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Open-File Report 02-99

Comparison of Aeromagnetic Data with Geologic Map Data

Detailed comparisons of the mapped geology with the aeromagnetic data were made to establish the magnetic signatures of the various rock units. For most of the area, the correlations were made using 1:48,000 scale geologic maps (Drewes, 1971; Simons, 1974) overlain with contour maps of the aeromagnetic anomaly and complemented with color shaded relief images of the aeromagnetic anomaly at the same scale. This combination proved to be very useful for observing relationships both in the field and in the office. The western part of the area is not covered by detailed mapping and in this part of the study area correlation was made with 1:125,000 scale mapping (Drewes, 1980; Gettings and Houser, 1997). The geologic map of Plate 1 is from 1:125,000 scale mapping and does not include the detail of the 1:48,000 maps but is used here for convenience of presentation. Plate 2 shows the shaded relief aeromagnetic data overlain with the line work from the map of Plate 1. By comparing the two plates it is easy to locate the extent of the various geologic units on the aeromagnetic map. Areas or anomalies of particular interest to be discussed below are numbered on Plate 2.

One caveat must be borne in mind in studying Plate 2 and subsequent plates in this report with geologic linework overlain on them. The geologic base was hand digitized from the map of Drewes (1980) over a period of several years by students, and some registration errors are contained in the data. These errors are not systematic and appear to be small rotations and translations of different digitizing sessions relative to one another. Consequently, the linework is registered to the aeromagnetic map correctly in some areas and not in others. The overlays in this report are the result of carefully fitting ("rubber sheeting") the linework to points on the aeromagnetic map determined by overlaying the aeromagnetic data on the original paper map of Drewes (1980). The resulting fit is reasonably good in most areas but some linework is as much as 500 m off from the correct point on the aeromagnetic map. The majority of the linework is believed by the author to be within 300m of its correct location. Correlations with geologic units are not affected by this problem because the associations were made using accurate overlays of the aeromagnetic map on the 1:48,000 and 1:125,000 scale maps and did not make use of the digital linework. The linework overlain on plates 2-7 contains a few spurious lines. The lines present on pl. 1 are to be taken as correct. Appendix 1 contains Postscript files of contour maps of the parts of the aeromagnetic survey which overlay the 1:48,000 maps of Drewes (1971) and Simons (1974). Appendix 1 also contains files to allow plotting 1:175,000 scale versions of color plates 1-7.

In general, there is a good correlation between exposed geologic units and the aeromagnetic anomaly field, with many of the mapped faults displaying a strong magnetic signature, and a number of magnetic anomalies strongly suggest faults not shown on the geologic map. In some places, faults mapped on the surface have no magnetic expression and cut across magnetic features such as anomaly minima and gradients. The aeromagnetic anomaly field is highly varied in the study area and is due to a large variety of lithologies with highly variable magnetization. Aeromagnetic anomalies are very large and have steep gradients in several areas of the map (for example, numbers 1, 6, 9, and 14 on pl. 2) with positive anomalies in excess of 2,000 nanoTesla (nT) (no. 1 on pl. 2) and negative anomalies of –500 nT or more (north of no. 9, pl. 2) indicating strongly magnetic rocks near the surface.

Faults mapped in the basin fill (west of nos. 4 and 16, pl. 2, and pl. 1) do not exhibit short wavelength anomalies indicative of magnetic sources in the surficial basin fill sediments such as are observed in the Albuquerque basin (Grauch, 1997). Instead, their magnitude and wavelength reflect the fault offset of deeper magnetic sources, typically at the bedrock-basin fill interface (Hegmann, 1998; Baldyga, 2000). Generally, the basin fill has behaved as a brittle material and the mapped surface scarp of the fault is approximately the up-dip projection of the fault defined by the aeromagnetic and gravity data, rather than directly above the edge of the upthrown block as would be expected for unconsolidated material (Baldyga, 2000). The Santa Cruz basin fill is mainly consolidated conglomerate with only a relatively small amount of unconsolidated material (Gettings and Houser, 1997).

