Regional Tilt of the Mount Stuart Batholith, Washington, Determined

Brandon and Ague's reply follows after this discussion, which is included for completenesss.

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia: Discussion and reply

Discussion

Scott R. Paterson* Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740 Robert B. Miller Department of Geology, San Jose State University, San Jose, California 95192-0102

INTRODUCTION and roughly parallel to paleohorizontal; rock probably began during emplacement and (3) tilting of the batholith did not occur during certainly was active during cooling, suggesting Ague and Brandon (1996) calculated a best-fit cooling. We suggest that all three of these as- that tilting was occurring during cooling of the surface through Al-in-hornblende paleobaromet- sumptions may fail for the Mount Stuart batholith. Paterson et al. (1994) specifically ric data to determine the paleohorizontal in the batholith. First, there remain considerable noted that cordierite porphyroblasts in the Mount Stuart batholith, North Cascades, Wash- problems with calculations of crystallization hanging wall of the Tumwater Mountain shear ington, and thus attempted to provide a means to pressures in this batholith, as outlined by zone (which deforms and displaces the north- better interpret paleomagnetic results obtained Anderson (1996, 1997). Anderson, using min- east margin of the batholith) are partially from this pluton. Ague and Brandon (1996) then eral equilibria, and Francis et al. (1996), using pseudomorphed by staurolite, sillimanite, and used these results to reach conclusions about the oxygen isotopic data from the batholith and kyanite. These mineral relations indicate a tectonics of the northwestern Cordillera. We host rocks, concluded that mineral composi- pressure increase, analogous to the loading think that this is a useful approach and applaud tions and calculated paleopressures in the history of the Chiwaukum Schist. Since this their attempt to refine this technique. However, batholith reflect both magmatic and subsolidus shear zone was active during and only for a application of this technique to the Mount Stuart processes. We have concerns about whether short time after emplacement, this pressure in- batholith remains problematic for reasons out- any barometric data in the Mount Stuart crease (and thus loading) must have occurred lined in the following, and thus raises uncertain- batholith should currently fit a single plane. prior to or during emplacement of the 93 Ma ties about their tectonic conclusions. We address We have documented that much of the phase of the batholith. the following issues: (1) Ague and Brandon’s at- batholith was folded during final crystalliza- tempt to determine a paleobarometric surface in tion and that folding and ductile shear contin- REGIONAL RELATIONSHIPS the Mount Stuart batholith; (2) regional relation- ued in four domains during subsolidus cooling ships that may bear on possible tilt directions; of the batholith (Miller and Paterson, 1992, The only “regional relationships” that Ague (3) direction of motion of the Windy Pass thrust; 1994; Paterson et al., 1994). Although the and Brandon (1996, p. 482) offered in support of and (4) interpretation of existing paleomagnetic magnitude of displacement in these domains is their proposed tilt direction is that “From north- data. We hope that our comment emphasizes the generally unknown, at least in one case northwest to south-southeast across the Mount complexity of data sets from the Mount Stuart re- (Tumwater Mountain shear zone described Stuart area, one moves in an [stratigraphic] up- gion and thus the need for caution when inter- by Miller and Paterson, 1992, 1994) it was section direction from the Chiwaukum Schist preting its geologic history. enough to change pressures across the zone through the Ingalls ophiolite into the uncon- by >2 kbar (Paterson et al., 1994; see also formably overlying sedimentary strata of the PALEOBAROMETRIC SURFACES Magloughlin, 1994). Existing paleomagnetic Swauk Formation.” However, the contact be- and structural data (see the following) also in- tween the Chiwaukum Schist and Ingalls Com- Ague and Brandon (1996) noted that for dicate that the batholith may have undergone plex is the Windy Pass fault, which was active their technique to work the following three as- subsolidus rotations, but not as a single rigid during emplacement of the Mount Stuart sumptions must be met: (1) reliable measure- block (e.g., Beck et al., 1981; Lund et al., batholith, and likely displayed major ramps ments of crystallization pressures must be 1993, Paterson et al., 1994). Data summarized rather than forming a single, subhorizontal planar available; (2) barometric surfaces are planar in Paterson et al. (1994) and Davidson and surface (Paterson et al., 1994). The Ingalls- Evans (1995) indicate that a pressure increase Swauk contact represents a basin margin that *E-mail: [email protected] caused by loading of the batholith and its host formed well after emplacement, and the Swauk

