Data Repository Introduction. New physical property data #

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1 #21131 Physical property data and geophysical models to accompany Rooted Brooks Range ophiolite: Implications for Cordilleran terranes by Saltus, Hudson, Karl, and Morin. Data Repository Introduction This data compilation and summary of geophysical modeling accompanies our report (Saltus et al., 21) on the implications of a tectonic reinterpretation of the mafic and ultramafic rocks of the Copter Peak and Misheguk Mountain allochthons in the Noatak region of northwestern Alaska. We describe and present magnetic susceptibility and density measurements made on mafic and ultramafic rocks from the study area. We then present a series of magnetic and gravity models to illustrate geophysical constraints on the geological interpretation of these rocks. In contrast with the prevailing geological interpretations for the area, we find that the basalt, gabbro, and ultramafic rocks are thick and intermixed at depth. New physical property data Measured and inferred densities and magnetic susceptibilities provide bounds for potential field models of the Copter Peak and Misheguk Mountain allochthons in the Noatak region. Table 1 lists densities and magnetic susceptibilities measured on hand samples collected on and around Asik Mountain in June 2 (assisted by Gil Mull); sample locations shown are shown on Figures 1 and 2. The samples designated Mv are all from Asik Mountain. Tables contains magnetic susceptibility measurements made by us on alluvial basalt boulders in seven drainages in the Maiyumerak Mountains basalt massif; these measurement locations are shown on Figure 2. Table 3 is a summary of the density and magnetic susceptibility information combined with measurements on rocks from the Siniktanneayak complex (Morin and Moore, 1996) to the northeast and Baird Mountains to the east. We measured magnetic susceptibilities using a hand-held KAPPAMETER susceptibility meter manufactured by GEOFYZIKA a.s. in the Czech Republic. The meter has a sensitivity of 1 x - SI. Before every rock measurement the meter was calibrated by taking a free space measurement. The values reported are the apparent susceptibility values direct from the instrument readout. Depending on the size of the rock sample and the surface roughness, these values may underestimate true susceptibility by up to 1%. Care was taken to make measurements on flat surfaces of samples greater than cm in thickness and diameter to minimize this underestimation. Densities were measured by weighing dry (Wa) and saturated (Ws) rock samples in air and weighing the saturated sample suspended in water (Ww). Weights were measured on a calibrated scale to an accuracy of better than. gm. Three densities were calculated: (1) grain density = Wa/(Wa-Ww), (2) saturated bulk density = Ws/(Ws-Ww), and (3) dry bulk density = Wa/(Ws-Ww). We used the saturated bulk densities for modeling. Densities are accurate to better than.1 g/cm 3.