Relations Between Mapped Rock Units and Magnetic Anomalies

The largest positive anomaly in the study area is centered on the Grosvenor Hills Volcanics (Drewes, 1971; no. 1, pl. 2); a sequence comprised of Oligocene lower rhyolitic and upper rhyodacitic members. The sequence is intruded by several 26-28 Ma rhyodacite and granodiorite laccoliths, dikes and plugs (Drewes, 1971). The exposed laccoliths (no. 1, pl. 2; pl. 1, and Drewes, 1971) are associated with reversely polarized anomalies characteristic of reversely magnetized thin sheets. These anomalies are of the order of 500 nT amplitude and superposed on a large circular positive anomaly of about 1700 nT peak to trough intensity, about 10 km diameter, and approximately centered 2km southwest of no. 1 on pl. 2. The circular anomaly is interpreted to be due to a subsurface intrusion based on the following evidence. First, the circular anomaly shape is larger than the laccoliths and is normal in polarity whereas the laccoliths are reversed; second, the anomaly does not correlate closely with any one unit on the geologic map; and third, the fault pattern on the geologic map (pl. 1) is that of doming and most of the upper part of the Grosvenor Hills Volcanics upper member has been eroded away in the area contained by the anomaly, suggesting doming and uplift. Assuming the circular anomaly to be a thick intrusive, the models of Andreasen and Zietz (1969) give an effective susceptibility of 0.009 cgs/cc, which is quite magnetic. However, if it is assumed that the rocks possess significant remanent magnetization, a susceptibility of 0.003-0.005 cgs/cc gives Konigsberger ratios Q of 2.2-0.9. These are typical values implying the induced and remanent magnetizations are about equal and the intrusion has a susceptibility typical of a fresh basalt. Although no sites were measured in this study on the Grosvenor Hills laccoliths, susceptibilities in the Laramide Josephine Canyon diorite and Gringo Gulch pluton are about 0.003 cgs/cc (table 1). Since the anomalies associated with these bodies (nos. 2 and 11, pl. 2) are less intense than those of the Gosvenor Hills anomaly, it is reasonable to assume that the source of the circular anomaly has a susceptibility in the range 0.003-0.005 cgs/cc and a Q of 1-2. If the laccoliths are assumed to be 250 m thick (the thickest exposed section, Drewes, 1971), the models of Andreasen and Zietz (1969) imply an effective susceptibility of 0.005cgs/cc for these rocks. These susceptibility and Q estimates are in good agreement with average values for rock types of andesitic to basaltic compositions (Clark, 1997 and 1999).

A group of north-northwest positive anomalies are associated with the Laramide intrusive rocks, generally of dioritic composition (for example no. 2, pl. 2). The anomalies are not as intense as those of the Grosvenor Hills but they are nevertheless large amplitude within the study area. These rocks include the Josephine Canyon Diorite, the Madera Canyon granodiorite, and the main phase of the Gringo Gulch pluton (Drewes, 1971; Vugteveen and others, 1981). They are the northern end of a narrow belt of intrusives that have strong positive magnetic anomalies and which extend southward into Sonora, Mexico (Finn and others, 1999). Field measurements of the magnetic susceptibility of these rocks (table 1) show that they have large susceptibilities (0.001-0.003 cgs/cc), and arguments similar to those above for the Grosvenor Hills intrusion suggest they have a significant remanent magnetization in a normal direction with Q values of the order of 1. The distribution of these rocks is important because intrusions of intermediate to silicic composition of Laramide age are often associated with the large porphyry copper deposits of this region of Arizona.