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685 DISCUSSION AND REPLY

Formation was unlikely to have extended for any PALEOMAGNETIC DATA main incompatible or poorly understood. How- significant distance laterally over the Ingalls ever, we also believe that the greatest understand- Complex and Chiwaukum Schist (cf. Taylor As noted by Ague and Brandon (1996), ing of the geology of this region will eventually et al., 1988). Furthermore, significant segments Lund and coworkers (Lund et al., 1993, 1994; come from an integration of these data sets rather of the Ingalls-Swauk contact have been modified Paterson et al., 1994) collected new paleomag- than an emphasis on a single set. by Eocene or younger faulting (Tabor et al., netic data from the Mount Stuart batholith. This Ague and Brandon (1996), emphasizing a sin- 1982; Miller et al., 1990). Thus, the Chiwaukum- study (as stated in Paterson et al., 1994) was gle data set, reached conclusions about pa- Ingalls-Swauk complex is largely a structural se- able to reproduce, and is completely compatible leobarometric surfaces in the Mount Stuart quence that in part reflects lateral variations in with, the Beck et al. (1981) results. We thus find batholith, possible tilt directions, direction of mo- geology and may easily record effects other than it surprising that Ague and Brandon chose to ac- tion of the Windy Pass thrust, and interpretations tilting. In addition, one may also move from the cept only the older and less-extensive results of of existing paleomagnetic data that we feel are Chiwaukum Schist through the Ingalls ophiolite Beck et al. (1981). In addition, we urge caution not justified given evaluation of all data sets. and into overlying Tertiary sedimentary strata in in interpretation of either data set. Magnetic However, we support two of the main points of a southwest, southeast, east, and northeast direc- remanence values are weak in this batholith and their paper: (1) when certain assumptions are tion rather than only to the south-southeast, as questions remain about the magnetic carrier, the met, calculation of paleobarometric surfaces is a stated by Ague and Brandon. timing of magnetic remanence, and the signifi- useful tool for evaluating geologic and paleo- We were disappointed that Ague and Brandon cance of patterns of the paleomagnetic values. magnetic data; and (2) available data do not re- (1996) did not discuss regional structural pat- However, initial conclusions using both data quire large tilts of the Mount Stuart batholith, terns, metamorphism, and in part geochronologic sets are intriguing. In contrast to statements thus permitting large latitudinal displacement data sets, which have been used previously to ar- made by Ague and Brandon (1996), these data since Cretaceous time. gue for a northeast-up tilt (e.g., Beck et al., 1981; indicate that (1) paleomagnetic results are avail- Haugerud, 1987). Without at least some mention able for the northern loaded part of the REFERENCES CITED of these data sets, and discussion of why they batholith; (2) widespread pyrrhotite and in local conclude that their paleobarometric data set is domains magnetite dominate the magnetic min- Ague, J. J., and Brandon, M. T., 1996, Regional tilt of the Mount Stuart batholith, Washington, determined using more reliable, we feel that it leaves readers with eralogy; (3) paleomagnetic results preserve both aluminum-in-hornblende barometry: Implications for the an overly simplistic view of regional relation- normal polarity (southeast “unloaded” domain) northward translation of Baja British Columbia: Geolog- ical Society of America Bulletin v. 108, p. 471–488. ships and thus possible tilt scenarios. It would be and reversed polarity (northwest “loaded” do- Anderson, J. L., 1996, Status of thermobarometry in granitic particularly helpful to know the nature and loca- main); and (4) the paleomagnetic results require batholiths: Transactions of the Royal Society of Edinburgh, tion of structure(s), given their conclusion of a vertical axis rotation of 20°–40° between at Earth Sciences v. 87, p. 125–138. Anderson, J. L., 1997, Regional tilt of the Mount Stuart northeast-trending tilt axis, that caused their pro- least two crustal blocks, separated by a poorly batholith, Washington, determined using aluminum-in- posed tilt so that others may look for and exam- defined northwest-striking structure (Beck hornblende barometry: implications for northward trans- ine the structure(s). et al., 1981; Lund et al., 1993; Paterson et al., lation of Baja British Columbia: Discussion: Geological Society of America Bulletin, v. 109, p. 1223–1227. 