2 Our 12 laboratory and over 7 field measurements indicate that the Copter Peak basalts have magnetic susceptibilities ranging from essentially zero to a maximum of about x -3 SI with an average value of 1 x -3 SI. Density measurements on 4 basalt samples range from 2.8 to 2.98 g/cm 3 with an average value of 2.8 g/cm 3. Forty-one laboratory magnetic susceptibility measurements on gabbros of the Misheguk Mountain allochthon have values ranging from essentially zero to 113 x -3 SI with an average of about 3 x -3 SI. Density measurements on 2 gabbros show a range of 2.82 to 3.22 g/cm 3 with an average of 3.3 g/cm 3. Six laboratory measurements on ultramafic rocks from Asik Mountain show magnetic susceptibilities ranging from.6 to 49 x -3 SI with an average value of 14 x -3 SI. Eight density measurements on ultramafic rocks from Asik Mountain range from 3. to 3.24 g/cm 3 with an average value of 3.18 g/cm 3. Gravity and magnetic profile models A series of two-dimensional magnetic and gravity models along aeromagnetic profiles #1 and #2 (Saltus et al., 21, Figure 2) demonstrate some of the volumetric and structural constraints that these data impose on geologic interpretations of the Copter Peak and Misheguk Mountain allochthons. Figure G1 shows two models that demonstrate the difficulty of fitting the aeromagnetic data on profile #1 with a thin, synformal body. In model 1.1, the best-fitting uniform magnetic susceptibility was calculated for a synformal body with a horizontal base at about 3 km depth. The calculated effect of this body fails to match the observed data in two key ways: (1) the complex shape in the center of the body is not matched, and (2) the flanks of the anomaly are too steep and don t match the smooth flanks of the observed data. In model 1.2 we preserved the thin synformal shape required by previous geologic interpretations and allowed the magnetic susceptibility to vary laterally within steep-sided domains. We used an automatic inversion method to calculate the best-fitting susceptibility values. Even if we allow the susceptibilities to vary freely, the model still fails to match the broad flanks of the observed anomaly. The laterally variable domains do allow us to fit the complex shape of the center of the anomaly, but the susceptibilities required are greater than those we observed in any of our measured rock properties from the region. Figure G2 shows a series of models that demonstrate the inability to fit the aeromagnetic anomaly on profile #2 with a uniformly magnetized body. This series of models were calculated in the pseudogravity domain. The pseudogravity transform (Baranov, 197) is a mathematical way to convert a magnetic anomaly into a gravity-like anomaly that may be more conducive to interpretation (Blakely, 199). This technique is advantageous here because: (1) it reduces the sensitivity of the models to shallow effects, and (2) it removes a dependence on magnetic field direction. The appropriate physical property in the pseudogravity domain is pseudodensity. Pseudodensity can be converted mathematically to an equivalent magnetic susceptibility. For each of the 6 uniformly magnetized bodies in figure G2 we label the modeled pseudodensity value (D) and the equivalent susceptibility value (S). The equivalent susceptibilities range from 17 to 112 x -3 SI. Each of the model bodies extends to the surface. The bottom interface of each body is adjusted to fit the broad northern bulge of the aeromagnetic anomaly. Models 2.1 and 2.2 fit the steep-sided southern high over Asik Mountain but the others do not. Given

3 that the mean measured susceptibility for the Copter Peak basalts is 1 x -3 SI and that of the Misheguk Mountain gabbros is about 3 x -3 SI, it is difficult to accept models 2.1 to 2.3 that use average susceptibilities that are 2 to 3 times this amount. The best compromise is perhaps provided by model 2.4 with an assumed susceptibility of 41 x -3 SI. This model extends to 8 km depth and still fails to match the highest and steepest part of the Asik Mountain high. Model 2.6 is an attempt to fit the anomalies with a uniformly magnetized body with a susceptibility equal to the measured value for the Copter Peak basalts. This produces a body that is significantly thicker than 2 km, and does not provide a good fit to the Asik Mountain high. These modeling results for uniformly magnetized bodies are all unsatisfactory and demonstrate the need for multi-body models to satisfy the geologic, physical property, and aeromagnetic data constraints. Figure G3 shows several multi-body models that illustrate the range of possible geophysical solutions along profile #1. In each case, body A is set to the measured average magnetic susceptibility of the Copter Peak basalts as exposed along this profile. Several scenarios for the susceptibility and shape of the deeper, more highly magnetized body B, are illustrated in the models. In model 3.1 we require the base of body B to be approximately horizontal and allow body B to assume a relatively high susceptibility (nearly twice the average we measured for the Misheguk Mountain gabbro). Model 3.2 shows a scenario in which the bottom of body B is allowed to vary and we flatten the interface between the two source bodies. Model 3.3 shows a model that results if we use the measured average susceptibility of the Misheguk Mountain gabbro for body B. This series of models demonstrates that we can fit the observed anomalies if we place more magnetic rocks beneath the Copter Peak basalts. If these rocks are like the Misheguk Mountain gabbros, the overall assemblage must be quite thick, probably greater than 8 km. Figure G4 shows two combined gravity and magnetic models along profile #2, which crosses the gravity and magnetic high at Asik Mountain. In these models each body is assigned both a magnetic susceptibility and a density value and the calculated effects must match both the observed aeromagnetic and ground-based gravity data. Densities are listed as values (in g/cm 3 ) relative to the surrounding country rock. To convert to absolute densities, add these values to the assumed background density of 2.7 g/cm 3. To fit the data, we were required to create models with four separate geophysical bodies. In both models, body A has magnetic susceptibility equal to that measured for the Copter Peak basalts, but has a lower density as required by the gravity values in the northern part of the profile. These lower gravity values are partially caused by the affects of lowdensity sediments in the Noatak basin to the west of the profile. In both models, body B has density and susceptibility values that reflect our measured values for the Copter Peak basalts. To match the Asik Mountain gravity and magnetic highs we were required in both models to include the very magnetic, dense, and shallow body C. This body has magnetic susceptibility at the maximum end of any we measured in the region and a density higher than any we measured. For model 4.1, body D has density and magnetic susceptibility at the upper end of those we measured on gabbros. For model 4.2, body D has density and susceptibilities that reflect the average values we obtained in our measurements. In both models the shape of the bottom of the complex is not well constrained there are modeling trade-offs with the shape of the interface between