The Laramide Josephine Canyon Diorite forms the core of the San Cayetano Mountains horst block and its magnetic anomaly forms a nearly north-south ridge in the magnetic field (between nos. 3 and 5, pl. 2). A small stock, the granodiorite and rhyodacite porphyry of the San Cayetano Mountains (Drewes, 1971) is reversely polarized (no. 3, pl. 2). The stock is dated radiometrically (lead-alpha method) at 27 Ma (Drewes, 1971). This age is essentially identical (27.5 Ma, potassium-argon method, Drewes, 1971) with the rhyodacite laccoliths in the Grosvenor Hills intrusion which are also reversely polarized.
The Grosvenor Hills volcanics (Drewes, 1971) contain rocks of both normal and reversed polarities. The rhyolite (older) member is normally polarized (no. 4, pl. 2), while both units of the rhyodacite (younger) member and the laccoliths and dikes intruding them (no. 1, pl. 2) appear to be reversely polarized (nos. 5 and 6 and especially 4 km to the southeast of no. 5, pl. 2). The granodiorite and rhyodacite porphyry stock of the San Cayetano Mountains appears to be contemporaneous with the extrusion of the rhyodacite member of the Grosvenor Hills volcanics and the intrusion of the laccoliths and dikes.

The Jurassic Squaw Gulch Granite (and quartz monzonite) of Drewes (1971) is present over a large area of the central portion of the survey, from the north end of the San Cayetano Mountains north to Cottonwood Canyon, east almost to Temporal Gulch and south nearly to Patagonia (fig. 1). This body is of batholithic proportions and forms the basement for the Cretaceous volcanic rocks of the Salero Formation (Drewes, 1971) extruded on it. It is reversely polarized (for example, no. 7 on pl. 2). Note the north-northwest extension of this unit in the geologic linework from the "7" on pl. 2; this is the worst registration error found in the linework – the granite should overlie the magnetic low 300-500 m to the east. From the "7" southwards the contact is correctly located. The Salero formation (Drewes,1971) consists of a lower member of mainly dacitic flows and tuff breccias which are reversely polarized, overlain by a rhyodacite welded tuff member which is normally polarized. Between the two lies an exotic block member (Drewes, 1971) or megabreccia which is probably reversely polarized (no. 8, pl. 2; no. 3, table 1) but has some areas of ambiguous magnetic signature on the map of Drewes (1971). Although the area of the Salero Formation just east of the Grosvenor Hills (no. 1, pl. 2) is undivided on the map of Drewes (1971), from the magnetic anomaly it appears to be dominantly reversed, that is, the lower and exotic block member. The large magnetic low (no. 9, pl. 2) on the north side of the large circular anomaly is a compound anomaly, made up in part by the dipole minimum of the (normal) circular anomaly and in part due to the (reversed) Squaw Gulch Granite forming the bedrock in this area.

The Gringo Gulch pluton (no. 11, pl. 2) has a normally polarized anomaly, although in detail it is distorted in the area of outcrops of microgranodiorite (Drewes, 1971) and this late phase unit is reversely polarized as indicated by the measurements (nos. 8-9, table 1; Hagstrum, 1994). The age of the pluton is now accepted as the older of those reported by Drewes (1971) and the volcanic rocks intruded by it are Lower Paleocene (Laramide) in age (Vugteveen and others, 1981). As reported by Vugteveen and others (1981), the Gringo Gulch volcanics are reversely polarized (southwest of no. 11, pl. 2 and no. 12, pl. 2); however the uppermost unit appears to be normally polarized (no. 13, pl. 2). These volcanics apparently include the extrusive rocks of Red Mountain to the south (fig. 1), although this is ambiguous because of the positive anomaly caused by the Red Mountain porphyry (no. 18, pl. 2) which contains disseminated magnetite and is normally polarized. In addition the volcanic rocks of the Red Mountain area are highly altered and oxidized (Corn, 1975) and thus not very magnetic in any case. The large anomaly east of Red Mountain (east of no. 18, pl. 2) is due to Laramide intrusive rocks of Saddle Mountain (fig. 1) similar to the Josephine Canyon diorite. Similar anomalies can be seen to the south of Red Mountain (no. 18, pl. 2) and are almost certainly caused by similar Laramide intrusives at depth because there is no consistent correlation between exposed units (Simons, 1974) and the strong positive anomalies. In this area, (the western Patagonia Mountains, fig. 1) the horst of Precambrian and Jurassic rocks shown on pl. 1 and corresponding to strong positive magnetic anomalies (north and south of no. 19, pl. 2) may have as its source of uplift the intrusion of the magnetic Laramide rocks.