1994, 1996), which has not been recognized in Beck, M. E., Jr., Burnmester, R. F., and Schoonover, R., 1981, DISPLACEMENT OF WINDY PASS FAULT any previous mapping (e.g., Tabor et al., 1982, Paleomagnetism and tectonics of the Cretaceous Mount Stuart Batholith of Washington: Translation or tilt?: Earth 1987; Paterson et al., 1994). At this time these and Planetary Science Letters, v. 56, p. 336–342. Ague and Brandon (1996) assumed that the data do not support or refute the tilt suggested Davidson, G. F., and Evans, B. W., 1995, Kinetically-controlled Windy Pass fault is a single, planar, “near-hori- by Ague and Brandon (1996), but they strongly medium P metamorphism in the Chiwaukum Schist, southwestern North Cascades crystalline core, Washing- zontal” structure “that extended across the en- support the conclusion that tilt and/or rotation ton: Geological Society of America Abstracts with Pro- tire Mount Stuart area.” On the basis of these as- scenarios for this batholith are not simple grams, v. 27, no. 7, p. A-262. sumptions, they suggested northeast-directed and did not occur as a single block rotation Haugerud, R. A., 1987, Argon geochronology of the Ten Peak pluton and untilting of the Wenatchee Block, North Cas- motion for this fault because it cut up their (Paterson et al., 1996). cades range, Washington: Eos (Transactions, American paleobarometric contours to the northeast. We Geophysical Union), v. 68, p. 1814. Lund, S. P., Paterson, S., and Anderson, L., 1993, Paleomag- question this conclusion because the northeast- CONCLUSIONS netism and rock magnetism of the Mount Stuart ern segments of the Windy Pass fault lie at the Batholith: Reassessment of discordant paleomagnetic re- base of detached klippe either present in a >40° Our work to date and that of others indicate sults: Eos (Transactions, American Geophysical Union), v. 74, p. 206. plunging synform, or uplifted in the hanging that the geology of the Mount Stuart region Lund, S. P., Paterson, S., and Anderson, L., 1994, Paleomag- wall of the Tumwater Mountain shear zone. formed by complex processes, resulting in com- netism and rock magnetism of the Mount Stuart Furthermore, the fault is certainly not planar; plex data sets. For example, there is an intriguing Batholith: Reassessment of discordant paleomagnetic results: Geological Society of America Abstracts with Pro- the original geometry was significantly modi- but poorly understood correlation between the grams, v. 26, no. 7, p. A-460. fied by folding during northeast-southwest con- zones of elevated oxygen isotope values noted by Magloughlin, J. F., 1994, The Rock Lake shear zone: A geo- barometric discontinuity in the Nason terrane and impli- traction and batholith emplacement (Miller, Francis et al. (1996), the domains of subsolidus cations for the timing of metamorphism and the mecha- 1985; Paterson et al., 1994). Ague and Brandon deformation in the batholith noted by Miller and nism of crustal thickening: Geological Society of also ignored a great deal of structural data and Paterson (1994), and the zones of remanent mag- America Abstracts with Programs, v. 26, no. 7, p. A-187. Miller, R. B., 1985, The ophiolitic Ingalls Complex, North Cas- regional considerations summarized by Miller netism preserved in magnetite instead of cades, Washington: Geological Society of America Bul- (1985) and Paterson et al. (1994), which suggest pyrrhotite noted by Lund et al. (1993). There is letin, v. 96, p. 27–42. an uncertain but probable north direction of also an excellent correlation between loaded re- Miller, R. B., and Paterson, S. R., 1992, Tectonic implications of syn- and post-emplacement deformation of the Mount early motion with some late southwest displace- gions (marked by late staurolite-kyanite assem- Stuart batholith for mid-Cretaceous orogenesis in the ment. Some discussion of why they disregarded blages) and the region where reversed paleomag- North Cascades, Canadian Journal of Earth Sciences, v. 29, p. 479–485. these observations would help in evaluating netic results are preserved. We encourage readers Miller, R. B., and Paterson, S. R., 1994, The transition from their conclusions. to treat all these data sets with caution: some re- magmatic to high- temperature solid-state deformation:

686 Geological Society of America Bulletin, May 1998 DISCUSSION AND REPLY

Implications from the Mount Stuart batholith, Washing- Sciences, University of Washington, p. 2F-1–2F-47. M. J., and Zartman, R. E., 1987, Geologic map of the ton: Journal of Structural Geology, v. 16, p. 853–865. Paterson, S. R., Lund, S., Miller, R. B., and Teruya, L., 1996, Chelan 30′ × 60′ quadrangle, Washington: U.S. Geologi- Miller, R. B., Johnson, S. Y., and McDougall, J. W., 1990, Dis- Tertiary rotations in the Cascades core, Washington: The cal Survey Map I-1661, scale: 1:100 000. cordant paleomagnetic poles from the Canadian coast importance of the Entiat fault: Geological Society of Taylor, S. B., Johnson, S. Y., Fraser, G. T., and Roberts, J. W., plutonic complex: Regional tilt rather than large displace- America Abstracts with Programs, v. 28, no. 5, p. 99. 1988, Sedimentation and tectonics of the lower and mid- ment?: Comment: Geology, v. 18, p. 1164–1165. Tabor, R. W.,Waitt, R. B., Frizzell,V.A., Swanson, D.A., Byerly, dle Eocene Swauk Formation in eastern Swauk basin, Paterson, S. R., Miller, R. B., Anderson, J. L., Lund, S., G. R., and Bently, R. D., 1982, Geologic map of the central Cascades, central Washington: Canadian Journal Bendixen, J., Taylor, N., and Fink, T., 1994. Emplacement Wenatchee quadrangle, central Washington: U.S. Geologi- of Earth Sciences, v. 25, p. 1020–1036. and evolution of Mount Stuart batholith, in Swanson, cal Survey Map I-1311, scale 1:100 000. D. A., and Haugerud, R. A., eds., Geologic field trips in Tabor, R. W., Frizzell, V.A., Whetten, J. T., Waitt, R. B., Swan- MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 3, 1996 the Pacific Northwest: Seattle, Department of Geological son, D. A., Byerly, G. R., Booth, D. B., Hetherington, MANUSCRIPT ACCEPTED JULY 24, 1997

Mark T. Brandon* Department of Geology and Geophysics, Yale University, Jay J. Ague† } P.O. Box 208109, New Haven, Connecticut 06520-8109