4 bodies B and D but, the overall thickness is about 8 km. Similarly, in both models a multi-body solution is required to fit all available data. Conclusions Analysis of a series of magnetic and gravity models indicates that the Copter Peak basalts are very thick and are underlain by regionally more magnetic and locally more dense rocks. When model susceptibilities and densities are constrained by our measurements on basalt, gabbro, and ultramafic rocks in the region, they produce total thickness for the mafic and ultramafic complex of at least 8 km. There are insufficient additional constraints to produce a single preferred geophysical model for the subsurface character of the basalt, gabbro, and ultramafic rocks but the available data and models that indicate these rocks are thick and intermixed at depth are robust. References Baranov, V., 197, A new method for interpretation of aeromagnetic maps: pseudogravimetric anomalies: Geophysics, v. 22, p Blakely, R.J., 199, Potential Theory in Gravity and Magnetic Applications: Cambridge University Press, New York, 441 p. Morin, R.L., and Moore, T.E., 1996, Gravity models of the Siniktanneayak maficultramafic complex, western Brooks Range, Alaska: Evidence for thrust emplacement of Brooks Range ophiolites, in Moore, T.E., and Dumoulin, J.A., eds., Geological Studies in Alaska by the U.S. Geological Survey, 1994: U.S. Geological Survey Bulletin 212, p Saltus, R.W., Hudson, T.L, Karl, S.M., and Morin, R.L., 21, Rooted Brooks Range ophiolite: Implications for Cordilleran terranes: Geology, v. XX, n. YY, p. X-Y.

Figure 1 - Asik Mountain sample sites AH18 AH17 67 3' N Asik Mountain

5 Figure 1 - Asik Mountain sample sites AH18 AH ' N Asik Mountain AH16 AH1 AH9 AH8 AH7 AH6 AH AH AH4 AH11 AH3 AH1 AH2 AH14 AH12 AH ' W km

6 Figure 2 - Maiyumerak Mountains sample sites x AH24 x AH23 x AH2 x AH26 x AH ' N x AH22 AH21 AH 2 19 AH x AH28 km 162 W