The biotite-hornblende granodiorite of Paleocene age (Simons, 1974) has a similar large positive anomaly except where the rocks are altered in an east-northeast trending belt (no. 19, pl. 2) that includes the mining areas of Providencia and Sycamore Canyons and Soldier basin (Simons, 1974).

In the northern part of the study area, strong anomalies are present beneath an area of Precambrian granite, Cretaceous sedimentary, and Tertiary intrusive rock outcrop (no. 14, pl. 2). Once again, the anomalies do not correlate well spatially with Precambrian outcrop and the source of the magnetic anomaly may be a mostly buried Laramide intrusion that has "rafted" the Precambrian rocks upward, similar to the north end of the Whetstone Mountains (Gettings, 1996). The area of the magnetic low is much larger than the mining districts and the areas of alteration mapped by Simons (1974), and the anomaly may identify an underlying porphyry system whose upward apophyses are the mining districts.
The Elephant Head quartz monzonite (Drewes, 1971; no. 15, pl. 2) is reversely polarized (also shown by Hagstrum, 1994) and yields a generally low magnetic anomaly field in the northwest end of the Santa Rita Mountains. However, a positive anomaly (no. 16, pl. 2) is due to Josephine Canyon diorite, a small outcrop of which is mapped (Drewes, 1971) beneath the anomaly maxima.

In the Tumacacori Mountains (fig. 1), the oldest volcanic rocks are andesite flows (Ta, pl. 1); these rocks appear to be normally polarized and continuous at depth in a northwest trending belt (southwest of no. 17, pl. 2). The bulk of the range is made up of younger rhyolitic volcanic rocks, predominantly ash-flow tuffs that are reversely polarized. The Jurassic granite mapped at the north end of the range does not have a strongly correlated magnetic anomaly; there is some suggestion from the map pattern that it could be reversely polarized. The Jurassic granite of Mount Benedict (no. 20, pl. 2) appears to be normally polarized, although it also may be intruded below judging from the mapped fault pattern which is consistent with doming and the fact that it is heavily intruded with dikes (Simons, 1974).

From the above discussion and noting that some of the reversely polarized rocks (table 1; also Hagstrum, 1994) indicate paleomagnetic directions that are not parallel (or anti-parallel) to the present day induction direction, it is clear that in much of the survey area, remanent magnetization is an important, sometimes dominant, component of the magnetic anomaly. Because of this, transformations often performed on aeromagnetic survey data, such as reduction to the pole and various forms of filtering and depth-to-source estimation can only be used with great caution as they are not valid for magnetic fields with remanent magnetization directions other than that of the inducing field. In this survey, many of the magnetic anomaly minima are due to remanent magnetization directions other than the induction direction and thus are not due to nonmagnetic rocks as is often assumed. This renders automatic depth-to-source calculations useless for methods that assume negative anomalies are due to greater depth-to-magnetic-source. This includes the finding of horizontal gradients often performed on potential field data to locate body edges (e.g. Blakely and Simpson, 1986) because the computation for aeromagnetic data requires the reduction to the pole transformation.