INTRODUCTION well suited for this task and has provided us with the displacement history of the Windy Pass thrust, a robust data set that can be interpreted independ- and (4) reliability and interpretation of the Mount Our use of Al-in-hornblende (AH) barometry to ent of other geologic information. Stuart paleomagnetic data. determine regional tilting of granitic plutons Our primary tectonic interpretation is that the (Ague and Brandon, 1992, 1996) has generated Mount Stuart, at its present level of exposure, pre- PALEOBAROMETRIC SURFACES much argument and disagreement. Most of this ex- serves a set of nearly planar isobaric surfaces that change has been informal, so we welcome this op- were formed when the batholith cooled through Paterson and Miller suggest that all three as- portunity for a published discussion of our work. its solidus and that those surfaces currently dip sumptions underlying our planar-tilt model may We focus here on geologic and tectonic issues about 7° to the southeast (133°). This interpreta- fail for the Mount Stuart batholith. The quoted raised in Paterson and Miller’s discussion. In a tion represents a well-defined testable hypothesis. headings below refer to Paterson and Miller’s separate discussion,Anderson (1997) argued that We maintain that it is fully consistent with our AH summary descriptions of our assumptions. the AH barometer may commonly fail to produce data and the published geology of the region. Our 1. “Reliable measurements of crystallization reliable pressure estimates, a point underscored AH data provide information about the cumula- pressures must be available.” We evaluated this in Paterson and Miller’s discussion. We rely on tive tilt of the batholith, but nothing about its in- assumption at great length (Ague and Brandon, Ague and Brandon (1997) and Ague (1997) to cremental tilt history. A full synthesis of the tilt 1996, 1997). We remain confident of our results address this part of the disagreement. history of the batholith would require a massive because careful attention was paid to the estab- Paterson and Miller’s discussion concerns two integration of igneous and metamorphic barome- lished guidelines for the AH barometer: bulk rock themes: (1) our planar-tilt interpretation is too try, thermochronology, paleomagnetic data, and with a tonalitic composition and rim compositions simple given the complex history of the Mount Cenozoic stratigraphy, a task well beyond the of unaltered hornblende in textural equilibrium Stuart batholith, and (2) we have tended to gloss scope and objective of our paper. with other phases of the specified critical assem- over certain geologic relationships that might un- Much of Paterson and Miller’s discussion fo- blage. We cannot vouch for Anderson’s (1997) re- dermine our interpretation. We suspect that these cuses on what they see as conflicts between our sults (as variously reported in Paterson et al., 1994; disagreements are further fueled by differences in AH–based results and regional geologic relation- Anderson and Smith, 1995) because he generally the scale of the problems that our groups are ships. None of their points is new to us. We have disregarded the accepted guidelines for the barom- studying. Paterson, Miller, and coworkers have given considerable thought to each and have de- eter. His AH determinations came from samples been studying the processes of pluton emplace- cided that there is no available geologic evidence that had a wide variety of bulk compositions and ment as revealed by local deformation and meta- that refutes our planar-tilt hypothesis. Paterson commonly lacked such critical minerals as potas- morphism of the Mount Stuart batholith and sur- and Miller acknowledge that much of their evi- sium feldspar and sphene. Furthermore, it is un- rounding country rock (e.g., Miller and Paterson, dence is ambiguous with respect to the tilt ques- clear whether he was careful to avoid hydrother- 1992, 1994; Paterson et al., 1994). Our work has tion, but conclude that our interpretation is likely mal alteration in the hornblendes. His nominal focused on a regional-scale study of the pressure flawed, given the large number of contrary points pressure estimates are different from ours in that field in the batholith at the time of emplacement that they have identified. We fail to see how the they do not follow a planar trend and show local with the goal of using this information to restore evidence becomes any stronger in aggregate. We variations, especially near the northeast margin of a batholith-scale paleomagnetic data set (Beck support these broad statements below with a the batholith (e.g., Paterson et al., 1994, Fig. 11). et al., 1981). We believe that AH barometry is point-by-point reply to the four specific issues Ague and Brandon (1992) recognized that the raised by Paterson and Miller, i.e., (1) Estimation northeast lobe of the Mount Stuart showed exten- *e-mail: [email protected] of paleobarometric surfaces, (2) conflicts with re- sive subsolidus alteration, so we are not surprised †[email protected] gional geologic relationships, (3) inferences about that Paterson et al. (1994), Anderson (1996,