7 Table 1 Office measured susceptibilities on hand samples from Noatak 2 fieldtrip Saltus & Hudson, 11 July 2 model susceptibilities model densities Population categories: SI x -3 cgs x -3 Inferred lithology(ies) dead <2 metamorphosed gabbros, some basalts basalt 2.9 low 2<x< (mean ).4 most basalts, most umafic gabbro 3.1 med <x<8 (mean 2) 2 2. some basalts, some gabbros umafic 3.2 high 8<x (mean ) 8. some gabbros Weights (gm) Densities (g/cm3) in air in water saturated grain sat bulk dry bulk Site Lithology(Travis, Gil) Mag category Susceptibilities (SI x -3) SUS MEAN Site Wa Ww Ws D1 D2 D3 DEN MEAN AH18 basalt dead AH AH19 basalt low 2 7 AH AH19 basalt dead.28.4 AH19 AH2 basalt med AH2 AH2a basalt basalt AH2a basalt AH23 basalt high,med,dead ave 1.46 AH ave AH2 basalt & gabbro med, low stddev AH2.9 std dev AH2b diabase count 12 AH2b count AH1B gabbro low 7.7 max 93.6 AH1B AH1b gabbro med AH1b AH2a gabbro AH2a AH2b gabbro AH2b AH3 gabbro med, low AH3 AH3a gabbro AH3a AH4 gabbro low, dead AH4 AH4a gabbro AH4a AH gabbro med AH AH6 gabbro high AH AH7(1) gabbro high, med AH7(1) AH7(2) gabbro high 93.7 AH7(2) AH8 gabbro med, low AH AH gabbro med, low AH AH13 gabbro med, low AH13 AH13a gabbro AH13a AH13b gabbro AH13b AH13c gabbro AH13c AH13d gabbro AH13d AH13himag gabbro high 87.3 AH13himag AH14 gabbro med AH14 AH14 gabbro high,med AH14 AH14c gabbro AH14c AH1b gabbro AH1b AH21 gabbro med, low AH AH21 gabbro med, low AH21 AH2a gabbro AH2a AH26 gabbro low,dead AH AH28 gabbro med AH Mv8 gabbro med 38.6 Mv8 Mv9 gabbro high gabbro Mv9 gabbro AH7 gabbro ave 37.4 AH ave AH2 gabbro & dunite dead.24.2 stddev 3.47 AH2. std dev AH16A(1) gabbro & mafic med, dead count 41 AH16A(1) 2 count AH16A(2) gabbro & mafic low,dead max 113 AH16A(2) AH16Axtra gabbro & mafic low,dead min AH16Axtra AH16B gabbro & mafic low, dead AH16B AH1 gabbro & umafic med AH1 AH1big gabbro & umafic high AH1big AH1himag gabbro & umafic high 91 AH1himag Mv4 gabbro cumulate low Mv AH17 gabbro metamophic med, low AH17 Mv2 gabbro peroxenite med Mv Mv3 gabbro serpentine low Mv AH11 gabbro(?) low AH Mv7 mafic low 6.16 Mv7 Mv16 mafic igneous dead.34 Mv16 Mv1 mafic schistose med,low Mv1 Mv2 mafic schistose med,low Mv2 Mv3 SW Asik Mtn dead.11 Mv3 AH1A umafic med, low AH1A AH1B umafic AH1B AH3b umafic AH3b AH12 umafic low AH AH14b umafic AH14b AH1a umafic AH1a AH2b umafic umafic AH2b umafic AH4b umafic or pyx-gabbro ave 14.4 AH4b ave Mv W. Asik Mtn dead.6 stddev Mv.8 std dev count 6 8 count

8 Table 2.1 Creek bottom susceptibility measurements - Noatak Quad, Alaska, 1 June 2 Rick Saltus and Travis Hudson Site #1 (also AH22) LON LAT ELEV (ft) 87 Susceptibilities (SI x -3) Histogram bins Bin Frequency More Series Average Std dev Median Geomean

9 Table 2.2 Creek bottom susceptibility measurements - Noatak Quad, Alaska, 1 June 2 Rick Saltus and Travis Hudson Site #2 (also AH23) LON LAT ELEV (ft) 9 Susceptibilities (SI x -3) Histogram bins Bin Frequency More Series Average Std dev Median Geomean

10 Table 2.3 Creek bottom susceptibility measurements - Noatak Quad, Alaska, 1 June 2 Rick Saltus and Travis Hudson Site #3 (also AH24) LON LAT ELEV (ft) 9 Susceptibilities (SI x -3) Histogram bins Bin Frequency More Series Average Std dev Median Geomean

11 Table 2.4 Creek bottom susceptibility measurements - Noatak Quad, Alaska, 1 June 2 Rick Saltus and Travis Hudson Site #4 (also AH2) LON LAT ELEV (ft) 94 Susceptibilities (SI x -3) Histogram bins Bin Frequency More Series Average Std dev Median Geomean

