of +63~ Both Ht and Mt components have normal polarity.

~ indicates moderate overlap between Ht and Mt at moderate field strength. Ht comprises almost all of the remanent magnetization above 40 mT. The T'b spectrum 11

IS dominated by Ht, however below 550D C. a broad Mt component Is also recognized. SAMPLE Rm 14-JC • • • 1. 4 • • • • 1. 2 • • 1.0 27 • • • • • • • ·+ • • • • • • • 90 • ,, • • 8 • • • . 6 • • • l. •

• 2 180 Stereo plot of directions 200 400 600 TEMP. 0 c Normalized moment

.igure 10

N w 0 ····································-······································································--·-·-············································--··+-··-·---························ H RMT-143C

-200

• -800 -600 -400 -200 0 200 Modified Zij~erJ'ld diagram SAMPLE _Rm_l4-4A • • • • • • • 1. 2 • • 270 • • • • ·+ • • • • • • • • 9(t 1. • " • ... . • • • • 6 • • • 4 •

• 2 Stereo plot directions

20 40 60 80 100 AF PEAK FIELD (mT) Normalized moment

Figure 11 200--~~~~~~~~~~~~~~~~~~,-~~~-, • N w 0 ···-···································································-···············································-···-·······································. f········································

H Rlv1A-144A

·=

-200 -

c Mo

-400 -

-600 -

D 1 -WOL------"--'----~------~------' • -800 -600 -400 -200 0 200 Modified Z~jder~d digrarr.

! - -~------_____J SAMPLE We 11-lc

• • • • • • 1. 2 • • • •••• +• • • • • • • • • 1. 0 27

• • • J ": .:.l • •

:~~ 180 Stereo plot directions l I 200 400 600 TEMP. 0 c Normalized moment

.igure 12

w 0 ···-·· · T

H WCT-111C / . !

-200

-400

•

-600

• 200 -800 -600 -400 -200 0 Modified ZijderJ°ld diagram SAMPLE We 11-28

• • • • • • • • • •••••• +• .... 'r. : • • "' .• .. • •

• L

• 2 Stereo plot directions

20 40 60 80 100 AF PEAK FIELD (mT) Normalized moment

• Figure 13 N w 0 ········································-······························-······································································································t·································

-200 WCA-1128 ,,..•

-400

-600 N··------·

-800 D

• 0 200 -1000 -800 -600 -400 -200 e. Modified Zijder~ld diagram SAMPLE We 22-lA

• • 1.4 • • • • 1. 2 •N • • 1.0 27 .... • ·+ • • • • . • • • • • 8 • 1· • • • 6 • • • 4

180 .2 i'---_____, 200 400 600 TEMP. 0 c

• N w 0 ········································································································-·-·-······-----············································································ H WCA-222A

-200

-400

-600 .P

-800 D

• -800 -600 -400 -200 0 -1200 -1000 SAMPLE We 22-2A

• • • • • 1. 2 • • •. • • • + • • • • • • • • 9 •

. 6

• 2

20 40 60 80 100 AF PEAK FIELD (mT)

• N

100 w ··-····················-····-·········--·-···- ··-·-··········----·················································o······································-··-············-···-·····-··· H -100 D a YJ-~t)~·}- ~').-?-')-~ -300 0 WCA-222A ...... ·~ ... -500 t:'. •fiJ D c D • -70QL--~~~._~~~-'-~~~-'-~~~-L--~~~...J--~~~~ -1200 -1000 -800 -600 -400 -200 0 SAMPLE Rg 15-4A

• • 1. 4 • • • 1. 2 • • • 1.0 27 • • • • ·+ ••• • • • • r}I • • • 8 • • • • 6 .0 0 • 0 • . 4 •

• 2 180 Stereo plot directions 200 400 600 TEMP. 0 c Normalized moment

• Figure 16 N w 0 ········································-··················································-·········-···-··-··· . .

-200 RGT-154A

-400

-600

-800 D • -1000 -800 -600 -400 -200 0 200 Modified Zijder~ld diagram SAMPLE_ Rg 15-SA

• • • • • • • 1. 2 • 270 •••• + • • • • • • • • 9· 1•N • 1. • • • • • • . 6 • • . 4

.2 Stereo plot directions 20 40 60 80 100 AF PEAK FIELD (mT) Normalized moment

• Figure 17 N

-200 RGA-155A

-400

-600

-800 ... 0 "~ • -1000 -1000 -800 -600 -400 -200.. 0 200 Modified Zijderv°1.d diagram SAMPLE Rg 22-lB

• • 1.4 • • • 1.2 • • • 1.0 27 • • • • + • • • • 9 • •,I . 8 •• •,..;..

. 6 • • • • 4 • • 2 180

200 400 600 Stereo plot directions TEMP. 0 c Normalized moment

Figure 18

• 600

400

200 RGT-2218 w o ··············-···-~-----·-···········-····-···········--····················-·-·······-·····r-····················································-····························

a ~ -200 a ( oJI

-400

-600

-800 0

• -1000 -800 -600 -400 -200 0 200 400 600 800

Modified ZijderJ'ld diagrams

1 • • • • • • • 1. 2 • 270 • • •••••• + • • • • • • • • l. ...

. 6

.2 Stereo plot directions

20 40 60 80 100 AF PEAK FIELD (mT)

Normalized moment

Figure 19

• 600

400

-600 ,/ -800 0

• -1000 -800 -600 -400 -200 0 200 400 600 800 Modified ZijderJ°ld diagram 12

References

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