Trends in the Magnetic Survey Data and Their Relation to Geologic Features

Plate 3 displays a set of trend lines drawn by the author using image manipulation software. The trend lines were drawn from visual inspection on the computer screen (21" monitor) using the computer’s "mouse" on the shaded relief image of the aeromagnetic data (pl. 2). The geologic linework was superimposed afterward as a separate layer. Because of parallax effects on the onscreen image, some lines may not be located exactly where they would be had they been drawn on a smaller scale map and digitized. Trends which appeared visually obvious to the author were drawn, and although and attempt was made to be thorough for trends 2 km or longer in length, certainly some were overlooked, particularly those roughly parallel to a nearby trend already drawn. Comparison with the geologic linework shows that many of the magnetic anomaly trends coincide with geologic contacts or faults or closely parallel them. Phillips (2002) discusses some of the reasons trends may parallel a structure or magnetic boundary but be offset from them. In this study offsets as much as 500 m between a trend line and the structure may be seen on pl. 3. Many of the offsets are due to using the observed (dipole) magnetic anomaly data rather than a reduced-to-the-pole version; however, as discussed above, the reduction-to-the-pole transformation is invalid for much of the dataset. Some faults in the Santa Cruz basin area either have no magnetic expression or such a small one that it was not detected by the author. On the other hand, several faults within basin fill have extensions in the sub-basin bedrock even though the fault has no surface trace along the extension.

Four lines on pl. 3 are heavier in weight than the others, and mark what are believed to be important boundaries based on evidence presented below. The heavyweight boundaries separate the map of pl. 2 into an eastern area of high amplitude, "rough" anomalies, and a western area of "smoother ", lower amplitude anomalies. The northwest-trending heavyweight line in the southwest of the map area follows the Santa Cruz fault (Gettings and Houser, 1997) and its extensions. The heavyweight line trending northwest in the northeast part of the map follows a magnetic minima that crosses many structures and is part of a regional magnetic boundary extending both northwest and southeast outside the map area (Finn and others, 1999). This regional-scale magnetic low may mark the accretionary boundary of arc terranes from the southwest and the associated arc magmatism during Late Triassic and Early Jurassic times (Dickinson, 1989). The boundary is offset to the southwest near the north central edge of the map; this is a somewhat arbitrary decision on the part of the author and may be artificial. The final heavyweight line on Plate 3 trends approximately N30oE from the Santa Cruz fault at 31o30’ N, 111o E to the north edge of the map. This line and the Santa Cruz fault line define a boundary within which all of the high amplitude anomalies of the Santa Rita and Patagonia Mountains are contained (the areas marked "A" on pl. 3). This northeast-trending boundary is quite well defined in the aeromagnetic field (see pl. 2), but does not correspond to any single feature on the geologic map (pl. 1). An unresolved question is why the amplitudes of anomalies should change so abruptly across this boundary from less intense to the northwest to more intense to the southeast. Rocks causing the large positive anomaly in the extreme northwest part of the area (UTM 495 km east, 3515 km north, pl. 3) and to the north of the study area are similar age and composition to the Laramide diorites southeast of the boundary, but do not cause as large amplitude anomalies. If they are simply more deeply buried outside of block "A" (pl. 3) then why is the boundary not a fault? The heavyweight lines trending northwest (pl. 3), and others that parallel them, though not as continuously, are believed to be deep seated structures which have their origin in the Triassic-Jurassic subduction system that accreted the crust in the study area. Many of these structures have been reactivated through time as faults and loci of magmatism. The heavyweight line trending northwest from UTM 3497 km north, 531 km east (pl. 3) includes the Sawmill Canyon fault zone, believed by Drewes (1981) to be a controlling structure throughout the geologic history of the area.

In order to quantify the trends analysis and determine if they were clustered, the trends of plate 3 were digitized. Curved trends were digitized in segments which were approximately straight. From the resulting dataset segment azimuth, length, and coordinates of the segment center were computed. Figure 2 displays several plots made from this dataset. Figure 2a is a histogram of azimuth of the segments. The distribution is tri-modal, with the most frequent trends being about N40oW, the second most frequent about N65oE, and the least frequent approximately N5-10oE. A narrow peak occurs at N83oW; these correspond to a unique cluster from the doming faults in the Grosvenor Hills volcanics in the center of the map area. The northwest trends correspond both to the original structures aquired during crustal accretion and to subsequent reactivation, particularly the Mid-Tertiary magmatism and extensional deformation event (Dickinson, 1989) which resulted in extension oriented along an axis N60oE. In this area, much of the extension has been accomplished on northwest trending normal faults in addition to some low-angle faults (Gettings, 1994). Moreover, extension was not uniform over the whole area resulting in shear motion on N60oE block boundaries. Within the study area, the Santa Rita block apparently extended farther than the block to the southeast containing the Patagonia Mountains. This resulted in a rotational component extending and rotating the southern Santa Rita block in a clockwise sense, providing a tensional regime for emplacement of the Alto dike swarm and related dikes (Drewes, 1996) which host the lead-silver carbonate vein deposits of the area. Two major faults which bound the Patagonia graben (fig. 1, pl. 1, and UTM 524 km east, 3487 km north on pl. 3) appear to be a boundary between these blocks. Offset of the magnetic anomalies due to the Laramide diorites across these faults suggest about 2.5 km of left-lateral motion. The deep (about 300 m, Cisneros and others, 1999) but areally small basin at the town of Patagonia (fig. 1) may be a pull-apart basin resulting from this process.