Geological Society of America Bulletin, May 1998 687 DISCUSSION AND REPLY

1997) and Francis et al. (1996) have come to the 3. “Tilting of the batholith did not occur during margin of the block is overlapped by a thick, well- same conclusion. Our paper showed that hy- cooling.” This summary of our assumption is mis- stratified sequence of Paleogene sediments and drothermal alteration of hornblende was patchy, leading. In our words (Ague and Brandon, 1996), volcanic rocks (e.g., Swauk Formation, Teanaway and that with careful use of back-scattered elec- we assumed that “the batholith underwent no sig- basalts, and Roslyn Formation described in Tabor tron imaging, primary igneous compositions nificant tilting until after it cooled through the et al., 1987). Published mapping (Frizzell et al., could be recovered. magnetic block temperature....”When restoring 1984; Tabor et al., 1987) clearly shows that this The alteration history of the northeast lobe of paleomagnetic data, the AH–determined paleo- sequence was deposited unconformably on the the Mount Stuart is important for understanding horizontal is only useful if the batholith did not tilt Mount Stuart block. The unconformity is marked the tectonic history of the entire batholith, but significantly between the closure time of the AH by Ni-rich lateritic sediments derived from weath- it is not relevant to restoring the controversial barometer and the blocking time of the measured ering of the underlying Jurassic Ingalls ophiolite paleomagnetic data of Beck et al. (1981), which magnetization. This assumption is difficult to (iron sandstone unit of Frizzell et al., 1984; Tabor came from the relatively unaltered southern half evaluate because the blocking temperature for the et al., 1987). At a regional scale, the Paleogene se- of the batholith. Paterson and Miller do not ad- Beck et al. (1981) magnetization remains poorly quence youngs to the south and ultimately dips dress this important distinction. constrained (see discussion of paleomagnetism in beneath the Miocene flood basalts of the Colum- 2. “Barometric surfaces are planar and roughly Ague and Brandon, 1996, for details). However, bia River Plateau. Paterson and Miller are correct parallel to paleohorizontal.” Paterson and Miller’s concordant K-Ar ages for hornblende and biotite in saying that the Paleogene sequence is folded main point is that “much of the batholith was pairs from the western and southern parts of the and faulted and that the basal unconformity is lo- folded during final crystallization and that folding Mount Stuart indicate rapid cooling in those areas cally faulted as well, but these features are impor- and ductile shear continued in four domains during (Ague and Brandon, 1996, Fig. 2). About half of tant only at the local scale. They do not change the subsolidus cooling of the batholith. . . .” Closure of the paleomagnetic sites of Beck et al. (1981) fact that the Paleogene sequence has a general the AH barometer is thought to occur at or slightly come from those parts of the batholith. There was southward dip at the regional scale. We (Ague and below the solidus (see Ague and Brandon, 1996, probably not enough time at those sites for much Brandon, 1996) also noted that the Windy Pass for details), so magmatic structures are not relevant tilting to occur between closure of the AH barom- thrust and Ingalls ophiolite show a similar gener- to our study. etry and blocking of the reported magnetization. ally southward dip (Miller, 1985). Paterson et al. (1994, p. 2F–12F) stated, “Most Our intent in citing these relationships is to of the Mount Stuart batholith only shows minor REGIONAL RELATIONSHIPS show that the southeast dip of the Mount Stuart subsolidus deformation (e.g., undulose extinction, batholith is not totally unexpected. However, it is deformation twins) and recrystallizations (e.g., In Ague and Brandon (1996), we argued that important to note that available geologic data pro- hornblende and biotite locally replaced by seri- our tilt interpretation for the Mount Stuart bath- vide no hard evidence concerning the tilt geome- cite). Exceptions occur in three domains along the olith was reasonably consistent with the sur- try of the batholith. Many authors have attempted northeast margin of the batholith . . . and one do- rounding geology at a regional scale. Paterson to deduce tilt of the batholith using the patterns main within the batholith where gently dipping and Miller question our arguments on this mat- of metamorphism and cooling ages within the magmatic foliations exist. . . .” (See Miller and ter. We hope that the expanded discussion below Mount Stuart block (Beck et al., 1981; Haugerud, Paterson, 1992, 1994, for further details.) helps to clarify the disagreement. 1987; Umhoefer and Magloughlin, 1990; Miller Using Figure 14 in Paterson et al. (1994), we The Mount Stuart batholith and associated et al., 1990; Beck, 1991). We believe that these estimate that these deformed domains account for Mesozoic country rock (e.g., Ingalls ophiolite and approaches are problematic because the Mount <5% of the batholith. The three domains along the Chiwaukum Schist) collectively make up a fault- Stuart block has a complex metamorphic and northeast margin of the batholith are not relevant bounded block, which we call the Mount Stuart thermal history. The most significant event is the to issues about deformation within the batholith. block (Haugerud, 1987, used the term Wenatchee Late Cretaceous load-related metamorphism that The Tumwater Mountain shear zone lies along the block). We contend that the southern half of the postdated intrusion of the Mount Stuart batholith northeast margin of the batholith and is included Mount Stuart block—from the northern margin of and affected the northeast lobe of the batholith in one of these domains. The only deformed do- the Mount Stuart batholith to the southern limit of and the surrounding Chiwaukum Schist (see main within the batholith (Pioneer Creek) is de- the Ingalls ophiolite—has seen relatively little in- Ague and Brandon, 1996, for summary). Follow- scribed as having a strong magmatic foliation and ternal deformation since the emplacement of the ing Evans and Berti (1986) and Brown and a weak subsolidus fabric (Paterson et al., 1994). batholith. Our inference is based on the preserva- Walker (1993), we argued that a tectonic or mag- None of these details are relevant to our study. tion of planar isobaric surfaces in the Mount Stu- matic load caused prograde metamorphism of the Paterson and Miller also stated, “We have art and the relatively static nature of the post-in- Chiwaukum Schist and widespread hydrothermal concerns about whether any barometric data in trusion load-related metamorphism that affected alteration of the northeast lobe. This load is com- the Mount Stuart batholith should currently fit a the Chiwaukum Schist (Evans and Berti, 1986; pletely gone now, so one can only speculate single plane.” We were also worried that the Brown and Walker, 1993; Paterson et al., 1994). whether it was a higher thrust sheet (Evans and batholith may have a more complex subsolidus The block is bounded by major north- and north- Berti, 1986; McGroder, 1991) or a massive high- internal structure. We closely examined (Ague west-striking high-angle faults on its east side level intrusion (Brown and Walker, 1993). The and Brandon, 1996) the spatial distribution and (e.g., Leavenworth fault) and along at least part of critical point for us is that metamorphic isograds the magnitude of the misfit between our AH data its west side (e.g., Evergreen, and Deception Pass in the Chiwaukum Schist and cooling ages in the and the best-fit planar-tilt model. We also exam- faults) (e.g., see Fig. 2a in Ague and Brandon, Chiwaukum Schist and the northern part of the ined more restricted parts of the batholith to see 1996, or Fig. 1 in Paterson et al., 1994). We ac- Mount Stuart batholith are related primarily to if we could detect differential tilts. None of these knowledge that relationships are poorly exposed this younger event and provide no direct evidence tests revealed the presence of more complex re- on the west side of the block and remain disputed of the cumulative tilt experienced by the batholith. gional-scale structure, such as broad folds or (Miller et al., 1990; Butler et al., 1989, 1990; It is interesting to note that this loading event ap- widely spaced faults. Tabor et al., 1993). Nonetheless, the southern pears to have caused no detectable distortion of