12 Table 2. Creek bottom susceptibility measurements - Noatak Quad, Alaska, 1 June 2 Rick Saltus and Travis Hudson Site # (also AH26) LON LAT ELEV (ft) Susceptibilities (SI x -3) Histogram bins Bin Frequency More Series Average Std dev Median Geomean

13 Table 2.6 Creek bottom susceptibility measurements - Noatak Quad, Alaska, 1 June 2 Rick Saltus and Travis Hudson Site #6 (also AH27) LON LAT ELEV (ft) 12 Susceptibilities (SI x -3) Histogram bins Bin Frequency More Series Average Std dev Median Geomean

14 Table 2.7 Creek bottom susceptibility measurements - Noatak Quad, Alaska, 1 June 2 Rick Saltus and Travis Hudson Site #7 (also AH28) LON LAT ELEV (ft) 9 Susceptibilities (SI x -3) Histogram bins Bin Frequency More Series Average Std dev Median Geomean

15 Table 3 - Physical property measurements Format: AVERAGE±STD DEV(#SAMPLES) DENSITY (gm/cm^3) basalt gabbro umafic Noatak 2.8±.9(4) 3.3±.(2) 3.18±.8(8) Siniktanneyak 2.74±.12(7) 2.91±.8(13) 2.96±.17(4) SUSCEPT (SI x -3) basalt gabbro umafic Noatak 1.±27.2(12) 37.4±3.(41) 14.4±18.9(6) Maiyumerak 1.1±16.2(7) Siniktanneyak 2.3±.6(7) 9.1±13.4(13) 2.39±2.1(4) Sue Karl rx 2.1±2.9().6±.7(8) EXPLANATION Noatak = Hand samples collected and measured by Saltus and Hudson (see Table 1) Maiyumerak = Field measurements by Saltus and Hudson (see Table 2) Siniktanneyak = Hand samples collected and measured by Morin Sue Karl rx = Baird Mountains samples collected by Sue Karl, susceptibilities measured by Saltus

16 F IG UR E G 1 - T hin layer magnetic models along profile #1. Magnetics (nt ) Depth (km) 2-2 north =Obs erved, V.E.=1 model 1.1 C alculated: =model 1 =model V.E.=1 model Distance (km) south 6 3 3

17 F IG UR E G 2 - Uniformly magnetized body models along profile #2. P seudog ravity (pseudo-mg als) Asik Mtn profile (profile #2) S outh 2.1 =Obs erved, =C alculated North D=, Model S 2.1, =.112 D=, S =112 Model 2.2, D=, S =67 Model 2.3, D=3, S =2 Model 2.4, D=2, S =41 Depth (km) Model 2., D=1, S =27 Model 2.6, D=., S = Distance (km) V.E.=1

1 A S =1 B S =7 magnetic susceptibilities in S I x -3 V.E.

18 F igure G 3 - T wo-body models along profile #1 Magnetics (nt ) north =Obs erved, C alculated: model 3.1 model 3.2 model 3.3 south Depth (km) 1 Model 3.1 A S =1 B S =7 magnetic susceptibilities in S I x -3 V.E.=1 A S =1 Depth (km) 1 B S =3 2 2 Model 3.2 V.E.=1 Depth (km) A S =1 B S = Model Distance (km) V.E.=1

19 F igure G 4 - Multi-body magnetic and gravity models along profile #2 Magnetics (nt ) south =Obs erved, C alculated: model 4.1 model 4.2 north G ravity (mg als) =Obs erved, C alculated: model 4.1 model 4.2 Depth (km) A C B D=.6, S =8 D=, S =1 D D D=.3, S =8 model D=-.8, S =1 V.E.=1 Depth (km) A C B D=, S =1 D=.6, S =8 D D=.1, S =3 model D=-.8, S =1 V.E.= Distance (km)

3D Geometry of the Xade Complex inferred from Gravity and Magnetic Data

Geophysical Case Histories 3D Geometry of the Xade Complex inferred from Gravity and Magnetic Data 1. British Geological Survey, Edinburgh, United Kingdom Paper 92 Pouliquen, G. [1], Key, R. [1] ABSTRACT