Figure 2. a) Histogram of trend segment azimuths digitized from Plate 3. b) Rose diagram of trend azimuths. c) Plot of trend segment length versus trend segment azimuth.

Figure 2b displays a rose diagram of the observed trends. In this diagram all segments were given equal weight, so it is essentially a radial histogram. The north-trending cluster of trends is clearly displayed on this plot. These trends correspond to the latest deformation, representing east-west extension resulting in north trending faults of the Late Cenozoic Basin and Range episode (Dickinson, 1989).

Figure 2c is a plot of segment length versus segment azimuth. Although it is somewhat misleading because long continuous segments that were not straight were broken into smaller straight line segments, the plot still places a lower bound on trend length as a function of azimuth. The plot shows clearly that the longest straight segments are oriented N40oW, and segments in the 1-4 km length range are essentially uniformly distributed. Segments in the 5-10 km length range occur mainly at the azimuths of the peaks in the azimuth distribution (fig. 2a).

Another technique, closely related to textural measures (discussed below) but difficult to quantify, is to use an image manipulation program to produce an image of the gridded data using random colors ("oil slick diagram", D. Clark, oral communication, 1999) as shown in plate 4. This often enables the visual detection of trends in the data as they cross areas of widely varying anomaly texture because it removes the dominating effect of anomaly amplitudes and polarities. Trends that are difficult to trace across "magnetically flat" areas in the data can sometimes be estimated with this technique. A number of trends so identified are labeled on plate 4 using lower case letters. The letter "a" identifies the northeast trending boundary discussed above dividing high amplitude anomalies to the southeast from lower amplitude ones to the northwest. Lines labeled "b" are northwest trending boundaries discussed above. Note that the continuity of these boundaries is easier to see in this display (pl. 4) and that there are several more than identified on plate 3. Not all such boundaries are labeled on plate 4. The line labeled "c" on plate 4 corresponds to the boundary including the Sawmill Canyon fault zone discussed above and may represent an approximate northeastern boundary of Jurassic crust in this area. The lines labeled "d" outline the belt of Laramide diorite intrusives, and the line labeled "e" includes the southern boundary of the Santa Rita block passing through the town of Patagonia, and its extension to the southwest. The visualization of the data shown in plate 4 also shows some areas of essentially uniform "texture" in color, size, and shape of features. Several of these areas have been outlined and numbered on plate 4. the areas numbered "1" correspond to the volcanic rocks of the Tumacacori Mountains and their northward extension in the subsurface. The outline of this texture in the Tumacacori Mountains area of plate 4 includes areas along the eastern edge of the volcanic field which are buried beneath basin fill of the Santa Cruz Valley. The outline in the southwest corner of the study area labeled "2" is an area discussed in more detail below. The southwest third of the outlined area is Cretaceous rhyolite lava flows, tuffs, and volcaniclastic sediments (Drewes, 1980) and is thought to be part of a Cretaceous caldera by Lipman and Sawyer (1985). As shown below, the two thirds of the outlined area to the northeast may be the (buried) remainder of the caldera, based on the continuity of textural measures of the magnetic field.

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