688 Geological Society of America Bulletin, May 1998 DISCUSSION AND REPLY the isobaric surfaces within the batholith, which about thrust ramps without some stratigraphic or Paterson et al. (1994) claimed that they repli- implies that the batholith moved as a rigid body paleohorizontal reference. The footwall and hang- cated the study of Beck et al. (1981) because they during emplacement and removal of the higher ing wall of the Windy Pass thrust are a polyphase- sampled near some of the Beck et al. sites, but level load. deformed schist and serpentinized ultramafite, re- there is no assurance that the same lithologies Paterson and Miller encourage us to specify the spectively. were sampled or that the replicated sites had the nature and location of structures responsible for Paterson and Miller question our proposal of a same magnetization. Thus, we considered it un- the proposed southeast-down tilting. First, we ramp structure in the Windy Pass thrust, but their wise to judge the published work of Beck et al. note that a tilt axis is not a structure, but rather a summary is misleading. We stated that “the Windy (1981) using the preliminary results of Lund et al. geometric pole of rotation. Therefore, our defini- Pass thrust and superjacent Ingalls ophiolite prob- (1993, 1994). tion of a tilt axis does not predict any specific ably originated as a near-horizontal structural se- Most of Paterson and Miller’s criticisms of the structure, in much the same way that a fold axis quence that extended across the entire Mount Stu- Beck et al. (1981) paleomagnetic data (numbered does not predict the geometry and location of a art area.” Thus, the isobaric surfaces in the Mount 1–3 in their Discussion) were already addressed fold. Second, it should be obvious that our esti- Stuart batholith can be used, in a rough way, to in- in several long paragraphs in our paper (Ague and mated tilt for the Mount Stuart is the sum of sev- fer the climb direction of the Windy Pass thrust Brandon, 1996, p. 483–484). The last comment eral poorly understood incremental tilts, including relative to paleohorizontal (Fig. 2a in Ague and (number 4 in their Discussion) is incorrect on sev- the load-related metamorphism, postmetamor- Brandon, 1996). The western exposed end of the eral counts. There is a small discordance in paleo- phic exhumation, the formation of regional angu- thrust starts at the 7 km contour and then climbs magnetic directions for the eastern and western lar unconformities at the base and top of the rapidly to the northeast to the 6 km contour, which sites of Beck et al. (1981), but Beck et al. noted Paleogene sequence, and the modern uplift of the it follows for some 10 to 20 km to the east. This that the discordance was just at the limit of statis- Cascade Range. It would be naive on our part to relationship suggests a ramp-flat geometry, the tical significance. So differential motion within ascribe all of the observed tilt to a single event and Windy Pass thrust climbing to the northeast rela- the batholith is suggested, but not required, by the a single structure. tive to our estimated isobaric surfaces for the paleomagnetic data. Furthermore, the discordance Given this disclaimer, we suggest that much Mount Stuart batholith. This geometry implies between the two sets of directions does not re- of the tilt might be due to the uplift of the Mount top-to-the-northeast motion of the Ingalls ophiolite quire a vertical-axis rotation. The discordance Stuart block. This style of deformation has been over the Chiwaukum Schist, which was the inter- could have been produced by rotation on any axis referred to as “piano-key tectonics,” a term of pretation favored by McGroder (1991) in his re- that lies in the plane that bisects the two sets of di- J. Rodgers (1996, personal commun.). Fault- gional-scale cross section. rections. For example, a +10° rotation around a bounded sedimentary sequences east and west Paterson and Miller argue that the top-northeast horizontal axis lying at about 165° would bring of the Mount Stuart block have dropped down, motion on the Windy Pass thrust is inconsistent the eastern site directions into coincidence with while the Mount Stuart block has rotated up by with “a great deal of structural data and regional the western site directions. south-down tilting of the block. This deforma- considerations summarized by Miller (1985) and tion probably started in Eocene time and may Paterson et al. (1994) which suggest an uncertain CONCLUSIONS have been reactivated during late Cenozoic up- but probable north direction of early motion with lift of the modern Cascade Range. some late southwest displacement.” In our paper, The main objective of our study was to restore we acknowledged Miller’s (1985) preference for the paleomagnetic data of Beck et al. (1981) in DISPLACEMENT OF WINDY PASS a top–north slip direction on the Windy Pass order to estimate the northward offset of Baja FAULT thrust but omitted further review because the ex- British Columbia (Cowan et al., 1997). Wynne isting structural data seemed vague and inconclu- et al. (1995) published a high-quality paleomag- Paterson and Miller cite Paterson et al. (1994) sive regarding the main transport direction associ- netic study for Baja British Columbia based on for the conclusion that the Windy Pass thrust “was ated with ophiolite obduction. Our exchange here sedimentary and volcanic rocks of the Tatlow active during emplacement of the Mount Stuart highlights the need for a modern kinematic analy- syncline in southwest British Columbia (located batholith, and likely displayed major ramps rather sis of this important structure. ~475 km north of Mount Stuart). They isolated a than forming a single, subhorizontal planar sur- well-defined high-coercivity prefolding magnet- face....”We agree that the strong magmatic and PALEOMAGNETIC DATA ization, which can be restored to paleohorizontal weak subsolidus deformation recognized by using local bedding. If Mount Stuart was Miller and Paterson (1994) and Paterson et al. Paterson and Miller question that we did not incorrectly restored, then we should see a differ- (1994) in the Pioneer Creek domain of the Mount use more recent paleomagnetic data from the ence in the northward offsets estimated by these Stuart batholith might have been formed by mo- Mount Stuart batholith. We chose to rely on the studies. For instance, a degree of inclination er- tion on the Windy Pass thrust. This deformation older study of Beck et al. (1981) because it was ror would produce an error in the estimated seems relatively minor compared to the very large fully published. The work of Lund et al. is avail- northward offset of ~98 km. Wynne et al.’s (1995) slip that occurred during obduction of the Ingalls able only in abstract form (Lund et al., 1993, results indicate a northward offset of 3000 ± 500 ophiolite, which overlies the Windy Pass thrust. In 1994). Paterson et al. (1994) provided a brief km, which is in very good agreement with our fact, deformation in the Pioneer Creek domain summary but directional data for sites and de- estimate of 3100 ± 600 km. Recent paleomag- could be due to reactivation of the Windy Pass magnetization plots were not reported. A mod- netic results of Ward et al. (1997) indicate a sim- thrust after ophiolite obduction. ern paleomagnetic study of the Mount Stuart ilar northward offset (~3500 km) for Campanian Paterson et al. (1994) are cited as the source for batholith is clearly needed, including step-wise strata (~80 Ma) of the Nanaimo Group, which observations about ramp structures along the demagnetization and component analysis. Per- was deposited on the southwest part of the Baja Windy Pass thrust, but we found no discussion haps the work of Lund et al. will fill this gap, but British Columbia block. there of this point. Furthermore, we do not see we cannot evaluate their results until they are We conclude that our results from the Mount how Paterson and Miller can make inferences fully published. Stuart batholith show that AH barometry can be

