Margot McMechan mmcmecha@nrcan.gc

1. Transverse Structure and Tectonic Inversion Mosque Mountain area, southeastern Skeena Fold Belt Earth Sciences Sector Northern Resources Development Margot McMechan mmcmecha@nrcan.gc.ca

2. LOCATION Mosque Mtn area At NW end southwestern Sustut basin of preservation Evenchick, 2003

3. STRATIGRAPHY • Paleocurrents lower Tango Creek SW directed(Eisbacher, 1974) • Basin of preservation • Major change in level sub-Sustut unconformity across Mosque Mountain area

4. MosqueMtn Evenchick et al., 2003 Anomalous area with transverse northeast to east and orogen parallel northwest compressive structures

5. STRUCTURAL STYLE • Similar for both northwest and northeast to east trends • FOLDS - dominantly detachment folds, formed above blind detachments, wavelength largely controlled by thickness dominant member; different levels of detachment Longer wavelength folds where thick sandstones occur in Tango, and sst/cgl at base Brothers Peak Thick sandstones near base Tango Thick sandstone/conglomerate base Brothers Peak, at Mosque Mountain

6. NW-trending folds common throughout area • NE- to E-trending folds occur mainly in eastern part of area Nearly orthogonal detachment folds Brothers Peak Fm, south of Mosque Mtn

7. Area where NE to E trending folds more common • Fold interference evident on stereograms (widely dispersed poles, calt FA nearly orthogonal to mapped fold) • In o/c as variable plunges and unusual orientation

8. REVERSE FAULTS • Can be difficult to recognize • NW, N, NE and E-trends • Asymmetry subparallel hanging and/or footwall folds indicate: • NE-directed motion on NW trending faults; • NW or SE-directed motion on NE trending faults View to S

9. RELATIVE TIMING RELATIONSHIPS NE-trending crenulations of steep E limb SE plunging fold. View to N View to NW • Mainly from relationships between nearly orthogonal folds and reverse faults a a

10. View to SE • NO consistent timing relationship • NE of Mosque Mtn at least 3 episodes NE-trending compression separated by movement on NW-trending structures

11. SHORTENING • NE directed approx 30% • NW-SE directed 5-10% • Regional 44% NE directed, NE Skeena Fold Belt (Evenchick, 1991) B’ A Late dextral oblique-slip and normal faults MosqueMtn B A’

12. EARLY PRE-SUSTUT STRUCTURE

13. POSSIBLE ORIGINS COMPRESSIVE STRUCTURAL TRENDS NORTHWEST-TRENDS • Express NE-directed shortening • Consistent with northeastward expansion of orogenic wedge (Critical Taper Model of Davis et al., 1983; Evenchick 2001)

14. NORTHEAST- TO EAST- TRENDING • Express NW- to N-directed shortening highly oblique to northeastward expansion of orogenic wedge • Potential explanations • En-echelon strike-slip faulting • Rotation about a vertical axis • Oblique ramp • Basement features View to SW

15. Rotation about a vertical axis En-echelon strike-slip faults Pre-mid Cretaceous (95 Ma) Both have geometric and timing problems Evenchick, 2003

16. Oblique Ramps - cause out of plane stresses - oblique folds, faults, cleavages, joints in hanging wall Surface geology, depth to magnetic basement calculations (Lowe et al., in press) rule out

17. Constant thickness Thin Thick Structure curves out to foreland where sedimentary section thickens significantly (ie Helena Embayment, Crowsnest Deflection) Basement Features- transverse sedimentary taper Such thickening occurs under Mosque Mtn area Sotoet al., 2002

18. Basement Features – effecting preferred zones of detachment Bowser Sedimentary taper, and distribution of preferred zones of detachment in Bowser could cause northeastward swing in structural trends, - help explain why less shortening in Sustut south of Mosque Mtn area

19. Basement Features- inversion of pre-Sustut “basement uplift” NW end pre-Sustut basement uplift sloped wrong way for compressive out-of-plane deformation with NE-directed transport Bowser Bowser Inversion above NE-directed thrust resulted in -localization NW end Sustut preservational basin - potential development oblique structures Mosque Mtn area along higher detachments

20. A STRUCTURAL MODEL • Skeena Fold Belt characterized by detachment folding above multiple detachments and blind thrusts • Mosque Mtn area nearly orthogonal trends – defm on alternatively active detachments and (blind) thrusts • NE- to E-trends out-of-plane stress due to inversion or reactivation north end basement uplift or detachment deflection at N end • NW-trends formed at times or above detachments that lacked those influences (Evenchick, 1991)

21. SUMMARY • Anomalous nearly orthogonal compressive structures, parallel and transverse to the regional structural trend, formed alternatively during post early Maastrichtian deformation of the Mosque Mtn area • The NW end of a pre-Albian pre-Sustut “basement” uplift underlies the Mosque Mtn area • Structural inversion of the pre-Sustut “basement” uplift produced a SE-facing monocline that localized the NW limit of preserved Sustut strata to the Mosque Mtn area and caused detachment deflection, and out-of-plane stresses to form anomalous transverse structures in the Mosque Mtn area