Hydrothermally enhanced magnetization at the center of the Haughton impact structure?

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In 2011, the discovery of shatter cones confirmed the 28 km diameter Tunnunik complex impact structure, Northwest Territories, Canada. This study presents the first results of ground-based electromagnetic, gravimetric, and magnetic surveys over this impact structure. Its central area is characterized by a~10 km wide negative gravity anomaly of about 3 mGal amplitude, roughly corresponding to the area of shatter cones, and associated with a positive magnetic field anomaly of~120 nT amplitude and 3 km wavelength. The latter correlates well with the location of the deepest uplifted strata, an impact-tilted Proterozoic dolomite layer of the Shaler Supergroup exposed near the center of the structure and intruded by dolerite dykes. Locally, electromagnetic field data unveil a conductive superficial formation which corresponds to an 80-100 m thick sand layer covering the impact structure. Based on the measurements of magnetic properties of rock samples, we model the source of the magnetic anomaly as the magnetic sediments of the Shaler Supergroup combined with a core of uplifted crystalline basement with enhanced magnetization. More classically, the low gravity signature is attributed to a reduction in density measured on the brecciated target rocks and to the isolated sand formations. However, the present-day fractured zone does not extend deeper than~1 km in our model, indicating a possible 1.5 km of erosion since the time of impact, about 430 Ma ago. PLAIN LANGUAGE SUMMARYThis study reveals the geophysical signature of the buried structure of an eroded impact crater, Tunnunik, located in Northwest Territories, Canada. A positive magnetic anomaly was detected at the center, showing the uplift of some deep geological formations and the possible presence of strongly magnetized basement. A negative gravimetric anomaly is also observed, mostly corresponding to the fracturing/brecciation of the impacted rocks inside the crater. Using numerical models constrained by laboratory measurements on rock samples, the physical properties and geometry of the buried geological formations are estimated. An important implication of this study is the link between the geophysical remains of an impact crater and the postimpact erosion. KEY POINTSA 3 mGal negative gravity anomaly and a 120 nT positive magnetic anomaly were measured over the center of the Tunnunik impact structure. A numerical model constrained by laboratory measurements on rock samples suggests an uplifted magnetic crystalline basement. The fracturing/brecciation extends down to 1 km in depth.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Geophysical signature of the Tunnunik impact structure, Northwest Territories, Canada

Quesnel

Zylberman

Rochette

et al. 2020

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“…*The average impact is computed from sites TUN07, 20, 21, 29, 31, 33, 36 (see text). for post-tilting remagnetization of these sedimentary rocks are thermal magnetization during cooling from impact-related heating, shock magnetization during pressure release immediately after passage of the impact shock wave, or chemical magnetization resulting from postimpact hydrothermal activity (e.g., Zylberman et al 2017). Shock remanent magnetization is excluded as the source of the impact-related magnetization because the release of the shock wave occurs before tilting, so that a shock remanence would pass a tilt test.…”

Section: Discussionmentioning

confidence: 99%

A Paleozoic age for the Tunnunik impact structure

Lepaulard

Gattacceca

Swanson‐Hysell

et al. 2019

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We report paleomagnetic directions from the target rocks of the Tunnunik impact structure, as well as from lithic impact breccia dikes that formed during the impact event. The target sedimentary rocks have been remagnetized after impact‐related tilting during a reverse polarity interval. Their magnetization is unblocked up to 350 °C. The diabase dikes intruding into these sediments retained their original magnetization which unblocks above 400 °C. The impact breccia records a paleomagnetic direction similar to that of the overprints in the target sedimentary rocks. The comparison of the resulting virtual geomagnetic pole for the Tunnunik impact structure with the apparent polar wander path for Laurentia combined with biostratigraphic constraints from the target sedimentary rocks is most consistent with an impact age in the Late Ordovician or Silurian, around 430–450 Ma, soon after the deposition of the youngest impacted sedimentary rocks. Our results from the overprinted sedimentary rocks and diabase dikes imply that the postimpact temperature of the studied rocks was about 350 °C.

“…Another study, by Osinski and Ferri ere (2016), of the HIS shatter cones demonstrated that their distribution is an indicator of the extent of the apparent crater diameter and obliquity of the impact, and that they play a role in reducing the strength of target rocks, thus enhancing crater collapse. Further studies of the HIS investigated the central positive magnetic anomaly (~900 nT), and concluded that impact-generated hydrothermal alteration crystallized magnetite in the uplifted basement rock, which was subsequently concentrated in the near surface, producing the very strong anomaly observed presently (Quesnel et al 2013;Zylberman et al 2017).…”

Section: History Of Haughton Investigationsmentioning

confidence: 99%

A novel geomatics method for assessing the Haughton impact structure

Patterson

Ernst

2020

Terrestrial impact structures are typically modified by erosion, burial, and tectonic deformation. Their systematic morphologies are typically reconstructed through a combination of geological and topographic mapping, satellite imagery, and geophysical surveys. This study applies a novel geomatics approach to assessment of the morphology of the extensively studied Haughton impact structure (HIS), Devon Island, Nunavut, in order to test its potential to improve the accuracy and quality of future impact structure reconstruction. This new methodology integrates HIS lithological data, in the form of digitized geologic mapping, with a digital elevation model, within diametrically opposed, wedge-shaped couplets, and plots these data as pseudo cross sections that capitalize on the radial symmetry of the impact structure. The pseudo cross sections provide an accurate reconstruction of the nearsurface stratigraphic sequences and terraces in the faulted annulus of the modified crater rim. The resultant pseudo cross sections support current interpretations regarding the 10-12 km diameter of the transient cavity, and successfully reproduce the visible outer ring and intermediate uplifted zone within the central basin. Observed positions of vertical offsets suggest that the extent of impact deformation extends beyond the current estimates of the apparent crater rim to radial distances of between 14 and 15 km. Finally, the new geomatics approach provides an improved determination of the spatial, stratigraphic, and temporal relationships among the Haughton Formation lake sediments, allochthonous crater-fill impactites, and uplifted target rocks, and indicates that there was a not-insignificant time gap between the formation of the impact structure and the deposition of the sediments.