Geological Society of America Bulletin, May 1998 689 DISCUSSION AND REPLY used to provide an independent measure of the Beck, M. E., Jr., 1991, Some thermal and paleomagnetic conse- Society of America Bulletin, v. 103, p. 189–209. quences of tilting a batholith: Tectonics, v. 11, p. 297–302. Miller, R. B., 1985, The ophiolitic Ingalls Complex, north-cen- regional-scale structure of a large batholith. We Beck, M. E., Jr., Burmester, R. F., and Schoonover, R., 1981, tral Cascade Mountains, Washington: Geological Society thank Paterson and Miller for their critical com- Paleomagnetism and tectonics of the Cretaceous Mount of America Bulletin, v. 96, p. 27–42. Stuart Batholith of Washington: Translation or tilt?: Earth Miller, R. B., and Paterson, S., 1992, Tectonic implications of ments, which have allowed us a context and a fo- and Planetary Science Letters, v. 56, p. 336–342. syn- and post-emplacement deformation of the Mount rum to clarify some important questions about Brown, E. H., and Walker, N., 1993, A magma-loading model Stuart batholith for mid-Cretaceous orogenesis in the our work. for Barrovian metamorphism in the southeast Coast Plu- North Cascades: Canadian Journal of Earth Sciences, tonic Complex, British Columbia and Washington: Geo- v. 29, p. 479–485. logical Society of America Bulletin, v. 105, p. 479–500. Miller, R. B., and Paterson, S., 1994, The transition from mag- ACKNOWLEDGMENTS Butler, R., Gehrels, G., McClelland, W., May, S., and Klepacki, matic to high-temperature solid-state deformation: Impli- D., 1989, Discordant paleomagnetic poles from the Cana- cations from the Mount Stuart batholith, Washington: dian coast plutonic complex: Regional tilt rather than Journal of Structural Geology, v. 16, p. 853–865. This work was supported by National Science large displacement?: Geology, v. 17, p. 691–694. Miller, R. B., Johnson, S., and McDougall, J., 1990, Discordant Foundation grant EAR-9106652. Butler, R., Gehrels, G., McClelland, W., May, S., and Klepacki, paleomagnetic poles from the Canadian Coast Plutonic D., 1990, Discordant paleomagnetic poles from the Cana- Complex: Regional tilt rather than large displacement?: dian coast plutonic complex: Regional tilt rather than Comment: Geology, v. 18, p. 1164–1165. REFERENCES CITED large displacement?: Reply: Geology, v. 18, p. 801–802. Paterson, S., Miller, R. B., Anderson, J. L., Lund, S., Bendixen, Cowan, D. S., Brandon, M. T., and Garver, J. I., 1997, Geologic J., Taylor, N., and Fink, T., 1994, Emplacement and evo- Ague, J. J., 1997, Thermodynamic calculation of emplacement tests of hypotheses for large coastwise displacements: A lution of the Mount Stuart batholith, in Swanson, D., and pressures for batholithic rocks, California: Implications critique illustrated by the Baja British Columbia contro- Haugerud, R., eds., Guides to field trips, Geological So- for the aluminum-in-hornblende barometer: Geology, versy: American Journal of Science, v. 297, p. 117–173. ciety of America Annual Meeting: Seattle, Washington, v. 25, p. 563–566. Evans, B. W., and Berti, J. W., 1986, A revised metamorphic p. 2F-1–2F-48. Ague, J. J., and Brandon, M. T., 1992, Tilt and northward offset history for the Chiwaukum Schist, North Cascades, Tabor, R. W., Frizzell, V.A., Jr., Whetten, J. T., Waitt, R. B., Jr., of Cordilleran batholiths resolved using igneous barome- Washington: Geology, v. 14, p. 695–698. Swanson, D. A., Byerly, G. R., Booth, D. B., Hethering- try: Nature, v. 360, p. 146–149. Francis, J., Anderson, J. L., and Morrison, J., 1996, Oxygen iso- ton, M. J., and Zartman, R. E., 1987, Preliminary geo- Ague, J. J., and Brandon, M., 1996, Regional tilt of the Mount topic zonation of a Cordilleran batholith: Eos (Transac- logic map of the Chelan 30′ × 60′ quadrangle, Washing- Stuart batholith, Washington, determined using alu- tions, American Geophysical Union), v. 77, p. S290. ton: U.S. Geological Survey Map I-1661, scale minum-in-hornblende barometry: Implications for north- Frizzell, V. A., Jr., Tabor, R. W., Booth, D. B., Ort, K. M., and 1:100 000, 1 sheet, 33 p. ward translation of Baja British Columbia: Geological Waitt, R. B., Jr., 1984, Preliminary geologic map of the Tabor, R., Frizzell, V.A., Booth, D., Waitt, R., Whetten, J., and Society of America Bulletin, v. 108, p. 471–488. Snoqualmie Pass quadrangle, Washington: U.S. Geolog- Zartman, R., 1993, Geologic map of the Skykomish River Ague, J. J., and Brandon, M., 1997, Regional tilt of the Mount ical Survey Open-File Map OF-84–693, scale 1:100 000, 30′ × 60′ quadrangle, Washington: U.S. Geological Sur- Stuart batholith, Washington, determined using alu- 1 sheet, 42 p. vey Map I-1963, scale 1:100 000, 1 sheet, 42 p. minum-in-hornblende barometry: Implications for north- Haugerud, R., 1987, Argon geochronology of the Ten Peak plu- Umhoefer, P., and Magloughlin, J., 1990, Discordant paleo- ward translation of Baja British Columbia: Reply: Geo- ton and untilting of the Wenatchee Block, North Cascades magnetic poles from the Canadian Coast Plutonic Com- logic Society of America Bulletin, v. 109, p. 1223–1227. range, Washington: Eos (Transactions, American Geo- plex: Regional tilt rather than large-scale displacement: Anderson, J. L., 1996, Status of thermochronology in granitic physical Union), v. 68, p. 1814. Comment: Geology, v. 18, p. 800–801. batholiths, in Brown, M., Candela, P. A., Peck, D. L., Lund, S., Paterson, S., and Anderson, J. L., 1993, Paleomag- Ward, P. D., Hurtado, J. M., Kirschvink, J. L., and Verosub, K., Stephens, W. E., Walker, R. J., and Zen, E-an., eds., The netism and rock magnetism of the Mount Stuart 1997, Measurements of the Cretaceous paleolatitude of third Hutton symposium on the origin of granites and re- Batholith: Reassessment of discordant paleomagnetic re- Vancouver Island: Consistent with the Baja–British Co- lated rocks: Geological Society of America Special Paper sults: Eos (Transactions, American Geophysical Union), lumbia hypothesis, Science, v. 277, p. 1642–1645. 315, p. 125–138. v. 74, p. 206. Wynne, P., Irving, E., Maxon, J., and Kleispehn, K., 1995, Pa- Anderson, J. L., 1997, Regional tilt of the Mount Stuart Lund, S., Paterson, S., and Anderson, J. L., 1994, Paleomag- leomagnetism of the Upper Cretaceous strata of Mount batholith, Washington, determined using aluminum-in- netism and rock magnetism of the Mount Stuart Tatlow: Evidence for 3000 km of northward displacement hornblende barometry: Implications for northward trans- Batholith: Reassessment of discordant paleomagnetic re- of the eastern Coast Belt, British Columbia: Journal of lation of Baja British Columbia: Discussion: Geological sults: Geological Society of America Abstracts with Pro- Geophysical Research, v. 100, p. 6073–6092. Society of America Bulletin, v. 109, p. 1223–1227. grams, v. 26, no. 7, p. A-460. Anderson, J. L., and Smith, D., 1995, The effects of tempera- McGroder, M. F., 1991, Reconciliation of two-sided thrusting, ture and oxygen fugacity on the Al-in-hornblende barom- burial metamorphism, and diachronous uplift in the Cas- MANUSCRIPT RECEIVED BY THE SOCIETY JULY 2, 1997 eter: American Mineralogist, v. 80, p. 549–559. cades of Washington and British Columbia: Geological MANUSCRIPT ACCEPTED JULY 9, 1997

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